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>> On 8/2/24 07:42, Petr Vorel wrote: >>>> On 2024-08-01, Petr Vorel wrote: >>>>> Hi all, >>>>>> This is a patch-set that implements fchmodat2() syscall coverage. >>>>>> fchmodat2() has been added in kernel 6.6 in order to support >>>>>> AT_SYMLINK_NOFOLLOW and AT_EMPTY_PATH in fchmodat(). >>>>>> There's no man pages yet, so please take the following links as >>>>>> main documentation along with kernel source code: >>>>> I would hope that it'd be at least Christian's fork [1], but it's not there. >>>>> I suppose nobody is working on the man page. >>>>>> https://www.phoronix.com/news/fchmodat2-For-Linux-6.6 >>>>>> https://lore.kernel.org/lkml/20230824-frohlocken-vorabend-725f6fdaad50@brauner/ >>>>>> *********** >>>>>> * WARNING * >>>>>> *********** >>>>>> fchmodat2_02 fails with EOPNOTSUPP because of missing feature. >>>>> For a record, it's fchmodat2_01.c (from this patchset) which is failing (on >>>>> 6.10.1-1.g4c78d6f-default Tumbleweed and 6.6.21-0-lts Alpine, both x86_64 VMs). >>>>> Andrea, I would personally just skip test on EOPNOTSUPP (that's what we do in >>>>> LTP on EOPNOTSUPP). The question why is not supported and whether is going to be >>>>> fixed. >>>>> Looking into glibc change 65341f7bbe ("linux: Use fchmodat2 on fchmod for flags >>>>> different than 0 (BZ 26401)") one year old change from glibc-2.39 [2] it looks >>>>> just accepted behavior (glibc returns EOPNOTSUPP on symlink): >>>>> + /* Some Linux versions with some file systems can actually >>>>> + change symbolic link permissions via /proc, but this is not >>>>> + intentional, and it gives inconsistent results (e.g., error >>>>> + return despite mode change). The expected behavior is that >>>>> + symbolic link modes cannot be changed at all, and this check >>>>> + enforces that. */ >>>>> + if (S_ISLNK (st.st_mode)) >>>>> + { >>>>> __close_nocancel (pathfd); >>>>> - return ret; >>>>> + __set_errno (EOPNOTSUPP); >>>>> + return -1; >>>>> + } >>>>> Also musl also behaves the same on his fallback on old kernels [3] >>>>> (it started 10 years ago on 0dc48244 ("work around linux's lack of flags >>>>> argument to fchmodat syscall") when SYS_fchmodat was used and kept when this >>>>> year SYS_fchmodat2 started to be used in d0ed307e): >>>>> int ret = __syscall(SYS_fchmodat2, fd, path, mode, flag); >>>>> if (ret != -ENOSYS) return __syscall_ret(ret); >>>>> if (flag != AT_SYMLINK_NOFOLLOW) >>>>> return __syscall_ret(-EINVAL); >>>>> struct stat st; >>>>> int fd2; >>>>> char proc[15+3*sizeof(int)]; >>>>> if (fstatat(fd, path, &st, flag)) >>>>> return -1; >>>>> if (S_ISLNK(st.st_mode)) >>>>> return __syscall_ret(-EOPNOTSUPP); >>>>>> According to documentation, the feature has been implemented in >>>>>> kernel 6.6, but __in reality__ AT_SYMLINK_NOFOLLOW is not working >>>>>> on symbolic files. Also kselftests, which are meant to test the >>>>>> functionality, are not working and they are treating fchmodat2() >>>>>> syscall failure as SKIP. Please take a look at the following code >>>>>> before reviewing: >>>>>> https://github.com/torvalds/linux/blob/8f6a15f095a63a83b096d9b29aaff4f0fbe6f6e6/tools/testing/selftests/fchmodat2/fchmodat2_test.c#L123 >>>>> I see there is a kselftest workaround in 4859c257d295 ("selftests: Add fchmodat2 >>>>> selftest") [4], where fchmodat2 failure on symlink is simply skipped. >>>>> Aleksa, you're probably aware of this fchmodat2() failure on symlinks. Does >>>>> anybody work or plan to work on fixing it? LTP has policy to not cover kernel >>>>> bugs, if it's not expected to be working we might just skip the test as well. >>>> If I understand the bug report, the issue is that fchmodat2() doesn't >>>> work on symlinks? >>> Yes. >>>> This is intentional -- Christian fixed a tree-wide bug a while