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 <[email protected]> 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 <[email protected]>
> Cc: David Herrmann <[email protected]>
> Signed-off-by: Kir Kolyshkin <[email protected]>
> ---
> 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/>
On Sat, 22 Jan 2022, Aleksa Sarai wrote:
> Adding the maintainers of mm/{shmem,memfd}.c and fs/hugetlbfs/ just in
> case this was not intended behaviour.
Kir is correct - thanks - and it is intended behaviour. Not consciously
intended to make for a difficult manpage, but the implementation was
intended to be simple, so tmpfs and hugetlbfs do not internally
distinguish memfd objects from filesystem files - their filesystem
files simply start off with F_SEAL_SEAL to rule out any sealing.
>
> On 2022-01-21, Kir Kolyshkin <[email protected]> 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 <[email protected]>
> > Cc: David Herrmann <[email protected]>
> > Signed-off-by: Kir Kolyshkin <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
> > ---
> > 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/>
On 1/23/22 20:18, Hugh Dickins wrote:
> On Sat, 22 Jan 2022, Aleksa Sarai wrote:
>
>> Adding the maintainers of mm/{shmem,memfd}.c and fs/hugetlbfs/ just in
>> case this was not intended behaviour.
>
> Kir is correct - thanks - and it is intended behaviour. Not consciously
> intended to make for a difficult manpage, but the implementation was
> intended to be simple, so tmpfs and hugetlbfs do not internally
> distinguish memfd objects from filesystem files - their filesystem
> files simply start off with F_SEAL_SEAL to rule out any sealing.
>
Thanks Hugh, I agree.
>> On 2022-01-21, Kir Kolyshkin <[email protected]> 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 <[email protected]>
>>> Cc: David Herrmann <[email protected]>
>>> Signed-off-by: Kir Kolyshkin <[email protected]>
>
> Acked-by: Hugh Dickins <[email protected]>
Acked-by: Mike Kravetz <[email protected]>
--
Mike Kravetz