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@xxxxxxxxxx> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxx> Signed-off-by: Alexey Gladkov <legion@xxxxxxxxxx> Acked-by: Arnd Bergmann <arnd@xxxxxxxx> --- 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) { 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; + + 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
