Re: reverse debugging implementation

paawan oza
2010-08-25T06:30:24Z

Hi,

please find my inline.

Regards,
Oza.

----- Original Message ----
From: David McQuillan <dmcq@fano.co.uk>
To: gdb@sourceware.org
Sent: Sun, August 22, 2010 7:09:14 PM
Subject: reverse debugging implementation

Hi,

I see gdb now supports reverse debugging. This is something I implemented for
myself a number of years ago with my own simulators when I was working on
testing some compilers and other stuff. That all died a long time ago but some
of the ideas from it might be useful if you haven't done them already.

Firstly as far as I can see gdb saves all the state change for every
instruction. This is unnecessary, all you have to do is save the state of the
cpu every few thousand instructions and for each instruction only save the state
of memory locations before they are changed. Going back one instruction then
involves rolling back the memory changes, the registers are set up then go
forward n-1 instruction to go back one.

oza : so does it mean that instructions which changes memory you can take one
step backward, but instructions which have changed registers you can not step
back by one instruction but n-1. so we lost the ability to track back by
instruction, yes of course it is performance gain by the factor of n.

I also had debug instructions to mark the current spot, then one could go
between a few different different states easily to check on the difference.

I think practically everything else I had about what to do about library calls
is there - I intercepted system calls so they weren't redone just the old
results returned so I had to save both before and after looks. Special work had
to be done about allocating and freeing space and I only had serial files and no
special device access so you can see I had a fairly straightforward environment.

Oza: -> Is it possible to revoke all system calls, such as revoking sent signal
etc, doesnt it ned kernel change in all cases and probably making ptrace more
powerful. ?

If you used virtual memory tricks it might be possible to save the state of the
memory that way instead of even taking before looks. It would slow going
backwards a bit but I never noticed any particular delay going back an
instruction and machines have sped up and there's lots more memory nowadays.

Oza -> does it involve saving whole virtuall address space on disk or something
?

Cheers,
David McQuillan