[PATCH v4 0/5] Improve simple directory offset wrap behavior

[PATCH v4 0/5] Improve simple directory offset wrap behavior

[cel]

From: Chuck Lever <chuck.lever@oracle.com>

The purpose of this series is to construct a set of upstream fixes
that can be backported to v6.6 to address CVE-2024-46701.

My original plan was to add a cursor dentry. However, I've found a
solution that does not need one. In fact, most or all of the
reported issues are gone with 4/5. Thus I'm not sure 5/5 is
necessary, but it seems like a robust improvement.

Changes since v3:
- Series is no longer RFC
- Series passes xfstests locally and via NFS export
- Patch 2/5 was replaced; it now removes simple_offset_empty()
- 4/5 and 5/5 were rewritten based on test results
- Patch descriptions have been clarified

This series (still against v6.12) has been pushed to:

https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/log/?h=tmpfs-fixes

Next step is to try backporting these to v6.6 to see if anything
else is needed.

Chuck Lever (5):
  libfs: Return ENOSPC when the directory offset range is exhausted
  Revert "libfs: Add simple_offset_empty()"
  Revert "libfs: fix infinite directory reads for offset dir"
  libfs: Replace simple_offset end-of-directory detection
  libfs: Use d_children list to iterate simple_offset directories

 fs/libfs.c         | 158 ++++++++++++++++++++++-----------------------
 include/linux/fs.h |   1 -
 mm/shmem.c         |   4 +-
 3 files changed, 81 insertions(+), 82 deletions(-)

-- 
2.47.0

[PATCH v4 1/5] libfs: Return ENOSPC when the directory offset range is exhausted

[cel]

From: Chuck Lever <chuck.lever@oracle.com>

Testing shows that the EBUSY error return from mtree_alloc_cyclic()
leaks into user space. The ERRORS section of "man creat(2)" says:

>	EBUSY	O_EXCL was specified in flags and pathname refers
>		to a block device that is in use by the system
>		(e.g., it is mounted).

ENOSPC is closer to what applications expect in this situation.

Note that the normal range of simple directory offset values is
2..2^63, so hitting this error is going to be rare to impossible.

Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
Cc: <stable@vger.kernel.org> # v6.9+
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 46966fd8bcf9..bf67954b525b 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -288,7 +288,9 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 
 	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
 				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
-	if (ret < 0)
+	if (unlikely(ret == -EBUSY))
+		return -ENOSPC;
+	if (unlikely(ret < 0))
 		return ret;
 
 	offset_set(dentry, offset);
-- 
2.47.0

[PATCH v4 2/5] Revert "libfs: Add simple_offset_empty()"

[cel]

From: Chuck Lever <chuck.lever@oracle.com>

simple_empty() and simple_offset_empty() perform the same task.
The latter's use as a canary to find bugs has not found any new
issues. A subsequent patch will remove the use of the mtree for
iterating directory contents, so revert back to using a similar
mechanism for determining whether a directory is indeed empty.

Only one such mechanism is ever needed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c         | 32 --------------------------------
 include/linux/fs.h |  1 -
 mm/shmem.c         |  4 ++--
 3 files changed, 2 insertions(+), 35 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index bf67954b525b..b668a4f5bbc9 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -327,38 +327,6 @@ void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry)
 	offset_set(dentry, 0);
 }
 
-/**
- * simple_offset_empty - Check if a dentry can be unlinked
- * @dentry: dentry to be tested
- *
- * Returns 0 if @dentry is a non-empty directory; otherwise returns 1.
- */
-int simple_offset_empty(struct dentry *dentry)
-{
-	struct inode *inode = d_inode(dentry);
-	struct offset_ctx *octx;
-	struct dentry *child;
-	unsigned long index;
-	int ret = 1;
-
-	if (!inode || !S_ISDIR(inode->i_mode))
-		return ret;
-
-	index = DIR_OFFSET_MIN;
-	octx = inode->i_op->get_offset_ctx(inode);
-	mt_for_each(&octx->mt, child, index, LONG_MAX) {
-		spin_lock(&child->d_lock);
-		if (simple_positive(child)) {
-			spin_unlock(&child->d_lock);
-			ret = 0;
-			break;
-		}
-		spin_unlock(&child->d_lock);
-	}
-
-	return ret;
-}
-
 /**
  * simple_offset_rename - handle directory offsets for rename
  * @old_dir: parent directory of source entry
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3559446279c1..0698cf63346c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3434,7 +3434,6 @@ struct offset_ctx {
 void simple_offset_init(struct offset_ctx *octx);
 int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry);
 void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry);
-int simple_offset_empty(struct dentry *dentry);
 int simple_offset_rename(struct inode *old_dir, struct dentry *old_dentry,
 			 struct inode *new_dir, struct dentry *new_dentry);
 int simple_offset_rename_exchange(struct inode *old_dir,
diff --git a/mm/shmem.c b/mm/shmem.c
index 568bb290bdce..6ae963d42dbe 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3697,7 +3697,7 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
 
 static int shmem_rmdir(struct inode *dir, struct dentry *dentry)
 {
-	if (!simple_offset_empty(dentry))
+	if (!simple_empty(dentry))
 		return -ENOTEMPTY;
 
 	drop_nlink(d_inode(dentry));
@@ -3754,7 +3754,7 @@ static int shmem_rename2(struct mnt_idmap *idmap,
 		return simple_offset_rename_exchange(old_dir, old_dentry,
 						     new_dir, new_dentry);
 
