Re: [PATCH v3 3/7] shm: add memfd_create() syscall

[Hugh Dickins <hughd@google.com>]

On Fri, 13 Jun 2014, David Herrmann wrote:

> memfd_create() is similar to mmap(MAP_ANON), but returns a file-descriptor
> that you can pass to mmap(). It can support sealing and avoids any
> connection to user-visible mount-points. Thus, it's not subject to quotas
> on mounted file-systems, but can be used like malloc()'ed memory, but
> with a file-descriptor to it.
> 
> memfd_create() returns the raw shmem file, so calls like ftruncate() can
> be used to modify the underlying inode. Also calls like fstat()
> will return proper information and mark the file as regular file. If you
> want sealing, you can specify MFD_ALLOW_SEALING. Otherwise, sealing is not
> supported (like on all other regular files).
> 
> Compared to O_TMPFILE, it does not require a tmpfs mount-point and is not
> subject to quotas and alike. It is still properly accounted to memcg
> limits, though.

It's an important point, but unclear quite what "quotas and alike" means.
There's never been any quota support in shmem/tmpfs, but filesystem size
can be limited.  Maybe say "and is not subject to a filesystem size limit.
It is still properly accounted to memcg limits, though, and to the same
overcommit or no-overcommit accounting as all user memory."

> 
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

A comment or two below, but this is okay by me.  I'm not wildly excited
to be getting a new system call in mm/shmem.c.  I do like it much better
now that you've dropped the size arg, thank you, but I still find it an
odd system call: if it were not for the name, that you want so much for
debugging, I think we would just implement this with a /dev/sealable
alongside /dev/zero, which gave you your own object on opening (in the
way that /dev/zero gives you your own object on mmap'ing).

I haven't checked the manpage, I hope it's made very clear that
there's no uniqueness imposed on the name, that it's merely a tag
attached to the object.

But from a shmem point of view this seems fine: if everyone else
is happy with memfd_create(), it's fine by me.

> ---
>  arch/x86/syscalls/syscall_32.tbl |  1 +
>  arch/x86/syscalls/syscall_64.tbl |  1 +
>  include/linux/syscalls.h         |  1 +
>  include/uapi/linux/memfd.h       |  8 +++++
>  kernel/sys_ni.c                  |  1 +
>  mm/shmem.c                       | 72 ++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 84 insertions(+)
>  create mode 100644 include/uapi/linux/memfd.h
> 
> diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
> index d6b8679..e7495b4 100644
> --- a/arch/x86/syscalls/syscall_32.tbl
> +++ b/arch/x86/syscalls/syscall_32.tbl
> @@ -360,3 +360,4 @@
>  351	i386	sched_setattr		sys_sched_setattr
>  352	i386	sched_getattr		sys_sched_getattr
>  353	i386	renameat2		sys_renameat2
> +354	i386	memfd_create		sys_memfd_create
> diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
> index ec255a1..28be0e1 100644
> --- a/arch/x86/syscalls/syscall_64.tbl
> +++ b/arch/x86/syscalls/syscall_64.tbl
> @@ -323,6 +323,7 @@
>  314	common	sched_setattr		sys_sched_setattr
>  315	common	sched_getattr		sys_sched_getattr
>  316	common	renameat2		sys_renameat2
> +317	common	memfd_create		sys_memfd_create
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index b0881a0..0be5d4d 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -802,6 +802,7 @@ asmlinkage long sys_timerfd_settime(int ufd, int flags,
>  asmlinkage long sys_timerfd_gettime(int ufd, struct itimerspec __user *otmr);
>  asmlinkage long sys_eventfd(unsigned int count);
>  asmlinkage long sys_eventfd2(unsigned int count, int flags);
> +asmlinkage long sys_memfd_create(const char *uname_ptr, unsigned int flags);
>  asmlinkage long sys_fallocate(int fd, int mode, loff_t offset, loff_t len);
>  asmlinkage long sys_old_readdir(unsigned int, struct old_linux_dirent __user *, unsigned int);
>  asmlinkage long sys_pselect6(int, fd_set __user *, fd_set __user *,
> diff --git a/include/uapi/linux/memfd.h b/include/uapi/linux/memfd.h
> new file mode 100644
> index 0000000..534e364
> --- /dev/null
> +++ b/include/uapi/linux/memfd.h
> @@ -0,0 +1,8 @@
> +#ifndef _UAPI_LINUX_MEMFD_H
> +#define _UAPI_LINUX_MEMFD_H
> +
> +/* flags for memfd_create(2) (unsigned int) */
> +#define MFD_CLOEXEC		0x0001U
> +#define MFD_ALLOW_SEALING	0x0002U
> +
> +#endif /* _UAPI_LINUX_MEMFD_H */
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 36441b5..489a4e6 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -197,6 +197,7 @@ cond_syscall(compat_sys_timerfd_settime);
>  cond_syscall(compat_sys_timerfd_gettime);
>  cond_syscall(sys_eventfd);
>  cond_syscall(sys_eventfd2);
> +cond_syscall(sys_memfd_create);
>  
>  /* performance counters: */
>  cond_syscall(sys_perf_event_open);
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 1438b3e..e7c5fe1 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -66,7 +66,9 @@ static struct vfsmount *shm_mnt;
>  #include <linux/highmem.h>
>  #include <linux/seq_file.h>
>  #include <linux/magic.h>
> +#include <linux/syscalls.h>
>  #include <linux/fcntl.h>
> +#include <uapi/linux/memfd.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/pgtable.h>
> @@ -2662,6 +2664,76 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
>  	shmem_show_mpol(seq, sbinfo->mpol);
>  	return 0;
>  }
> +
> +#define MFD_NAME_PREFIX "memfd:"
> +#define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
> +#define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
> +
> +#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING)
> +
> +SYSCALL_DEFINE2(memfd_create,
> +		const char*, uname,

