[PATCH 04/46] mm/memfd: make f_pos accesses atomic

[sashiko-bot at kernel.org]

> From: Josef Bacik <josef at toxicpanda.com>
> 
> Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") made
> read(), write() and lseek() on a shared open file description of a regular
> file serialize on ->f_pos_lock.  The flag that asks for it,
> FMODE_ATOMIC_POS, is set in do_dentry_open().  A memfd is a regular file
> too, but its struct file comes from alloc_file_pseudo() and never passes
> through do_dentry_open(), so it does not get the flag:
> 
>   CPU0                                CPU1
>   write(memfd, "a", 1)                write(memfd, "b", 1)
>     pos = file->f_pos  (= 0)
>                                         pos = file->f_pos  (= 0)
>     vfs_write(..., &pos)
>                                         vfs_write(..., &pos)
> [ ... ]
> Fixes: 9183df25fe7b ("shm: add memfd_create() syscall")
> Cc: stable at vger.kernel.org
> Signed-off-by: Josef Bacik <josef at toxicpanda.com>
> [ david: minor description tweaks ]
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Sashiko has reviewed this patch and found no issues.

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