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: 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>
Message-Id: <f2a846ef495943c5d101011eebcf01179d0c7b61.1689092120.git.legion@kernel.org>
[brauner: pre reviews, do flag conversion in do_fchmodat() directly]
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/open.c b/fs/open.c
index 0c55c8e7f837d8ef03ab19b5645b40401205ee42..e52d78e5a33304b9e4b6e2fb92180df68b4842e7 100644 (file)
--- a/fs/open.c
+++ b/fs/open.c
@@ -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 @@ retry:
        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);
 }
 
 /*

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 584f404bf868f1f3b02fbaa57045ac4bfc4da86a..5690aef7b3a855db5309b4dbb7142323a4af0e0c 100644 (file)
--- 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, 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);