-	if (!simple_offset_empty(new_dentry))
+	if (!simple_empty(new_dentry))
 		return -ENOTEMPTY;
 
 	if (flags & RENAME_WHITEOUT) {
-- 
2.47.0

[PATCH v4 3/5] Revert "libfs: fix infinite directory reads for offset dir"

[cel]

From: Chuck Lever <chuck.lever@oracle.com>

The current directory offset allocator (based on mtree_alloc_cyclic)
stores the next offset value to return in octx->next_offset. This
mechanism typically returns values that increase monotonically over
time. Eventually, though, the newly allocated offset value wraps
back to a low number (say, 2) which is smaller than other already-
allocated offset values.

Yu Kuai <yukuai3@huawei.com> reports that, after commit 64a7ce76fb90
("libfs: fix infinite directory reads for offset dir"), if a
directory's offset allocator wraps, existing entries are no longer
visible via readdir/getdents because offset_readdir() stops listing
entries once an entry's offset is larger than octx->next_offset.
These entries vanish persistently -- they can be looked up, but will
never again appear in readdir(3) output.

The reason for this is that the commit treats directory offsets as
monotonically increasing integer values rather than opaque cookies,
and introduces this comparison:

	if (dentry2offset(dentry) >= last_index) {

On 64-bit platforms, the directory offset value upper bound is
2^63 - 1. Directory offsets will monotonically increase for millions
of years without wrapping.

On 32-bit platforms, however, LONG_MAX is 2^31 - 1. The allocator
can wrap after only a few weeks (at worst).

Revert commit 64a7ce76fb90 ("libfs: fix infinite directory reads for
offset dir") to prepare for a fix that can work properly on 32-bit
systems and might apply to recent LTS kernels where shmem employs
the simple_offset mechanism.

Reported-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 35 +++++++++++------------------------
 1 file changed, 11 insertions(+), 24 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index b668a4f5bbc9..461384fb6119 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -420,14 +420,6 @@ void simple_offset_destroy(struct offset_ctx *octx)
 	mtree_destroy(&octx->mt);
 }
 
-static int offset_dir_open(struct inode *inode, struct file *file)
-{
-	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
-
-	file->private_data = (void *)ctx->next_offset;
-	return 0;
-}
-
 /**
  * offset_dir_llseek - Advance the read position of a directory descriptor
  * @file: an open directory whose position is to be updated
@@ -441,9 +433,6 @@ static int offset_dir_open(struct inode *inode, struct file *file)
  */
 static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 {
-	struct inode *inode = file->f_inode;
-	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
-
 	switch (whence) {
 	case SEEK_CUR:
 		offset += file->f_pos;
@@ -457,8 +446,7 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 	}
 
 	/* In this case, ->private_data is protected by f_pos_lock */
-	if (!offset)
-		file->private_data = (void *)ctx->next_offset;
+	file->private_data = NULL;
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
@@ -489,7 +477,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
+static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 {
 	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
 	struct dentry *dentry;
@@ -497,21 +485,17 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
 	while (true) {
 		dentry = offset_find_next(octx, ctx->pos);
 		if (!dentry)
-			return;
-
-		if (dentry2offset(dentry) >= last_index) {
-			dput(dentry);
-			return;
-		}
+			return ERR_PTR(-ENOENT);
 
 		if (!offset_dir_emit(ctx, dentry)) {
 			dput(dentry);
-			return;
+			break;
 		}
 
 		ctx->pos = dentry2offset(dentry) + 1;
 		dput(dentry);
 	}
+	return NULL;
 }
 
 /**
@@ -538,19 +522,22 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
 static int offset_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct dentry *dir = file->f_path.dentry;
-	long last_index = (long)file->private_data;
 
 	lockdep_assert_held(&d_inode(dir)->i_rwsem);
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
-	offset_iterate_dir(d_inode(dir), ctx, last_index);
+	/* In this case, ->private_data is protected by f_pos_lock */
+	if (ctx->pos == DIR_OFFSET_MIN)
+		file->private_data = NULL;
+	else if (file->private_data == ERR_PTR(-ENOENT))
+		return 0;
+	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
 	return 0;
 }
 
 const struct file_operations simple_offset_dir_operations = {
-	.open		= offset_dir_open,
 	.llseek		= offset_dir_llseek,
 	.iterate_shared	= offset_readdir,
 	.read		= generic_read_dir,
-- 
2.47.0

[PATCH v4 4/5] libfs: Replace simple_offset end-of-directory detection

[cel]

From: Chuck Lever <chuck.lever@oracle.com>

According to getdents(3), the d_off field in each returned directory
entry points to the next entry in the directory. The d_off field in
the last returned entry in the readdir buffer must contain a valid
offset value, but if it points to an actual directory entry, then
readdir/getdents can loop.

