Setting registers

Setting registers

[Indira]

Hi,

I was writing a macro in which i was trying to the variable ebp and eip as,

set variable $ebp = *(int *)($sp+8)
set variable $eip = *(int *)($sp+36)

but, even though by doing this th eregister values dont get updated.
Why is this so? Can anyone help me on this?

Thanks in advance


-- 
Indu :)
Smile.... & Make others Smile :)

RE: Setting registers

[Dave Korn]

On 13 June 2007 11:28, Indira wrote:

> Hi,
> 
> I was writing a macro in which i was trying to the variable ebp and eip as,
> 
> set variable $ebp = *(int *)($sp+8)
> set variable $eip = *(int *)($sp+36)
> 
> but, even though by doing this th eregister values dont get updated.
> Why is this so? Can anyone help me on this?

  Dunno, works fine for me:

Breakpoint 1, main () at time.c:13
13      {       double t0 = get_time(), t1 = get_time();
(gdb) info regis
eax            0x10     16
ecx            0x401080 4198528
edx            0x4c     76
ebx            0x0      0
esp            0x23cc80 0x23cc80
ebp            0x23ccb8 0x23ccb8
esi            0x611001a0       1628438944
edi            0x401430 4199472
eip            0x40108b 0x40108b <main+11>
eflags         0x202    [ IF ]
cs             0x1b     27
ss             0x23     35
ds             0x23     35
es             0x23     35
fs             0x3b     59
gs             0x0      0
(gdb) set variable $ebp = *(int *)($sp+8)
(gdb) set variable $eip = *(int *)($sp+36)
(gdb) info regis
eax            0x10     16
ecx            0x401080 4198528
edx            0x4c     76
ebx            0x0      0
esp            0x23cc80 0x23cc80
ebp            0x61166a78       0x61166a78
esi            0x611001a0       1628438944
edi            0x401430 4199472
eip            0x2f     0x2f
eflags         0x202    [ IF ]
cs             0x1b     27
ss             0x23     35
ds             0x23     35
es             0x23     35
fs             0x3b     59
gs             0x0      0
(gdb)


  You're not by any chance trying to debug a core file are you?

    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....

Re: Setting registers

[Indira]

Yes I am debuggin a core file. We cant set registers while debuggin a core file?

On 6/13/07, Dave Korn <dave.korn@artimi.com> wrote:
> On 13 June 2007 11:28, Indira wrote:
>
> > Hi,
> >
> > I was writing a macro in which i was trying to the variable ebp and eip as,
> >
> > set variable $ebp = *(int *)($sp+8)
> > set variable $eip = *(int *)($sp+36)
> >
> > but, even though by doing this th eregister values dont get updated.
> > Why is this so? Can anyone help me on this?
>
>   Dunno, works fine for me:
>
> Breakpoint 1, main () at time.c:13
> 13      {       double t0 = get_time(), t1 = get_time();
> (gdb) info regis
> eax            0x10     16
> ecx            0x401080 4198528
> edx            0x4c     76
> ebx            0x0      0
> esp            0x23cc80 0x23cc80
> ebp            0x23ccb8 0x23ccb8
> esi            0x611001a0       1628438944
> edi            0x401430 4199472
> eip            0x40108b 0x40108b <main+11>
> eflags         0x202    [ IF ]
> cs             0x1b     27
> ss             0x23     35
> ds             0x23     35
> es             0x23     35
> fs             0x3b     59
> gs             0x0      0
> (gdb) set variable $ebp = *(int *)($sp+8)
> (gdb) set variable $eip = *(int *)($sp+36)
> (gdb) info regis
> eax            0x10     16
> ecx            0x401080 4198528
> edx            0x4c     76
> ebx            0x0      0
> esp            0x23cc80 0x23cc80
> ebp            0x61166a78       0x61166a78
> esi            0x611001a0       1628438944
> edi            0x401430 4199472
> eip            0x2f     0x2f
> eflags         0x202    [ IF ]
> cs             0x1b     27
> ss             0x23     35
> ds             0x23     35
> es             0x23     35
> fs             0x3b     59
> gs             0x0      0
> (gdb)
>
>
>   You're not by any chance trying to debug a core file are you?
>
>     cheers,
>       DaveK
> --
> Can't think of a witty .sigline today....
>
>


-- 
Indu :)
Smile.... & Make others Smile :)

Re: Setting registers

[Eli Zaretskii]

> Date: Fri, 15 Jun 2007 18:25:53 +0530
> From: Indira <yoursindu@gmail.com>
> Cc: gdb@sourceware.org
> 
> Yes I am debuggin a core file. We cant set registers while debuggin a core file?

No, you can't.  The program is dead, all you have is the last snapshot
of its memory.  Even if you could set registers (and other variables),
it would be of no practical use, since you cannot run the program.
All you can do is examine its memory and call stack.

