pidfds: add coredump_code field to pidfd_info

pidfds: add coredump_code field to pidfd_info

The struct pidfd_info currently exposes in a field called coredump_signal the
signal number (si_signo) that triggered the dump (for example, 11 for SIGSEGV).
However, it is also valuable to understand the reason why that signal was sent.
This additional context is provided by the signal code (si_code), such as 2 for
SEGV_ACCERR.

Add a new field to struct pidfd_info called coredump_code with the value of
si_code for the benefit of sysadmins who pipe core dumps to user-space programs
for later analysis. The following snippet illustrates a simplified C program
that consumes coredump_signal and coredump_code, and then logs core dump
signals and codes to a file:

    int pidfd = (int)atoi(argv[1]);

    struct pidfd_info info = {
        .mask = PIDFD_INFO_EXIT | PIDFD_INFO_COREDUMP,
    };

    if (ioctl(pidfd, PIDFD_GET_INFO, &info) == 0)
        if (info.mask & PIDFD_INFO_COREDUMP)
            fprintf(f, "PID=%d, si_signo: %d si_code: %d\n",
                info.pid, info.coredump_signal, info.coredump_code);

Assuming the program is installed under /usr/local/bin/core-logger, core dump
processing can be enabled by setting /proc/sys/kernel/core_pattern to
'|/usr/local/bin/dumpstuff %F'.

systemd-coredump(8) already uses pidfds to process core dumps, and it could be
extended to include the values of coredump_code too.

Signed-off-by: Emanuele Rocca <emanuele.rocca@arm.com>
Link: https://patch.msgid.link/acE52HIFivNZN3nE@NH27D9T0LF
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>