This patch introduces a specific fixed offset value that is placed
in the d_off field of the last entry in a directory. Some user space
applications assume that the EOD offset value is larger than the
offsets of real directory entries, so the largest possible offset
value is reserved for this purpose. This new value is never
allocated by simple_offset_add().

When ->iterate_dir() returns, getdents{64} inserts the ctx->pos
value into the d_off field of the last valid entry in the readdir
buffer. When it hits EOD, offset_readdir() sets ctx->pos to the EOD
offset value so the last entry is updated to point to the EOD marker.

When trying to read the entry at the EOD offset, offset_readdir()
terminates immediately.

It is worth noting that using a Maple tree for directory offset
value allocation does not guarantee a 63-bit range of values --
on platforms where "long" is a 32-bit type, the directory offset
value range is still 0..(2^31 - 1).

Fixes: 796432efab1e ("libfs: getdents() should return 0 after reaching EOD")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 38 ++++++++++++++++++++++----------------
 1 file changed, 22 insertions(+), 16 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 461384fb6119..fcb2cdf6e3f3 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -241,9 +241,16 @@ const struct inode_operations simple_dir_inode_operations = {
 };
 EXPORT_SYMBOL(simple_dir_inode_operations);
 
-/* 0 is '.', 1 is '..', so always start with offset 2 or more */
+/* simple_offset_add() allocation range */
 enum {
-	DIR_OFFSET_MIN	= 2,
+	DIR_OFFSET_MIN		= 2,
+	DIR_OFFSET_MAX		= LONG_MAX - 1,
+};
+
+/* simple_offset_add() never assigns these to a dentry */
+enum {
+	DIR_OFFSET_EOD		= LONG_MAX,	/* Marks EOD */
+
 };
 
 static void offset_set(struct dentry *dentry, long offset)
@@ -287,7 +294,8 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 		return -EBUSY;
 
 	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
-				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
+				 DIR_OFFSET_MAX, &octx->next_offset,
+				 GFP_KERNEL);
 	if (unlikely(ret == -EBUSY))
 		return -ENOSPC;
 	if (unlikely(ret < 0))
@@ -445,8 +453,6 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 		return -EINVAL;
 	}
 
-	/* In this case, ->private_data is protected by f_pos_lock */
-	file->private_data = NULL;
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
@@ -456,7 +462,7 @@ static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
 	struct dentry *child, *found = NULL;
 
 	rcu_read_lock();
-	child = mas_find(&mas, LONG_MAX);
+	child = mas_find(&mas, DIR_OFFSET_MAX);
 	if (!child)
 		goto out;
 	spin_lock(&child->d_lock);
@@ -477,7 +483,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
+static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 {
 	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
 	struct dentry *dentry;
@@ -485,7 +491,7 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 	while (true) {
 		dentry = offset_find_next(octx, ctx->pos);
 		if (!dentry)
-			return ERR_PTR(-ENOENT);
+			goto out_eod;
 
 		if (!offset_dir_emit(ctx, dentry)) {
 			dput(dentry);
@@ -495,7 +501,10 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 		ctx->pos = dentry2offset(dentry) + 1;
 		dput(dentry);
 	}
-	return NULL;
+	return;
+
+out_eod:
+	ctx->pos = DIR_OFFSET_EOD;
 }
 
 /**
@@ -515,6 +524,8 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
  *
  * On return, @ctx->pos contains an offset that will read the next entry
  * in this directory when offset_readdir() is called again with @ctx.
+ * Caller places this value in the d_off field of the last entry in the
+ * user's buffer.
  *
  * Return values:
  *   %0 - Complete
@@ -527,13 +538,8 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
-
-	/* In this case, ->private_data is protected by f_pos_lock */
-	if (ctx->pos == DIR_OFFSET_MIN)
-		file->private_data = NULL;
-	else if (file->private_data == ERR_PTR(-ENOENT))
-		return 0;
-	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
+	if (ctx->pos != DIR_OFFSET_EOD)
+		offset_iterate_dir(d_inode(dir), ctx);
 	return 0;
 }
 
-- 
2.47.0

[PATCH v4 5/5] libfs: Use d_children list to iterate simple_offset directories

[cel]

From: Chuck Lever <chuck.lever@oracle.com>

The mtree mechanism has been effective at creating directory offsets
that are stable over multiple opendir instances. However, it has not
been able to handle the subtleties of renames that are concurrent
with readdir.

