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

Alexey Gladkov
2023-07-26T13:46:15Z

On Wed, Jul 26, 2023 at 02:36:25AM +1000, Aleksa Sarai wrote:
> On 2023-07-11, Alexey Gladkov <legion@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@sifive.com>
> > Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
> > Signed-off-by: Alexey Gladkov <legion@kernel.org>
> > Acked-by: Arnd Bergmann <arnd@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/

> > {
> > 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