pthread_cancel fails with "RT33" with musl libc

[Luca Weiss]

From my testing it seems that QEMU built against musl libc crashes on pthread_cancel cancel calls - if the binary is also built with musl libc.

Minimal sample:

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
void* threadfunc(void* ignored) {
 while (1) {
  pause();
 }
 return NULL;
}
int main() {
 pthread_t thread;
 pthread_create(&thread, NULL, &threadfunc, NULL);
 sleep(1);
 pthread_cancel(thread);
 printf("OK, alive\n");
}

In an Alpine Linux aarch64 chroot (on an x86_64 host) the binary will just output RT33 and has exit code 161.

Using qemu-aarch64 on an x86_64 host results in the output (fish shell)
  fish: “qemu-aarch64-static ./musl-stat…” terminated by signal Unknown (Unknown)
or (bash)
  Real-time signal 2

and exit code 164.

It doesn't matter whether the binary is linked dynamically or static. You can see my test results in the following table:

| | QEMU glibc | QEMU musl |
|----------------------|------------|-----------|
| binary glibc dynamic | ✓ | ✓ |
| binary glibc static | ✓ | ✓ |
| binary musl dynamic | ✓ | ✗ |
| binary musl static | ✓ | ✗ |

Both QEMU builds are v5.1.0 (glibc v2.32 / musl v1.2.1)

I've uploaded all my compile and test commands (plus a script to conveniently run them all) to https://github.com/z3ntu/qemu-pthread_cancel . It also includes the built binaries if needed. The test script output can be found at https://github.com/z3ntu/qemu-pthread_cancel/blob/master/results.txt

Further links:
- https://gitlab.com/postmarketOS/pmaports/-/issues/190#note_141902075
- https://gitlab.com/postmarketOS/pmbootstrap/-/issues/1970

[Ariadne Conill (kaniini)]

This was a downstream regression in Alpine caused by an attempt to make older Go binaries work under emulation. We have reverted the patch there.

[Thomas Huth (th-huth)]

Ok, thanks, since this was a regressin in Alpine, I'm marking the bug as closed here.