[libvirt] [PATCH v3] util: Block SIGPIPE until execve in child process

[Eric Blake]

On 10/16/19 4:02 AM, Daniel P. Berrangé wrote:

> 
> The challenge here is that we're in between fork + execve and want signal
> handlers back to their defaults at time of execve.
> 
> If we set SIGPIPE to SIG_IGN and then execve() will that get reset back
> to SIG_DFL automatically ?

Sadly, no.  execve() does not change whether a signal is ignored or 
masked (ignored is more common - a number of CI systems have had issues 
where the child inherits SIGPIPE ignored because the parent forgot to 
reset it, but the child wasn't expecting it; but inheriting a signal 
masked is also a real issue), with the lone exception of SIGCHLD. 
However, execve() _does_ change a signal that is being caught in the 
parent into SIG_DFL post-exec.

That does mean, however, that it is viable to install a no-op SIGPIPE 
handler (SIGPIPE is generated but ignored, I/O gets the EPIPE as 
desired), then post-exec the new process will have SIG_DFL.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org