[PATCH v4 2/5] fs: Add fchmodat2()

[Aleksa Sarai]

On 2023-07-26, Alexey Gladkov <legion at kernel.org> wrote:
> On Wed, Jul 26, 2023 at 02:36:25AM +1000, Aleksa Sarai wrote:
> > On 2023-07-11, Alexey Gladkov <legion at kernel.org> wrote:
> > > On the userspace side fchmodat(3) is implemented as a wrapper
> > > function which implements the POSIX-specified interface. This
> > > interface differs from the underlying kernel system call, which does not
> > > have a flags argument. Most implementations require procfs [1][2].
> > > 
> > > There doesn't appear to be a good userspace workaround for this issue
> > > but the implementation in the kernel is pretty straight-forward.
> > > 
> > > The new fchmodat2() syscall allows to pass the AT_SYMLINK_NOFOLLOW flag,
> > > unlike existing fchmodat.
> > > 
> > > [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35
> > > [2] https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28
> > > 
> > > Co-developed-by: Palmer Dabbelt <palmer at sifive.com>
> > > Signed-off-by: Palmer Dabbelt <palmer at sifive.com>
> > > Signed-off-by: Alexey Gladkov <legion at kernel.org>
> > > Acked-by: Arnd Bergmann <arnd at arndb.de>
> > > ---
> > >  fs/open.c                | 18 ++++++++++++++----
> > >  include/linux/syscalls.h |  2 ++
> > >  2 files changed, 16 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/fs/open.c b/fs/open.c
> > > index 0c55c8e7f837..39a7939f0d00 100644
> > > --- a/fs/open.c
> > > +++ b/fs/open.c
> > > @@ -671,11 +671,11 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
> > >  	return err;
> > >  }
> > >  
> > > -static int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
> > > +static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, int lookup_flags)
> > 
> > I think it'd be much neater to do the conversion of AT_ flags here and
> > pass 0 as a flags argument for all of the wrappers (this is how most of
> > the other xyz(), fxyz(), fxyzat() syscall wrappers are done IIRC).
> 
> I just addressed the Al Viro's suggestion.
> 
> https://lore.kernel.org/lkml/20190717014802.GS17978@ZenIV.linux.org.uk/

I think Al misspoke, because he also said "pass it 0 as an extra
argument", but you actually have to pass LOOKUP_FOLLOW from the
wrappers. If you look at how faccessat2 and faccessat are implemented,
it follows the behaviour I described.

> > >  {
> > >  	struct path path;
> > >  	int error;
> > > -	unsigned int lookup_flags = LOOKUP_FOLLOW;
> > > +
> > >  retry:
> > >  	error = user_path_at(dfd, filename, lookup_flags, &path);
> > >  	if (!error) {
> > > @@ -689,15 +689,25 @@ static int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
> > >  	return error;
> > >  }
> > >  
> > > +SYSCALL_DEFINE4(fchmodat2, int, dfd, const char __user *, filename,
> > > +		umode_t, mode, int, flags)
> > > +{
> > > +	if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
> > > +		return -EINVAL;
> > 
> > We almost certainly want to support AT_EMPTY_PATH at the same time.
> > Otherwise userspace will still need to go through /proc when trying to
> > chmod a file handle they have.
> 
> I'm not sure I understand. Can you explain what you mean?

You should add support for AT_EMPTY_PATH (LOOKUP_EMPTY) as well as
AT_SYMLINK_NOFOLLOW. It would only require something like:

	unsigned int lookup_flags = LOOKUP_FOLLOW;

	if (flags & ~(AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW))
		return -EINVAL;

	if (flags & AT_EMPTY_PATH)
		lookup_flags |= LOOKUP_EMPTY;
	if (flags & AT_SYMLINK_NOFOLLOW)
		lookup_flags &= ~LOOKUP_FOLLOW;

	/* ... */

This would be effectively equivalent to fchmod(fd, mode). (I was wrong
when I said this wasn't already possible -- I forgot about fchmod(2).)

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