Re: [PATCH] fcntl.2: document F_GET_SEALS on tmpfs peculiarity

[Aleksa Sarai]

Adding the maintainers of mm/{shmem,memfd}.c and fs/hugetlbfs/ just in
case this was not intended behaviour.

On 2022-01-21, Kir Kolyshkin <kolyshkin@xxxxxxxxx> wrote:
> Currently, from the description of file sealing it can be deduced that
> unless the fd is a memfd, all sealing operations fail with EINVAL.
> 
> Apparently, it's not true for tmpfs or hugetlbfs -- F_GET_SEALS returns
> 1 (F_SEAL_SEAL) for an fd opened on these filesystems (probably because
> those are used to back memfd files).
> 
> Fix the description to mention that peculiarity. Not knowing this can
> result in incorrect code logic (see [1], where the code mistook a
> descriptor of a file opened on on tmpfs for a memfd).
> 
> While at it, clarify that fcntl does not actually return EINVAL, but
> sets errno to it (as it is usually said elsewhere).
> 
> [1] https://github.com/opencontainers/runc/pull/3342
> 
> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
> Cc: David Herrmann <dh.herrmann@xxxxxxxxx>
> Signed-off-by: Kir Kolyshkin <kolyshkin@xxxxxxxxx>
> ---
>  man2/fcntl.2 | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/fcntl.2 b/man2/fcntl.2
> index 7b5604e3a..f951b05ff 100644
> --- a/man2/fcntl.2
> +++ b/man2/fcntl.2
> @@ -1402,10 +1402,23 @@ file seals can be applied only to a file descriptor returned by
>  (if the
>  .B MFD_ALLOW_SEALING
>  was employed).
> -On other filesystems, all
> +On all other filesystems, except
> +.BR tmpfs (5)
> +and
> +.BR hugetlbfs ,
> +all
>  .BR fcntl ()
> -operations that operate on seals will return
> +operations that operate on seals will fail with
> +.I errno
> +set to
>  .BR EINVAL .
> +For a descriptor from a file on
> +.BR tmpfs (5)
> +or
> +.BR hugetlbfs ,
> +.B F_GET_SEALS
> +returns
> +.BR F_SEAL_SEAL .
>  .PP
>  Seals are a property of an inode.
>  Thus, all open file descriptors referring to the same inode share
> -- 
> 2.33.1
> 

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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