Instead of using the mtree to emit entries in the order of their
offset values, use it only to map incoming ctx->pos to a starting
entry. Then use the directory's d_children list, which is already
maintained properly by the dcache, to find the next child to emit.

One of the sneaky things about this is that when the mtree-allocated
offset value wraps (which is very rare), looking up ctx->pos++ is
not going to find the next entry; it will return NULL. Instead, by
following the d_children list, the offset values can appear in any
order but all of the entries in the directory will be visited
eventually.

Note also that the readdir() is guaranteed to reach the tail of this
list. Entries are added only at the head of d_children, and readdir
walks from its current position in that list towards its tail.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 77 ++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 57 insertions(+), 20 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index fcb2cdf6e3f3..398eac385094 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -243,12 +243,13 @@ EXPORT_SYMBOL(simple_dir_inode_operations);
 
 /* simple_offset_add() allocation range */
 enum {
-	DIR_OFFSET_MIN		= 2,
+	DIR_OFFSET_MIN		= 3,
 	DIR_OFFSET_MAX		= LONG_MAX - 1,
 };
 
 /* simple_offset_add() never assigns these to a dentry */
 enum {
+	DIR_OFFSET_FIRST	= 2,		/* Find first real entry */
 	DIR_OFFSET_EOD		= LONG_MAX,	/* Marks EOD */
 
 };
@@ -456,19 +457,43 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
-static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
+/* Cf. find_next_child() */
+static struct dentry *find_next_sibling_locked(struct dentry *parent,
+					       struct dentry *dentry)
 {
-	MA_STATE(mas, &octx->mt, offset, offset);
+	struct dentry *found = NULL;
+
+	hlist_for_each_entry_from(dentry, d_sib) {
+		if (!simple_positive(dentry))
+			continue;
+		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+		if (simple_positive(dentry))
+			found = dget_dlock(dentry);
+		spin_unlock(&dentry->d_lock);
+		if (likely(found))
+			break;
+	}
+	return found;
+}
+
+static noinline_for_stack struct dentry *
+offset_dir_lookup(struct file *file, loff_t offset)
+{
+	struct dentry *parent = file->f_path.dentry;
 	struct dentry *child, *found = NULL;
+	struct inode *inode = d_inode(parent);
+	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
+
+	MA_STATE(mas, &octx->mt, offset, offset);
 
 	rcu_read_lock();
 	child = mas_find(&mas, DIR_OFFSET_MAX);
 	if (!child)
 		goto out;
-	spin_lock(&child->d_lock);
-	if (simple_positive(child))
-		found = dget_dlock(child);
-	spin_unlock(&child->d_lock);
+
+	spin_lock(&parent->d_lock);
+	found = find_next_sibling_locked(parent, child);
+	spin_unlock(&parent->d_lock);
 out:
 	rcu_read_unlock();
 	return found;
@@ -477,30 +502,42 @@ static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
 static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 {
 	struct inode *inode = d_inode(dentry);
-	long offset = dentry2offset(dentry);
 
-	return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len, offset,
-			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
+	return dir_emit(ctx, dentry->d_name.name, dentry->d_name.len,
+			inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
+static void offset_iterate_dir(struct file *file, struct dir_context *ctx)
 {
-	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
+	struct dentry *dir = file->f_path.dentry;
 	struct dentry *dentry;
 
+	if (ctx->pos == DIR_OFFSET_FIRST) {
+		spin_lock(&dir->d_lock);
+		dentry = find_next_sibling_locked(dir, d_first_child(dir));
+		spin_unlock(&dir->d_lock);
+	} else
+		dentry = offset_dir_lookup(file, ctx->pos);
+	if (!dentry)
+		goto out_eod;
+
 	while (true) {
-		dentry = offset_find_next(octx, ctx->pos);
-		if (!dentry)
-			goto out_eod;
+		struct dentry *next;
 
-		if (!offset_dir_emit(ctx, dentry)) {
-			dput(dentry);
+		ctx->pos = dentry2offset(dentry);
+		if (!offset_dir_emit(ctx, dentry))
 			break;
-		}
 
-		ctx->pos = dentry2offset(dentry) + 1;
+		spin_lock(&dir->d_lock);
+		next = find_next_sibling_locked(dir, d_next_sibling(dentry));
+		spin_unlock(&dir->d_lock);
 		dput(dentry);
+
+		if (!next)
+			goto out_eod;
+		dentry = next;
 	}
+	dput(dentry);
 	return;
 
 out_eod:
@@ -539,7 +576,7 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 	if (ctx->pos != DIR_OFFSET_EOD)
-		offset_iterate_dir(d_inode(dir), ctx);
+		offset_iterate_dir(file, ctx);
 	return 0;
 }
 
-- 
2.47.0

Re: [PATCH v4 5/5] libfs: Use d_children list to iterate simple_offset directories

[Chuck Lever]

