single step exception

[y2bismil@engmail.uwaterloo.ca]

Okie, 

I think I see the problem.  Since I use pushfl and popfl which modify the esp,
there is a problem when the inline asm returns to C code.  GCC has no way of
knowing that the esp has been modified, so any accesses it does based on esp
would be wrong.

Even though I specify that esp has been modified, I don't think GCC takes that
into account.  Is there any way I can tell GCC that I have modified the esp by a
certain value?

Thanks,

Yamin

Quoting y2bismil@engmail.uwaterloo.ca:

> 
> 
> Hi all,
> 
> Okay, this is the issue.  I've isolated it way down to some very basic
> functionality, and made a reasonable test bed.  I'll post the code below.
> 
> When MYINLINE = 1, then the program executes correctly.  When MYINLINE is 0,
> then it stops with a single step exception.  I was stepping through the code
> and
> from my understanding there is something  definitely wrong when a function
> call
> is made instead of the inline code.  Bare in mind, my understaind could be
> what
> is wrong :)
> 
> >From my understand this is what happens.
> 
> pushf - pushes the eflags onto the stack, decrements esp
> mymemcpy - simple memcpy
> popf - pops the eflags from the stack into the eflgs register, increments
> esp
> 
> This is basically the program below.  When the inlining is not used, for
> some
> reason, the stack gets corrupted.  When the variables to be passed into
> memcpy
> are done, they overwrite the location where the eflags register was stored
> (from
> the pushf).  This then causes some things to mess up.  When inlining is used.
> 
> this nicely does not happen.
> 
> I always hesitate to say bug as its probably something i'm not specifying or
> some misunderstanding.
> Any ideas?  
> 
> Yamin
> 
> 
> *****************************************************
> main.cpp
> *****************************************************
> #include <io.h>
> #include <stdio.h>
> 
> #if 0
> #include "chead.h"
> #else
>   #define KS_DISABLE()  \
>       __asm__ __volatile__ ( \
>       "pushfl;" \
>       : \
>       : \
>       : "memory", "%esp");
> 
>    #define KS_ENABLE()  \
>       __asm__ __volatile__ ( \
>       "popfl;" \
>       : \
>       : \
>       : "memory", "%esp");
> 
> char * mymemcpy(void *dv, void *sv, int num)
> {
>     char * d = (char*)dv;
>     char * s = (char*)sv;
>     int i;
> 
>     for( i = 0; i < num; i++) *d++ = *s++;
> 
> }
> 
> 
> #endif
> 
> 
> 
> #define MYINLINE 1
> int main()
> {
> #if MYINLINE
> 	char *d;
>         char *s;
> 	int i;
> #endif
>         char mysrc[] = "HELLO YAMIN\r\n";
>         char mydst[32];
> 	printf ("starting popf test\r\n");
> 	KS_DISABLE();
> #if MYINLINE
> 	d = (char*) mydst;
>         s = (char*) mysrc;
>         for( i = 0; i < sizeof(mysrc); i++) *d++ = *s++;
> #else	
> 	mymemcpy( mydst, mysrc, sizeof(mysrc) );
> #endif
>         KS_ENABLE();
> 	printf ("ending popf test: %s\r\n", mydst);
> }
> *****************************************************
> 
> ----------------------------------------
> This mail sent through www.mywaterloo.ca
> 




----------------------------------------
This mail sent through www.mywaterloo.ca