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/>