id = 834
title = "linux-user: fails to deliver signals raised during pselect"
state = "closed"
created_at = "2022-01-23T01:02:55.512Z"
closed_at = "2022-03-24T20:08:18.931Z"
labels = ["Closed::Fixed", "kind::Bug", "linux-user"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/834"
host-os = "NixOS"
host-arch = "x86_64"
qemu-version = "qemu-x86_64 version 6.2.50 (v6.2.0-1117-gaeb0ae95b7)"
guest-os = "n/a"
guest-arch = "n/a"
description = """When run via qemu a program which blocks signals but unmasks them during `pselect` does not catch these signals when returning from `pselect`.

Used as reference on expected behavior: [The new pselect() system call](https://lwn.net/Articles/176911/)"""
reproduce = """A minimal test case below mimics behavior as encountered in the test suite of `p11-kit` ([link](https://github.com/p11-glue/p11-kit)) (which attempts to catch `SIGTERM` in a similar way and results in lingering processes after running the test suite).

```C
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/select.h>

static void handler(int sig)
{
\tputs("SIGNAL");
}

int main(int argc, char *argv[])
{
\tstruct sigaction sa;

\tfd_set rfds;
\tsigset_t emptyset, blockset;

\tsigemptyset (&blockset);
\tsigemptyset (&emptyset);
\tsigaddset (&blockset, SIGUSR1);

\tsa.sa_handler = handler;
\tsigemptyset(&sa.sa_mask);
\tsa.sa_flags = 0;
\tsigaction(SIGUSR1, &sa, NULL);

\tsigprocmask (SIG_BLOCK, &blockset, NULL);

\tFD_ZERO(&rfds);

\twhile(1) {
\t\tpselect(0, &rfds, NULL, NULL, NULL, &emptyset);
\t}

\treturn 0;
}
```

Running this without qemu should print _SIGNAL_ when sent `SIGUSR1`:

```
$ ./a.out &
[1] 1683587
$ kill -USR1 %1
$ SIGNAL
```

When run with `qemu-x86_64` however, it does not (also qemu's `-strace` confirms the signal isn't received whereas a strace of qemu shows it's in fact delivered).

The pselect call itself _is_ interrupted, but the signal goes missing."""
additional = "n/a"