Date: Thu, 29 Sep 2005 20:47:54 -0400
From: Daniel Jacobowitz <dan@debian.org>
To: John Reiser <jreiser@BitWagon.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: ptrace unexpected SIGTRAP (trace bit) on x86, x86_64  kernel 2.6.13.2
Message-ID: <20050930004754.GA26866@nevyn.them.org>
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On Thu, Sep 29, 2005 at 08:58:25AM -0700, John Reiser wrote:
> Ptrace is giving unexpected SIGTRAP (trace bit) in kernel 2.6.13.2
> on both x86 and x86_64.
> 
> The 8-instruction program below just execve()s itself over and over.
> When run under gdb, the first user-visible SIGTRAP is expected due to
> the 'int3'.  But the second user-visible SIGTRAP is unexpected, as
> there is no reason to trap.
> 
> Changing the line "nop; int3" to "nop; nop" gives a program that
> just spins merrily when run under /bin/bash.  But gdb sees a SIGTRAP,
> with the $pc pointing after the second 'nop'.  When run under strace
> (strace gdb ./execve; (gdb) run), the process spins merrily with
> no unexpected SIGTRAP.

Most likely GDB is receiving the SIGTRAP generated by execve().  I
don't know why it would appear to be after the int3 rather than before.
GDB has strictly limited support for programs which exec, mostly due to
thorny user interface issues.

-- 
Daniel Jacobowitz
CodeSourcery, LLC
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