ago[1] >>>> where some filesystems would change the mode of symlinks despite >>>> returning an error (usually EOPNOTSUPP) and IIRC a few others would >>>> happily change the mode of symlinks. >>> Ah, I've seen this in the past. Thanks a lot for reminding me. >>>> The current intended behaviour is to always return EOPNOTSUPP, and AFAIK >>>> there is no plan to re-enable the changing of symlink modes. EOPNOTSUPP >>>> was chosen because that's what filesystems were already returning. >>>> (While this is a little confusing, VFS syscalls return EINVAL for an >>>> unsupported flag, not EOPNOTSUPP.) >>>> The benefit of an AT_SYMLINK_NOFOLLOW flag is not just to to allow a >>>> syscall to operate on symlinks, it also allows programs to safely >>>> operate on path components without worrying about symlinks being >>>> followed (this is relevant for container runtimes, where we are >>>> operating on untrusted filesystem roots -- though in the case of >>>> fchmodat2(2) you would probably just use AT_EMPTY_PATH in practice). So >>>> an error here is actually what you want as a program that uses >>>> AT_SYMLINK_NOFOLLOW (since the actual operation is intentionally not >>>> supported by filesystems). >> Thanks for the explanation. I also have a question around this topic: >> AT_SYMLINK_NOFOLLOW has been added in order to avoid symlinks being >> followed. But if filesystems are not supporting it, why do we have this >> feature? > @Aleksa please correct me if I'm wrong. > > AFAIK (reading 5d1f903f75a8 commit message [1] and Aleksa's comments) previously > it was an idea which many of the filesystem implemented wrongly - a mess > regardless whether supported by the filesystem or not. I particularly like > changing the mode but fail EOPNOTSUPP. And because glibc and musl did EOPNOTSUPP > anyway (I found that as well), the best was just to follow this in kernel and > unify all filesystems behavior by disabling this in VFS. > >> Also, is it an unsupported feature only on certain filesystems? > Disabled in VFS => unsupported on all filesystems. Ah right, that's quite obvious indeed. >>> Thanks a lot for explaining the background! >>>> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d1f903f75a80daa4dfb3d84e114ec8ecbf29956 >>>>> I see a RFC UAPI related patchset [5] which touches include/uapi/linux/fcntl.h, >>>>> but AFAIK it's not related to this problem. >>>> Yeah this is unrelated, that patch is about clarifying how AT_* flags >>>> are allocated, not syscall behaviour. >>> Thanks! >>>>> Kind regards, >>>>> Petr >>> @Andrea, I guess we want something like this: >>> +++ testcases/kernel/syscalls/fchmodat2/fchmodat2_01.c >>> @@ -43,9 +43,10 @@ static void test_symbolic_link(void) >>> verify_mode(fd_dir, FNAME, S_IFREG | 0700); >>> verify_mode(fd_dir, SNAME, S_IFLNK | 0777); >>> - TST_EXP_PASS(fchmodat2(fd_dir, SNAME, 0640, AT_SYMLINK_NOFOLLOW)); >>> - verify_mode(fd_dir, FNAME, S_IFREG | 0700); >>> - verify_mode(fd_dir, SNAME, S_IFLNK | 0640); >>> + if (tst_kvercmp(6, 6, 0) >= 0) { >>> + TST_EXP_FAIL(tst_syscall(__NR_fchmodat2, fd_dir, SNAME, 0640, >>> + AT_SYMLINK_NOFOLLOW), EOPNOTSUPP); >>> + } >>> } >>> static void test_empty_folder(void) >> I think it makes more sense to filter out only filesystems which are not >> supporting this feature (see my comment above). > Due disabled in VFS since 5d1f903f75a8 all filesystems fail with EOPNOTSUPP, > thus failure in LTP (TBROK), which needs to be handled. Before 5d1f903f75a8 > some of them actually changed the mode (e.g. btrfs, ext4, xfs), but that's no > longer the case. And because it got backported to all stable/LTS, we can expect > this is the correct behavior. > > Kind regards, > Petr > >> Best regards, >> Andrea I guess we can just assume EOPNOTSUPP for the test. I will add the .tag and errno check. Thanks, Andrea