Re: gdb/2054: can't start X server under gdb

[Daniel Jacobowitz <drow at false dot org>]

The following reply was made to PR gdb/2054; it has been noted by GNATS.

From: Daniel Jacobowitz <drow@false.org>
To: Adam Jackson <ajackson@redhat.com>
Cc: gdb-gnats@sources.redhat.com
Subject: Re: gdb/2054: can't start X server under gdb
Date: Wed, 4 Jan 2006 11:46:09 -0500

 On Wed, Jan 04, 2006 at 11:28:24AM -0500, Adam Jackson wrote:
 > 
 > On Dec 20, 2005, at 11:34 AM, Daniel Jacobowitz wrote:
 > >
 > >Oh sorry - you have to not use strace -f with gdb, because only one
 > >process can ptrace a target at a time.  We want a trace of GDB, but not
 > >of the X server.
 > 
 > Right, forgot about that bit.  Updated trace is at:
 > 
 > http://people.freedesktop.org/~ajax/gdb-bug-2054/strace-of-gdb-2.txt
 
 So here's what happened - your strace unfortunately doesn't show some
 details, but I can guess them pretty confidently:
 
 ptrace(PTRACE_CONT, 31602, 0, SIG_0)    = 0
 waitpid(-1, [WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP], 0) = 31602
 
 We continue the parent, and it stops.  At this point it is reporting a
 fork.
 
 ptrace(0x4201 /* PTRACE_??? */, 31602, 0, 0xbff15d38) = 0
 
 We fatch the PID of the forked child.
 
 waitpid(31661, [WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP], 0) = 31661
 
 We wait for the forked child to stop.  Waitasec!  It didn't stop!  Not
 only didn't it stop, it reported an EXEC event (which we did request,
 contrary to what I said earlier in this bug report).
 
 So we should have seen a SIGSTOP from the newly forked child, but
 didn't.
 
 The code that is supposed to guarantee this is in kernel/fork.c, and
 seems to be intact in the latest kernel that I checked.  I'm stumped.
 Might want to make sure that's triggering.  Or, when GDB prints the
 error message, check /proc for the child process - does it have a
 pending SIGSTOP still?
 
 -- 
 Daniel Jacobowitz
 CodeSourcery