On 12/4/24 10:52 AM, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> The mtree mechanism has been effective at creating directory offsets
> that are stable over multiple opendir instances. However, it has not
> been able to handle the subtleties of renames that are concurrent
> with readdir.
> 
> Instead of using the mtree to emit entries in the order of their
> offset values, use it only to map incoming ctx->pos to a starting
> entry. Then use the directory's d_children list, which is already
> maintained properly by the dcache, to find the next child to emit.
> 
> One of the sneaky things about this is that when the mtree-allocated
> offset value wraps (which is very rare), looking up ctx->pos++ is
> not going to find the next entry; it will return NULL. Instead, by
> following the d_children list, the offset values can appear in any
> order but all of the entries in the directory will be visited
> eventually.
> 
> Note also that the readdir() is guaranteed to reach the tail of this
> list. Entries are added only at the head of d_children, and readdir
> walks from its current position in that list towards its tail.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>   fs/libfs.c | 77 ++++++++++++++++++++++++++++++++++++++++--------------
>   1 file changed, 57 insertions(+), 20 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index fcb2cdf6e3f3..398eac385094 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -243,12 +243,13 @@ EXPORT_SYMBOL(simple_dir_inode_operations);
>   
>   /* simple_offset_add() allocation range */
>   enum {
> -	DIR_OFFSET_MIN		= 2,
> +	DIR_OFFSET_MIN		= 3,
>   	DIR_OFFSET_MAX		= LONG_MAX - 1,
>   };
>   
>   /* simple_offset_add() never assigns these to a dentry */
>   enum {
> +	DIR_OFFSET_FIRST	= 2,		/* Find first real entry */
>   	DIR_OFFSET_EOD		= LONG_MAX,	/* Marks EOD */
>   
>   };
> @@ -456,19 +457,43 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>   	return vfs_setpos(file, offset, LONG_MAX);
>   }
>   
> -static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
> +/* Cf. find_next_child() */
> +static struct dentry *find_next_sibling_locked(struct dentry *parent,
> +					       struct dentry *dentry)

There might be a better name for this function.

It looks a lot like find_next_child(), but it acts more like
scan_positives(). It starts looking for positive dentries starting
at @dentry, thus it can return the dentry that was passed in @dentry.

find_positive_from_locked()  ??


>   {
> -	MA_STATE(mas, &octx->mt, offset, offset);
> +	struct dentry *found = NULL;
> +
> +	hlist_for_each_entry_from(dentry, d_sib) {
> +		if (!simple_positive(dentry))
> +			continue;
> +		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> +		if (simple_positive(dentry))
> +			found = dget_dlock(dentry);
> +		spin_unlock(&dentry->d_lock);
> +		if (likely(found))
> +			break;
> +	}
> +	return found;
> +}
> +
> +static noinline_for_stack struct dentry *
> +offset_dir_lookup(struct file *file, loff_t offset)
> +{
> +	struct dentry *parent = file->f_path.dentry;
>   	struct dentry *child, *found = NULL;
> +	struct inode *inode = d_inode(parent);
> +	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> +
> +	MA_STATE(mas, &octx->mt, offset, offset);
>   
>   	rcu_read_lock();
>   	child = mas_find(&mas, DIR_OFFSET_MAX);
>   	if (!child)
>   		goto out;
> -	spin_lock(&child->d_lock);
> -	if (simple_positive(child))
> -		found = dget_dlock(child);
> -	spin_unlock(&child->d_lock);
> +
> +	spin_lock(&parent->d_lock);
> +	found = find_next_sibling_locked(parent, child);
> +	spin_unlock(&parent->d_lock);
>   out:
>   	rcu_read_unlock();
>   	return found;
> @@ -477,30 +502,42 @@ static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
>   static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
>   {
>   	struct inode *inode = d_inode(dentry);
> -	long offset = dentry2offset(dentry);
>   
> -	return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len, offset,
> -			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
> +	return dir_emit(ctx, dentry->d_name.name, dentry->d_name.len,
> +			inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>   }
>   
> -static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> +static void offset_iterate_dir(struct file *file, struct dir_context *ctx)
>   {
> -	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> +	struct dentry *dir = file->f_path.dentry;
>   	struct dentry *dentry;
>   
> +	if (ctx->pos == DIR_OFFSET_FIRST) {
> +		spin_lock(&dir->d_lock);
> +		dentry = find_next_sibling_locked(dir, d_first_child(dir));
> +		spin_unlock(&dir->d_lock);
> +	} else
> +		dentry = offset_dir_lookup(file, ctx->pos);
> +	if (!dentry)
> +		goto out_eod;
> +
>   	while (true) {
> -		dentry = offset_find_next(octx, ctx->pos);
> -		if (!dentry)
> -			goto out_eod;
> +		struct dentry *next;
>   
> -		if (!offset_dir_emit(ctx, dentry)) {
> -			dput(dentry);
> +		ctx->pos = dentry2offset(dentry);
> +		if (!offset_dir_emit(ctx, dentry))
>   			break;
> -		}
>   
> -		ctx->pos = dentry2offset(dentry) + 1;
> +		spin_lock(&dir->d_lock);
> +		next = find_next_sibling_locked(dir, d_next_sibling(dentry));
> +		spin_unlock(&dir->d_lock);
>   		dput(dentry);
> +
> +		if (!next)
> +			goto out_eod;
> +		dentry = next;
>   	}
> +	dput(dentry);
>   	return;
>   
>   out_eod:
> @@ -539,7 +576,7 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
>   	if (!dir_emit_dots(file, ctx))
>   		return 0;
>   	if (ctx->pos != DIR_OFFSET_EOD)
> -		offset_iterate_dir(d_inode(dir), ctx);
> +		offset_iterate_dir(file, ctx);
>   	return 0;
>   }
>   


