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

[Aleksa Sarai]

On 2023-07-28, Aleksa Sarai <cyphar at cyphar.com> wrote:
> 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).)

... with the exception (as Christian mentioned) of O_PATH descriptors.
However, there are two counter-points to this:

 * fchownat(AT_EMPTY_PATH) exists but fchown() doesn't work on O_PATH
   descriptors *by design* (according to open(2)).
 * chmod(/proc/self/fd/$n) works on O_PATH descriptors, meaning this
   behaviour is already allowed and all that AT_EMPTY_PATH would do is
   allow programs to avoid depending on procfs for this.

FWIW, I agree with Christian that these behaviours are not ideal (and
I'm working on a series that might allow for these things to be properly
blocked in the future) but there's also the consistency argument -- I
don't think fchownat() is much safer to allow in this way than
fchmodat() and (again) this behaviour is already possible through
procfs.

Ultimately, we can always add AT_EMPTY_PATH later. It just seemed like
an obvious omission to me that would be easy to resolve.

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