Re: [PATCH v6 4/5] libfs: Replace simple_offset end-of-directory detection

[Chuck Lever <chuck.lever@oracle.com>]

On 12/23/24 11:30 AM, Liam R. Howlett wrote:
> * cel@kernel.org <cel@kernel.org> [241220 10:33]:
>> 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).
> 
> I have a standing request to have 32-bit archs return 64-bit values.  Is
> this another 'nice to have' 64 bit values on 32 bit archs?

It would be nice if the range of values that the mtree API handles were
the same on 32-bit and 64-bit platforms. I think that could reduce the
defect rate in mtree consumers.

But 32-bit is going away over time. I wonder how much such an effort
would pay off in the long run.


>> 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 8c9364a0174c..5c56783c03a5 100644
>> --- a/fs/libfs.c
>> +++ b/fs/libfs.c
>> @@ -245,9 +245,16 @@ 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)
>> @@ -291,7 +298,8 @@ 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 < 0))
>>   		return ret == -EBUSY ? -ENOSPC : ret;
>>   
>> @@ -447,8 +455,6 @@ 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);
>>   }
>>   
>> @@ -458,7 +464,7 @@ 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);
>> @@ -479,7 +485,7 @@ 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;
>> @@ -487,7 +493,7 @@ 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);
>> @@ -497,7 +503,10 @@ 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;
>>   }
>>   
>>   /**
>> @@ -517,6 +526,8 @@ 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
>> @@ -529,13 +540,8 @@ 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
>>
>>


-- 
Chuck Lever