reverse debugging implementation

paawan oza
Wed Aug 25 06:30:00 GMT 2010


please find my inline.


----- Original Message ----
From: David McQuillan <>
Sent: Sun, August 22, 2010 7:09:14 PM
Subject: reverse debugging implementation


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 

David McQuillan


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