crash after receiving SIGCHLD during system call

[David Mosberger]

Just to close on this thread: the reason for the crashes came from the
kernel not having Thumb enabled.  The user-level code wasn't supposed
to use Thumb, but it turns out that glibc-2.24 will use Thumb
instructions in the strlen(), strcmp(), and memcmp() functions even
when you don't ask for it (i.e., there is no CONFIG_THUMB or some such
enabled anywhere).

So, what happened was that user-level used Thumb and occasionally an
interrupt would happen in the middle of the Thumb code.  Now, if the
kernel ended up needing to send a signal (SIGCHLD in my case), it
would set up the signal frame but NOT clear the Thumb-mode bit (since
Thumb-support wasn't enabled).  Thus, when resuming user-level
execution, the CPU would try to execute the signal handler in Thumb
mode, which would then lead to the crash.

Mystery solved.

The code-size increase due to enabling Thumb support in the kernel and
since there is no way to stop user-level from using Thumb, it's really
necessary to always enable Thumb support in the kernel.

I do think it'd be a good idea to have a BUG_ON() for this case (i.e.,
attempting to deliver signal with Thumb-mode bit on, but Thumb-support
disabled), but that's up to the arm kernel maintainers, of course.

  --david