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

From: Alexey Gladkov <[email-masked]>
Date: Wed, 26 Jul 2023 15:45:56 +0200

On Wed, Jul 26, 2023 at 02:36:25AM +1000, Aleksa Sarai wrote:
> On 2023-07-11, Alexey Gladkov <[email-masked]> 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 <[email-masked]>
> > Signed-off-by: Palmer Dabbelt <[email-masked]>
> > Signed-off-by: Alexey Gladkov <[email-masked]>
> > Acked-by: Arnd Bergmann <[email-masked]>
> > ---
> >  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/[email-masked]/

> >  {
> >  	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?

-- 
Rgrds, legion