Re: ptrace single-stepping change breaks Wine

Linus Torvalds
2004-11-21T22:34:12Z

On Sun, 21 Nov 2004, Davide Libenzi wrote:
>
> I'd agree with Linus here. A signal handler is part of the application, so
> it should be single stepped in the same way other application code does.
> My original patch simply reenabled the flag before returning to userspace,
> and this had the consequence to single step into signal handlers too.

Hmmm.. I think I may have a test-case for the problem.

Lookie here:

#include <signal.h>
#include <sys/mman.h>

void function(void)
{
printf("Copy protected: ok\n");
}

void handler(int signo)
{
extern char smc;
smc++;
}

#define TF 0x100

int main(int argc, char **argv)
{
void (*fnp)(void);

signal(SIGTRAP, handler);
mprotect((void *)(0xfffff000 & (unsigned long)main), 4096, PROT_READ | PROT_WRITE);
asm volatile("pushfl ; orl %0,(%%esp) ; popfl"
: :"i" (TF):"memory");
asm volatile("pushfl ; andl %0,(%%esp) ; popfl"
: :"i" (~TF):"memory");
asm volatile("\nsmc:\n\t"
".byte 0xb7\n\t"
".long function"
:"=d" (fnp));
fnp();
exit(1);
}

Compile it, run it, and it should say

Copy protected: ok

Now, try to "strace" it, or debug it with gdb, and see if you can repeat
the behaviour.

Roland? Think of it as a challenge,

Linus