Unverified Commit 09da082b authored by Alexey Gladkov's avatar Alexey Gladkov Committed by Christian Brauner
Browse files

fs: Add fchmodat2() 

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: default avatarPalmer Dabbelt <palmer@sifive.com>
Signed-off-by: default avatarPalmer Dabbelt <palmer@sifive.com>
Signed-off-by: default avatarAlexey Gladkov <legion@kernel.org>
Acked-by: default avatarArnd Bergmann <arnd@arndb.de>
Message-Id: <f2a846ef495943c5d101011eebcf01179d0c7b61.1689092120.git.legion@kernel.org>
[brauner: pre reviews, do flag conversion in do_fchmodat() directly]
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 06a02139

fs/open.c

+17 −4
Original line number Diff line number Diff line
@@ -671,11 +671,18 @@ 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,

		       unsigned int flags)

{

	struct path path;

	int error;

	unsigned int lookup_flags = LOOKUP_FOLLOW;

	unsigned int lookup_flags;



	if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))

		return -EINVAL;



	lookup_flags = (flags & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;



retry:

	error = user_path_at(dfd, filename, lookup_flags, &path);

	if (!error) {
@@ -689,15 +696,21 @@ 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, unsigned int, flags)

{

	return do_fchmodat(dfd, filename, mode, flags);

}



SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename,

		umode_t, mode)

{

	return do_fchmodat(dfd, filename, mode);

	return do_fchmodat(dfd, filename, mode, 0);

}



SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)

{

	return do_fchmodat(AT_FDCWD, filename, mode);

	return do_fchmodat(AT_FDCWD, filename, mode, 0);

}



/*

include/linux/syscalls.h

+2 −0
Original line number Diff line number Diff line
@@ -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, unsigned 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);