-- 
Chuck Lever

Re: [PATCH v4 5/5] libfs: Use d_children list to iterate simple_offset directories

[Chuck Lever]

On 12/8/24 12:11 PM, Chuck Lever wrote:
> On 12/4/24 10:52 AM, cel@kernel.org wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>>
>> The mtree mechanism has been effective at creating directory offsets
>> that are stable over multiple opendir instances. However, it has not
>> been able to handle the subtleties of renames that are concurrent
>> with readdir.
>>
>> Instead of using the mtree to emit entries in the order of their
>> offset values, use it only to map incoming ctx->pos to a starting
>> entry. Then use the directory's d_children list, which is already
>> maintained properly by the dcache, to find the next child to emit.
>>
>> One of the sneaky things about this is that when the mtree-allocated
>> offset value wraps (which is very rare), looking up ctx->pos++ is
>> not going to find the next entry; it will return NULL. Instead, by
>> following the d_children list, the offset values can appear in any
>> order but all of the entries in the directory will be visited
>> eventually.
>>
>> Note also that the readdir() is guaranteed to reach the tail of this
>> list. Entries are added only at the head of d_children, and readdir
>> walks from its current position in that list towards its tail.
>>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>>   fs/libfs.c | 77 ++++++++++++++++++++++++++++++++++++++++--------------
>>   1 file changed, 57 insertions(+), 20 deletions(-)
>>
>> diff --git a/fs/libfs.c b/fs/libfs.c
>> index fcb2cdf6e3f3..398eac385094 100644
>> --- a/fs/libfs.c
>> +++ b/fs/libfs.c
>> @@ -243,12 +243,13 @@ EXPORT_SYMBOL(simple_dir_inode_operations);
>>   /* simple_offset_add() allocation range */
>>   enum {
>> -    DIR_OFFSET_MIN        = 2,
>> +    DIR_OFFSET_MIN        = 3,
>>       DIR_OFFSET_MAX        = LONG_MAX - 1,
>>   };
>>   /* simple_offset_add() never assigns these to a dentry */
>>   enum {
>> +    DIR_OFFSET_FIRST    = 2,        /* Find first real entry */
>>       DIR_OFFSET_EOD        = LONG_MAX,    /* Marks EOD */
>>   };
>> @@ -456,19 +457,43 @@ static loff_t offset_dir_llseek(struct file 
>> *file, loff_t offset, int whence)
>>       return vfs_setpos(file, offset, LONG_MAX);
>>   }
>> -static struct dentry *offset_find_next(struct offset_ctx *octx, 
>> loff_t offset)
>> +/* Cf. find_next_child() */
>> +static struct dentry *find_next_sibling_locked(struct dentry *parent,
>> +                           struct dentry *dentry)
> 
> There might be a better name for this function.
> 
> It looks a lot like find_next_child(), but it acts more like
> scan_positives(). It starts looking for positive dentries starting
> at @dentry, thus it can return the dentry that was passed in @dentry.
> 
> find_positive_from_locked()  ??
> 
> 
>>   {
>> -    MA_STATE(mas, &octx->mt, offset, offset);
>> +    struct dentry *found = NULL;
>> +
>> +    hlist_for_each_entry_from(dentry, d_sib) {
>> +        if (!simple_positive(dentry))
>> +            continue;
>> +        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
>> +        if (simple_positive(dentry))
>> +            found = dget_dlock(dentry);
>> +        spin_unlock(&dentry->d_lock);
>> +        if (likely(found))
>> +            break;
>> +    }
>> +    return found;
>> +}
>> +
>> +static noinline_for_stack struct dentry *
>> +offset_dir_lookup(struct file *file, loff_t offset)
>> +{
>> +    struct dentry *parent = file->f_path.dentry;
>>       struct dentry *child, *found = NULL;
>> +    struct inode *inode = d_inode(parent);
>> +    struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>> +
>> +    MA_STATE(mas, &octx->mt, offset, offset);
>>       rcu_read_lock();
>>       child = mas_find(&mas, DIR_OFFSET_MAX);
>>       if (!child)
>>           goto out;
>> -    spin_lock(&child->d_lock);
>> -    if (simple_positive(child))
>> -        found = dget_dlock(child);
>> -    spin_unlock(&child->d_lock);
>> +
>> +    spin_lock(&parent->d_lock);
>> +    found = find_next_sibling_locked(parent, child);
>> +    spin_unlock(&parent->d_lock);
>>   out:
>>       rcu_read_unlock();
>>       return found;
>> @@ -477,30 +502,42 @@ static struct dentry *offset_find_next(struct 
>> offset_ctx *octx, loff_t offset)
>>   static bool offset_dir_emit(struct dir_context *ctx, struct dentry 
>> *dentry)
>>   {
>>       struct inode *inode = d_inode(dentry);
>> -    long offset = dentry2offset(dentry);
>> -    return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len, 
>> offset,
>> -              inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>> +    return dir_emit(ctx, dentry->d_name.name, dentry->d_name.len,
>> +            inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>>   }
>> -static void offset_iterate_dir(struct inode *inode, struct 
>> dir_context *ctx)
>> +static void offset_iterate_dir(struct file *file, struct dir_context 
>> *ctx)
>>   {
>> -    struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>> +    struct dentry *dir = file->f_path.dentry;
>>       struct dentry *dentry;
>> +    if (ctx->pos == DIR_OFFSET_FIRST) {
>> +        spin_lock(&dir->d_lock);
>> +        dentry = find_next_sibling_locked(dir, d_first_child(dir));
>> +        spin_unlock(&dir->d_lock);
>> +    } else
>> +        dentry = offset_dir_lookup(file, ctx->pos);
>> +    if (!dentry)
>> +        goto out_eod;
>> +
>>       while (true) {
>> -        dentry = offset_find_next(octx, ctx->pos);
>> -        if (!dentry)
>> -            goto out_eod;
>> +        struct dentry *next;
>> -        if (!offset_dir_emit(ctx, dentry)) {
>> -            dput(dentry);
>> +        ctx->pos = dentry2offset(dentry);
>> +        if (!offset_dir_emit(ctx, dentry))
>>               break;
>> -        }
>> -        ctx->pos = dentry2offset(dentry) + 1;
>> +        spin_lock(&dir->d_lock);
>> +        next = find_next_sibling_locked(dir, d_next_sibling(dentry));
>> +        spin_unlock(&dir->d_lock);

