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Date: Mon, 16 Mar 2009 18:50:00 -0000
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From: Mark Kettenis <mark.kettenis@xs4all.nl>
To: aristovski@qnx.com
CC: gdb@sources.redhat.com
In-reply-to: <gpm2v2$n1o$1@ger.gmane.org> (message from Aleksandar Ristovski 	on Mon, 16 Mar 2009 13:40:49 -0400)
Subject: Re: [RFC] stepping over permanent breakpoint
References:  <gpm2v2$n1o$1@ger.gmane.org>
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> From: Aleksandar Ristovski <aristovski@qnx.com>
> Date:  Mon, 16 Mar 2009 13:40:49 -0400
> 
> Hello,
> 
> When there is a hard-coded breakpoint in code, like in this 
> example (for x86):
> 
> #include <stdio.h>
> 
> int main()
> {
>    __asm("       int $0x03\n");
>    printf("Hello World\n");
>    return 0;
> }
> 
> gdb on linux will appear to work correctly.

Well, on Linux, that instruction will not be interpreted as a
permanent breakpoint, just like on QNX.

> However, on systems that do not need pc adjustment after 
> break (like QNX) gdb will not be able to step over that 
> breakpoint unless user explicitly sets a breakpoint on top 
> of it.

The big question here is whether a breakpoint trap instruction should
always be interpreted as a permanent breakpoint in GDB or that it only
gets interpreted as such if you actually tell GDB about it.  Up until
now, we've always done the latter.  If you don't tell GDB, random
breakpoint trap instructions are handled as normal instructions and
you get to see whatever the architecture/OS does for these
instructions.

> I think that in case of linux it is actually working by 
> accident - because kernel does not back-up instruction 
> pointer after hard-coded breakpoint instruction was 
> executed. Gdb will receive SIGTRAP but will not really know why.
> 
> Attached patch fixes this for systems where 
> gdbarch_decr_pc_after_break (gdbarch) == 0

If you want to fix things, it should be fixed for *all* systems.