ptrace & /proc deadlock due to mm_for_maps cred_guard_mutex patch

[Tom Horsley]

The redhat bugzilla:

https://bugzilla.redhat.com/show_bug.cgi?id=520206#c2

describes a test program that demonstrates an apparent
deadlock ptrace can get into as a result of the
patch:

   commit 704b836cbf19e885f8366bccb2e4b0474346c02d
   Author: Oleg Nesterov <oleg@xxxxxxxxxx>
   Date:   Fri Jul 10 03:27:40 2009 +0200
   mm_for_maps: take ->cred_guard_mutex to fix the race with exec  

ptrace hangs attempting to read the newly exec'ed debugged
child process /proc/pid/maps file, and the child is obviously
never going to advance as long as it is stopped by ptrace.

I haven't seen any activity on the bugzilla, so since this
was a kernel.org patch I figured I'd mention it over here.

As the bugzilla says, it is possible to work around this by
single stepping the debugged process before reading the maps
file.
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