Recent coverity report:

*** CID 1602474:  Concurrent data access violations  (ATOMICITY)
/fs/libfs.c: 536 in offset_iterate_dir()
530
531     		ctx->pos = dentry2offset(dentry);
532     		if (!offset_dir_emit(ctx, dentry))
533     			break;
534
535     		spin_lock(&dir->d_lock);
 >>>     CID 1602474:  Concurrent data access violations  (ATOMICITY)
 >>>     Using an unreliable value of "dentry" inside the second locked 
section. If the data that "dentry" depends on was changed by another 
thread, this use might be incorrect.
536     		next = find_next_sibling_locked(dir, d_next_sibling(dentry));
537     		spin_unlock(&dir->d_lock);
538     		dput(dentry);
539
540     		if (!next)
541     			goto out_eod;

As far as I can tell, @dentry's list fields, which are the only fields
accessed in find_next_sibling_locked(), are protected by dir->d_lock. We
don't care about the other fields.

Not sure if this is a false positive. Is there an annotation that will
help clarify this situation?


>>           dput(dentry);
>> +
>> +        if (!next)
>> +            goto out_eod;
>> +        dentry = next;
>>       }
>> +    dput(dentry);
>>       return;
>>   out_eod:
>> @@ -539,7 +576,7 @@ static int offset_readdir(struct file *file, 
>> struct dir_context *ctx)
>>       if (!dir_emit_dots(file, ctx))
>>           return 0;
>>       if (ctx->pos != DIR_OFFSET_EOD)
>> -        offset_iterate_dir(d_inode(dir), ctx);
>> +        offset_iterate_dir(file, ctx);
>>       return 0;
>>   }
> 
> 


-- 
Chuck Lever

Re: [PATCH v4 5/5] libfs: Use d_children list to iterate simple_offset directories

[Al Viro]

On Sat, Dec 14, 2024 at 12:13:30PM -0500, Chuck Lever wrote:
> > > +/* Cf. find_next_child() */
> > > +static struct dentry *find_next_sibling_locked(struct dentry *parent,
> > > +                           struct dentry *dentry)
> > 
> > There might be a better name for this function.

There might be better calling conventions for it, TBH.
AFAICS, all callers are directly surrounded by grabbing/releasing
->d_lock on parent.  Why not fold that in, and to hell with any
mentionings of "locked" in the name...

