glibc 2.32 leaks errno in sched_rr_get_interval fallback

[Balint Reczey]

root@autopkgtest-lxd-rygyvj:/tmp/autopkgtest.Pui0CD/build.uqw/src# strace ./a.out
execve("./a.out", ["./a.out"], 0xfff2c8a0 /* 12 vars */) = 0
brk(NULL) = 0x1942000
uname({sysname="Linux", nodename="autopkgtest-lxd-rygyvj", ...}) = 0
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=24113, ...}) = 0
mmap2(NULL, 24113, PROT_READ, MAP_PRIVATE, 3, 0) = 0xf78ec000
close(3) = 0
openat(AT_FDCWD, "/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\3\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0\t\261\1\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=988544, ...}) = 0
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xf78ea000
mmap2(NULL, 1054788, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xf77c6000
mprotect(0xf78b3000, 61440, PROT_NONE) = 0
mmap2(0xf78c2000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xec000) = 0xf78c2000
mmap2(0xf78c6000, 6212, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xf78c6000
close(3) = 0
set_tls(0xf78eac90) = 0
mprotect(0xf78c2000, 8192, PROT_READ) = 0
mprotect(0x81b000, 4096, PROT_READ) = 0
mprotect(0xf78f2000, 4096, PROT_READ) = 0
munmap(0xf78ec000, 24113) = 0
sched_rr_get_interval_time64(1, 0xfff88678) = -1 ENOSYS (Function not implemented)
sched_rr_get_interval(1, {tv_sec=0, tv_nsec=0}) = 0
dup(2) = 3
fcntl64(3, F_GETFL) = 0x402 (flags O_RDWR|O_APPEND)
brk(NULL) = 0x1942000
brk(0x1963000) = 0x1963000
fstat64(3, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0x3), ...}) = 0
write(3, "error: Function not implemented\n", 32error: Function not implemented
) = 32
close(3) = 0
exit_group(0) = ?
+++ exited with 0 +++
root@autopkgtest-lxd-rygyvj:/tmp/autopkgtest.Pui0CD/build.uqw/src# cat test.c
#include <stdio.h>
#include <errno.h>
#include <sched.h>
int main() {
  struct timespec tp;
  errno = 0;
  sched_rr_get_interval(1, &tp);
  perror("error");
  return 0;
}

When sched_rr_get_interval_time64 is not implemented glibc falls back to sched_rr_get_interval syscall but does not clear errno.

This breaks 'unhide quick' and a few other unhide commands giving false positives as a result.

The problem is observed only on armhf testbeds.

[Balint Reczey (rbalint)]

vorlon | ok, agreed that it's not critical for glibc, feel free to untarget

[Launchpad Janitor (janitor)]

This bug was fixed in the package unhide - 20130526-4ubuntu1

---------------
unhide (20130526-4ubuntu1) groovy; urgency=medium

  * Work around Glibc 2.32 leaking errno from sched_rr_get_interval when
    sched_rr_get_interval_time64 syscall is not implemented (LP: #1896281)

 -- Balint Reczey <email address hidden> Fri, 18 Sep 2020 22:57:25 +0200

[Balint Reczey (rbalint)]

When sched_rr_get_interval_time64 is not implemented glibc falls back to sched_rr_get_interval syscall but does not clear errno.

This breaks 'unhide quick' and a few other unhide commands giving false positives as a result.

The problem is observed only on armhf in Ubuntu CI.

https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1896281

test.c:
#include <stdio.h>
#include <errno.h>
#include <sched.h>
int main() {
  struct timespec tp;
  errno = 0;
  sched_rr_get_interval(1, &tp);
  perror("error");
  return 0;
}

strace ./a.out
...
sched_rr_get_interval_time64(1, 0xfff88678) = -1 ENOSYS (Function not implemented)
sched_rr_get_interval(1, {tv_sec=0, tv_nsec=0}) = 0
dup(2) = 3
fcntl64(3, F_GETFL) = 0x402 (flags O_RDWR|O_APPEND)
brk(NULL) = 0x1942000
brk(0x1963000) = 0x1963000
fstat64(3, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0x3), ...}) = 0
write(3, "error: Function not implemented\n", 32error: Function not implemented
) = 32
...

...

[Andreas Schwab (schwab-linux-m68k)]

errno is only defined if sched_rr_get_interval returns -1.

[Balint Reczey (rbalint)]

root@gg-armhf:~# cat test.c
#include <stdio.h>
#include <errno.h>
#include <sched.h>
int main() {
  struct timespec tp;
  errno = 0;
  int ret = sched_rr_get_interval(1, &tp);
  printf("ret == %d\n", ret);
  perror("error");
  return 0;
}
root@gg-armhf:~# ./a.out
ret == 0
error: Function not implemented
root@gg-armhf:~#

[Adhemerval Zanella (adhemerval-zanella)]

I think this is similar (In reply to Balint Reczey from comment #2)
> root@gg-armhf:~# cat test.c
> #include <stdio.h>
> #include <errno.h>
> #include <sched.h>
> int main() {
> struct timespec tp;
> errno = 0;
> int ret = sched_rr_get_interval(1, &tp);
> printf("ret == %d\n", ret);
> perror("error");
> return 0;
> }
> root@gg-armhf:~# ./a.out
> ret == 0
> error: Function not implemented
> root@gg-armhf:~#

The errno value after a successful call does not give you any meaningful information. But I think this is similar to the recent stat y2038 support where it triggered some invalid used within glibc itself. I think it would be better to use INTERNAL_SYSCALL_CALL in such cases and only sets the errno in case of failure.