* Re: [PATCH] fcntl.2: document F_GET_SEALS on tmpfs peculiarity
[not found] ` <20220122090441.ktxh43lpgsd2dxj4@senku>
@ 2022-01-24 4:18 ` Hugh Dickins
2022-01-24 18:35 ` Mike Kravetz
0 siblings, 1 reply; 2+ messages in thread
From: Hugh Dickins @ 2022-01-24 4:18 UTC (permalink / raw)
To: Aleksa Sarai
Cc: Kir Kolyshkin, linux-man, Michael Kerrisk, David Herrmann,
Andrew Morton, Hugh Dickins, Mike Kravetz, linux-mm,
linux-kernel
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 <kolyshkin@gmail.com> 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@cyphar.com>
> > Cc: David Herrmann <dh.herrmann@gmail.com>
> > Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
> > ---
> > 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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