Re: [RFC] stepping over permanent breakpoint

[Aleksandar Ristovski <aristovski at qnx dot com>]

Mark Kettenis wrote:
> > 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.

Except on QNX gdb will not be able to continue or step over 
that instruction.

> > 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.

Yes, this is my dilemma. I think we could print more 
informative message, but I am not sure.

> > 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.  

What I proposed only brings in line those systems and makes 
them able to continue after hitting a permanent breakpoint 
(so on systems that normally do not need adjustment, I added 
advancing over the hardcoded breakpoint).