On Wed, Jul 26, 2023 at 02:36:25AM +1000, Aleksa Sarai wrote:
On 2023-07-11, Alexey Gladkov <legion@xxxxxxxxxx> 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@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)
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).
I just addressed the Al Viro's suggestion.
https://lore.kernel.org/lkml/20190717014802.GS17978@xxxxxxxxxxxxxxxxxx/
{
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.
I'm not sure I understand. Can you explain what you mean?
--
Rgrds, legion
