From 09da082b07bbae1c11d9560c8502800039aebcea Mon Sep 17 00:00:00 2001 From: Alexey Gladkov Date: Tue, 11 Jul 2023 18:16:04 +0200 Subject: 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 Signed-off-by: Palmer Dabbelt Signed-off-by: Alexey Gladkov Acked-by: Arnd Bergmann Message-Id: [brauner: pre reviews, do flag conversion in do_fchmodat() directly] Signed-off-by: Christian Brauner --- fs/open.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) (limited to 'fs/open.c') diff --git a/fs/open.c b/fs/open.c index 0c55c8e7f837..e52d78e5a333 100644 --- 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); } /* -- cgit v1.2.3 From 5daeb41a6fc9d0d81cb2291884b7410e062d8fa1 Mon Sep 17 00:00:00 2001 From: Aleksa Sarai Date: Fri, 28 Jul 2023 21:58:26 +1000 Subject: fchmodat2: add support for AT_EMPTY_PATH This allows userspace to avoid going through /proc/self/fd when dealing with all types of file descriptors for chmod(), and makes fchmodat2() a proper superset of all other chmod syscalls. The primary difference between fchmodat2(AT_EMPTY_PATH) and fchmod() is that fchmod() doesn't operate on O_PATH file descriptors by design. To quote open(2): > O_PATH (since Linux 2.6.39) > [...] > The file itself is not opened, and other file operations (e.g., > read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), > mmap(2)) fail with the error EBADF. However, procfs has allowed userspace to do this operation ever since the introduction of O_PATH through magic-links, so adding this feature is only an improvement for programs that have to mess around with /proc/self/fd/$n today to get this behaviour. In addition, fchownat(AT_EMPTY_PATH) has existed since the introduction of O_PATH and allows chown() operations directly on O_PATH descriptors. Signed-off-by: Aleksa Sarai Acked-by: Alexey Gladkov Message-Id: <20230728-fchmodat2-at_empty_path-v1-1-f3add31d3516@cyphar.com> Signed-off-by: Christian Brauner --- fs/open.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'fs/open.c') diff --git a/fs/open.c b/fs/open.c index e52d78e5a333..b8883ec286f5 100644 --- a/fs/open.c +++ b/fs/open.c @@ -678,10 +678,12 @@ static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, int error; unsigned int lookup_flags; - if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW)) + if (unlikely(flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))) return -EINVAL; lookup_flags = (flags & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW; + if (flags & AT_EMPTY_PATH) + lookup_flags |= LOOKUP_EMPTY; retry: error = user_path_at(dfd, filename, lookup_flags, &path); -- cgit v1.2.3