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

Aleksa Sarai
2023-07-25T16:37:09Z

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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).

> {
> 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.

> +
> + return do_fchmodat(dfd, filename, mode,
> + flags & AT_SYMLINK_NOFOLLOW ? 0 : LOOKUP_FOLLOW);
> +}
> +
> SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename,
> umode_t, mode)
> {
> - return do_fchmodat(dfd, filename, mode);
> + return do_fchmodat(dfd, filename, mode, LOOKUP_FOLLOW);
> }
>
> SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
> {
> - return do_fchmodat(AT_FDCWD, filename, mode);
> + return do_fchmodat(AT_FDCWD, filename, mode, LOOKUP_FOLLOW);
> }
>
> /*
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 584f404bf868..6e852279fbc3 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -440,6 +440,8 @@ asmlinkage long sys_chroot(const char __user *filename);
> asmlinkage long sys_fchmod(unsigned int fd, umode_t mode);
> asmlinkage long sys_fchmodat(int dfd, const char __user *filename,
> umode_t mode);
> +asmlinkage long sys_fchmodat2(int dfd, const char __user *filename,
> + umode_t mode, int flags);
> asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
> gid_t group, int flag);
> asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group);
> --
> 2.33.8
>

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

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