Re: Setting registers

[Paul Koning]

>>>>> "Eli" == Eli Zaretskii <eliz@gnu.org> writes:

 >> Date: Fri, 15 Jun 2007 18:25:53 +0530 From: Indira
 >> <yoursindu@gmail.com> Cc: gdb@sourceware.org
 >> 
 >> Yes I am debuggin a core file. We cant set registers while
 >> debuggin a core file?

 Eli> No, you can't.  The program is dead, all you have is the last
 Eli> snapshot of its memory.  Even if you could set registers (and
 Eli> other variables), it would be of no practical use, since you
 Eli> cannot run the program.  All you can do is examine its memory
 Eli> and call stack.

Sometimes it would be useful to be able to.  If the stack is corrupt,
or the return address (for processor types that have such a thing) and
you can figure out the correct values, it would be very handy to be
able to tell gdb "here is the right value" and let it use that for
subsequent analysis.

Right now, that's not possible.  So if I have a smashed stack, I have
to walk it by hand -- which means looking at local variables is
essentially undoable.  It's not always simply a matter of giving the
right SP to gdb, but often it is.

      paul

Re: Setting registers

[Daniel Jacobowitz]

On Fri, Jun 15, 2007 at 10:41:38AM -0400, Paul Koning wrote:
> Sometimes it would be useful to be able to.  If the stack is corrupt,
> or the return address (for processor types that have such a thing) and
> you can figure out the correct values, it would be very handy to be
> able to tell gdb "here is the right value" and let it use that for
> subsequent analysis.

We do get requests for this so frequently that I'm tempted to allow
modifiable registers... memory, though, would be hard.

-- 
Daniel Jacobowitz
CodeSourcery

Re: Setting registers

[Paul Koning]

>>>>> "Daniel" == Daniel Jacobowitz <drow@false.org> writes:

 Daniel> On Fri, Jun 15, 2007 at 10:41:38AM -0400, Paul Koning wrote:
 >> Sometimes it would be useful to be able to.  If the stack is
 >> corrupt, or the return address (for processor types that have such
 >> a thing) and you can figure out the correct values, it would be
 >> very handy to be able to tell gdb "here is the right value" and
 >> let it use that for subsequent analysis.

 Daniel> We do get requests for this so frequently that I'm tempted to
 Daniel> allow modifiable registers... memory, though, would be hard.

Not really, you'd just have to allow writing to the corefile.  I think
you'd want to disable that by default -- otherwise a slip of the
finger would mess up your carefully collected data.  But after an
explicit enable it could be done.

And it's necessary if you're trying to change a register for an inner
frame. 

       paul

Re: Setting registers

[Daniel Jacobowitz]

On Fri, Jun 15, 2007 at 11:21:37AM -0400, Paul Koning wrote:
> Not really, you'd just have to allow writing to the corefile.  I think
> you'd want to disable that by default -- otherwise a slip of the
> finger would mess up your carefully collected data.  But after an
> explicit enable it could be done.

Um, no.  We'd also have to maintain shadow buffers in memory for the
contents of RAM instead of reading it from disk.

-- 
Daniel Jacobowitz
CodeSourcery

Re: Setting registers

[Joel Brobecker]

> Not really, you'd just have to allow writing to the corefile.  I think
> you'd want to disable that by default -- otherwise a slip of the
> finger would mess up your carefully collected data.  But after an
> explicit enable it could be done.

Actually, this is already possible with "set write on".

-- 
Joel

Re: Setting registers

[Indira]

I just want to set the esp, eip and ebp registers and print backtrace.
Can i do so while debugging a core file?

On 6/15/07, Eli Zaretskii <eliz@gnu.org> wrote:
> > Date: Fri, 15 Jun 2007 18:25:53 +0530
> > From: Indira <yoursindu@gmail.com>
> > Cc: gdb@sourceware.org
> >
> > Yes I am debuggin a core file. We cant set registers while debuggin a core file?
>
> No, you can't.  The program is dead, all you have is the last snapshot
> of its memory.  Even if you could set registers (and other variables),
> it would be of no practical use, since you cannot run the program.
> All you can do is examine its memory and call stack.
>


-- 
Indu :)
Smile.... & Make others Smile :)

RE: Setting registers

[Dave Korn]

On 18 June 2007 13:01, Indira wrote:

> I just want to set the esp, eip and ebp registers and print backtrace.
> Can i do so while debugging a core file?

  I thought I'd seen those values of ($sp+8) and ($sp+36) before.

http://sources.redhat.com/ml/gdb/2007-04/msg00146.html
http://sources.redhat.com/ml/gdb/2007-04/msg00148.html

  What is it with you?  You asked this exact same stuff back in April, now
apparently you didn't believe the answer you were given so you've come back
under a false name to ask the exact same thing again.


    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....