Re: [PATCH v4 5/5] libfs: Use d_children list to iterate simple_offset directories

[Chuck Lever]

On 12/14/24 12:49 PM, Al Viro wrote:
> On Sat, Dec 14, 2024 at 12:13:30PM -0500, Chuck Lever wrote:
>>>> +/* Cf. find_next_child() */
>>>> +static struct dentry *find_next_sibling_locked(struct dentry *parent,
>>>> +                           struct dentry *dentry)
>>>
>>> There might be a better name for this function.
> 
> There might be better calling conventions for it, TBH.
> AFAICS, all callers are directly surrounded by grabbing/releasing
> ->d_lock on parent.  Why not fold that in, and to hell with any
> mentionings of "locked" in the name...

I've tried it both ways, couldn't make up my mind. I'll try it again.


-- 
Chuck Lever

Re: [PATCH v4 5/5] libfs: Use d_children list to iterate simple_offset directories

[Al Viro]

On Sat, Dec 14, 2024 at 02:22:41PM -0500, Chuck Lever wrote:
> On 12/14/24 12:49 PM, Al Viro wrote:
> > On Sat, Dec 14, 2024 at 12:13:30PM -0500, Chuck Lever wrote:
> > > > > +/* Cf. find_next_child() */
> > > > > +static struct dentry *find_next_sibling_locked(struct dentry *parent,
> > > > > +                           struct dentry *dentry)
> > > > 
> > > > There might be a better name for this function.
> > 
> > There might be better calling conventions for it, TBH.
> > AFAICS, all callers are directly surrounded by grabbing/releasing
> > ->d_lock on parent.  Why not fold that in, and to hell with any
> > mentionings of "locked" in the name...
> 
> I've tried it both ways, couldn't make up my mind. I'll try it again.

Single return there, so it's just a single spin_lock() on entry and
spin_unlock() on exit...  No idea if it'll make coverity any happier,
but it would be easier on human readers.

Re: [PATCH v4 0/5] Improve simple directory offset wrap behavior

[Christian Brauner]

On Wed, 04 Dec 2024 10:52:51 -0500, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> The purpose of this series is to construct a set of upstream fixes
> that can be backported to v6.6 to address CVE-2024-46701.
> 
> My original plan was to add a cursor dentry. However, I've found a
> solution that does not need one. In fact, most or all of the
> reported issues are gone with 4/5. Thus I'm not sure 5/5 is
> necessary, but it seems like a robust improvement.
> 
> [...]

Applied to the vfs-6.14.misc branch of the vfs/vfs.git tree.
Patches in the vfs-6.14.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-6.14.misc

[1/5] libfs: Return ENOSPC when the directory offset range is exhausted
      https://git.kernel.org/vfs/vfs/c/3569cc5260ac
[2/5] Revert "libfs: Add simple_offset_empty()"
      https://git.kernel.org/vfs/vfs/c/06ed2dfc3234
[3/5] Revert "libfs: fix infinite directory reads for offset dir"
      https://git.kernel.org/vfs/vfs/c/29bc7ff8920d
[4/5] libfs: Replace simple_offset end-of-directory detection
      https://git.kernel.org/vfs/vfs/c/d4849629a4b7
[5/5] libfs: Use d_children list to iterate simple_offset directories
      https://git.kernel.org/vfs/vfs/c/5ba9a91ae23f

Re: [PATCH v4 0/5] Improve simple directory offset wrap behavior

[Chuck Lever]

On 12/4/24 10:52 AM, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> The purpose of this series is to construct a set of upstream fixes
> that can be backported to v6.6 to address CVE-2024-46701.
> 
> My original plan was to add a cursor dentry. However, I've found a
> solution that does not need one. In fact, most or all of the
> reported issues are gone with 4/5. Thus I'm not sure 5/5 is
> necessary, but it seems like a robust improvement.
> 
> Changes since v3:
> - Series is no longer RFC
> - Series passes xfstests locally and via NFS export
> - Patch 2/5 was replaced; it now removes simple_offset_empty()
> - 4/5 and 5/5 were rewritten based on test results
> - Patch descriptions have been clarified
> 
> This series (still against v6.12) has been pushed to:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/log/?h=tmpfs-fixes
> 
> Next step is to try backporting these to v6.6 to see if anything
> else is needed.
> 
> Chuck Lever (5):
>    libfs: Return ENOSPC when the directory offset range is exhausted
>    Revert "libfs: Add simple_offset_empty()"
>    Revert "libfs: fix infinite directory reads for offset dir"
>    libfs: Replace simple_offset end-of-directory detection
>    libfs: Use d_children list to iterate simple_offset directories
> 
>   fs/libfs.c         | 158 ++++++++++++++++++++++-----------------------
>   include/linux/fs.h |   1 -
>   mm/shmem.c         |   4 +-
>   3 files changed, 81 insertions(+), 82 deletions(-)
> 

I've backported these, as a proof of concept, to origin/linux-6.6.y. You
can find that here:

https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/log/?h=nfsd-6.6.y

This series passes xfstests, including generic/736.

It would be a little cleaner if I could also backport da549bdd15c2
("dentry: switch the lists of children to hlist"), but that has similar
risks as backporting the Maple tree patches.


-- 
Chuck Lever