ptrace single-stepping change breaks Wine

Linus Torvalds torvalds at
Sun Nov 21 16:33:32 CST 2004

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;

	#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));

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,


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