crash after receiving SIGCHLD during system call

[Russell King - ARM Linux]

On Wed, May 17, 2017 at 10:04:32AM -0600, David Mosberger wrote:
> I added some instrumentation code to the SIGCHLD handler of lighttpd
> v1.4.45 and I have seen crashes after the SIGCHLD handler
> interrupted __libc_fork() and close().  The one constant so far is
> that the pc register in the signal handler machine-context
> (mcontext_t) points to the instruction after the "svc 0" instruction.

I don't think there's much to be read into that as signal delivery to
a process always occurs on the thread's exit path from kernel mode to
user mode.

This happens after:

- completion of an interrupt handler
- completion of a kernel syscall
- completion of a page fault

What would help is to enable CONFIG_DEBUG_USER and pass 'user_debug=31'
to the kernel.  It should be more verbose about the cause of the fault
from the kernel point of view.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.