Bug 934125 – <app-containers/docker-25: fchmodat2(..., AT_SYMLINK_NOFOLLOW) returns EPERM instead of ENOSYS on ppc64le, breaking GNU tar

• Status: CONFIRMED
• Product: Gentoo Release Media

[Description Michał Górny 2024-06-12 08:11:01 UTC]

Trivial reproducer:

FROM gentoo/stage3:ppc64le-openrc
RUN wget http://gentoo.mirror.web4u.cz/distfiles/00/Pebble-5.0.7.tar.gz \
 && tar -xf Pebble-5.0.7.tar.gz

At least on bogsucker, it fails with:

1.463 tar: Pebble-5.0.7/Pebble.egg-info: Cannot change mode to rwxr-xr-x: Operation not permitted
1.465 tar: Pebble-5.0.7/pebble/asynchronous: Cannot change mode to rwxr-xr-x: Operation not permitted
1.466 tar: Pebble-5.0.7/pebble/concurrent: Cannot change mode to rwxr-xr-x: Operation not permitted
1.468 tar: Pebble-5.0.7/pebble/pool: Cannot change mode to rwxr-xr-x: Operation not permitted
1.468 tar: Pebble-5.0.7/pebble: Cannot change mode to rwxr-xr-x: Operation not permitted
1.476 tar: Pebble-5.0.7/test: Cannot change mode to rwxr-xr-x: Operation not permitted
1.476 tar: Pebble-5.0.7: Cannot change mode to rwxr-xr-x: Operation not permitted
1.476 tar: Exiting with failure status due to previous errors


Apparently, it can't... chmod directories?  But I can't reproduce with plain mkdir+chmod.

FWICS tar uses mkdirat() instead.  Will try to write a simpler reproducer later.

[Comment 1 Michał Górny 2024-06-12 08:51:33 UTC]

Managed to get strace in there, and the relevant bit seems to be:

mkdirat(AT_FDCWD, "Pebble-5.0.7/Pebble.egg-info", 0700) = 0
[…]
utimensat(AT_FDCWD, "Pebble-5.0.7/Pebble.egg-info", [UTIME_OMIT, {tv_sec=1711060438, tv_nsec=754192400} /* 2024-03-21T22:33:58.754192400+0000 */], AT_SYMLINK_NOFOLLOW) = 0
fchownat(AT_FDCWD, "Pebble-5.0.7/Pebble.egg-info", 1000, 1000, AT_SYMLINK_NOFOLLOW) = 0
fchmodat2(AT_FDCWD, "Pebble-5.0.7/Pebble.egg-info", 0755, AT_SYMLINK_NOFOLLOW) = -1 EPERM (Operation not permitted)

[Comment 2 Michał Górny 2024-06-12 08:57:27 UTC]

Aaand, we have a winner:

```
#define _GNU_SOURCE 1
#include <fcntl.h>
#include <sys/stat.h>

int main() {
	fchmodat(AT_FDCWD, "foo", 0755, AT_SYMLINK_NOFOLLOW);

	return 0;
}
```


On host:

fchmodat2(AT_FDCWD, "foo", 0755, AT_SYMLINK_NOFOLLOW) = -1 ENOSYS (Function not implemented)

Inside docker:

fchmodat2(AT_FDCWD, "foo", 0755, AT_SYMLINK_NOFOLLOW) = -1 EPERM (Operation not permitted)

[Comment 3 Michał Górny 2024-06-12 08:59:08 UTC]

(and of course the problem is that GNU tar checks for ENOSYS there but treats EPERM as a real error)

[Comment 4 Michał Górny 2024-06-12 09:12:11 UTC]

Ok, so apparently this is fixed in -25.  Time to stabilize a newer version?