bin/58112: daemonized rump conceals output on crash

[campbell+netbsd%mumble.net@localhost]

>Number:         58112
>Category:       bin
>Synopsis:       daemonized rump conceals output on crash
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 04 17:20:01 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, 8, ...
>Organization:
The TestBSD Rumpdation
>Environment:
growing more extreme weather as we toast the planet
>Description:
rumpuser_daemonize -- e.g., rump_server without -s -- sends all output to /dev/null:

    122 		fd = open(_PATH_DEVNULL, O_RDWR);
    123 		if (fd == -1) {
    124 			error = errno;
    125 			goto out;
    126 		}
    127 		dup2(fd, STDIN_FILENO);
    128 		dup2(fd, STDOUT_FILENO);
    129 		dup2(fd, STDERR_FILENO);
    130 		if (fd > STDERR_FILENO)
    131 			close(fd);

https://nxr.netbsd.org/xref/src/lib/librumpuser/rumpuser_daemonize.c?r=1.8#122

That means output from kernel printf also goes to /dev/null, because with librumpuser it is printed via putchar.

That means running a rump_server in, e.g., automatic tests, such as https://nxr.netbsd.org/xref/src/tests/rump/rumpkern/t_sp.sh?r=1.17, conceals all output when it crashes, as it has been frequently doing for the t_sp:sigsafe test.
>How-To-Repeat:
cd /usr/tests/rump/rumpkern && atf-run t_sp

repeat until it fails, in some appropriate environment where it fails, e.g.:

https://releng.netbsd.org/b5reports/i386/2024/2024.04.04.09.19.42/test.html#rump_rumpkern_t_sp_sigsafe
>Fix:
Maybe we could have environment variables RUMP_STDOUT=/path/to/outfile RUMP_STDERR=/path/to/errfile which rumpuser_daemonize_begin redirects fds 1 and 2 to, if set, so that the output is captured somewhere.