Jann Horn suggested "const char __user *" rather than "const char *",
here and in syscalls.h, I think that's right (for sparse: compare
with sys_open, for example).

> +		unsigned int, flags)
> +{
> +	struct shmem_inode_info *info;
> +	struct file *file;
> +	int fd, error;
> +	char *name;
> +	long len;
> +
> +	if (flags & ~(unsigned int)MFD_ALL_FLAGS)
> +		return -EINVAL;
> +
> +	/* length includes terminating zero */
> +	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
> +	if (len <= 0)
> +		return -EFAULT;
> +	if (len > MFD_NAME_MAX_LEN + 1)
> +		return -EINVAL;
> +
> +	name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_TEMPORARY);
> +	if (!name)
> +		return -ENOMEM;
> +
> +	strcpy(name, MFD_NAME_PREFIX);
> +	if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
> +		error = -EFAULT;
> +		goto err_name;
> +	}
> +
> +	/* terminating-zero may have changed after strnlen_user() returned */
> +	if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
> +		error = -EFAULT;
> +		goto err_name;
> +	}
> +
> +	fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);

Perhaps we should throw O_LARGEFILE in there too?  So 32-bit is not
surprised when it accesses beyond MAX_NON_LFS.  I guess it's almost
a non-issue, since the file is in memory, so not expected to be very
large; but I seem to recall being caught out by a missing O_LARGEFILE
in the past, and a new interface like this might do better to force it.

But I'm not very sure of my ground here: please ask around, an fsdevel
person will have a better idea than me, whether it's best included.

> +	if (fd < 0) {
> +		error = fd;
> +		goto err_name;
> +	}
> +
> +	file = shmem_file_setup(name, 0, VM_NORESERVE);
> +	if (IS_ERR(file)) {
> +		error = PTR_ERR(file);
> +		goto err_fd;
> +	}
> +	info = SHMEM_I(file_inode(file));
> +	file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
> +	if (flags & MFD_ALLOW_SEALING)
> +		info->seals &= ~F_SEAL_SEAL;
> +
> +	fd_install(fd, file);
> +	kfree(name);
> +	return fd;
> +
> +err_fd:
> +	put_unused_fd(fd);
> +err_name:
> +	kfree(name);
> +	return error;
> +}
> +
>  #endif /* CONFIG_TMPFS */
>  
>  static void shmem_put_super(struct super_block *sb)
> -- 
> 2.0.0

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