segvrecover

segvrecover.c

#define _GNU_SOURCE /* for REG_RIP */

#include <signal.h>
#include <setjmp.h>

#include <stdio.h>

#ifdef _MSC_VER
/* Visual C */
#include <excpt.h>

#define METHOD "SEH"

int deref_i(int *p, int *failed)
{
	int result = 0;
	*failed = 1;
	__try {
		result = *p;
		*failed = 0;
	}
	__except (EXCEPTION_EXECUTE_HANDLER) {
		/**/
	}
	return result;
}

#else
/* fallback to signals */

// TODO: check availability of gcc/clang inline assembly
#if defined(__i386__) || defined(__x86_64__)

#define METHOD "IP++"

#if defined(__x86_64__)
#define IP_REG REG_RIP
#else
#define IP_REG REG_EIP
#endif

extern const int deref_asmsz_i;
void deref_L1_i();

static void segvhan(int signo, siginfo_t *info, void *vcontext)
{
	greg_t *p_ip;
	ucontext_t* context = (ucontext_t*)vcontext;
	p_ip = &context->uc_mcontext.gregs[IP_REG];
	if (deref_L1_i == (void*)*p_ip) {
		*p_ip += deref_asmsz_i;
		return;
	}
	// TODO: restore prev handler and raise
	// TODO: what if prev handler longjmps?
	raise(SIGTERM);
}

__attribute__((noinline))
int deref_i(int *p, int *p_failed) {
	int res, failed;
	asm ( "\n\t"
		".global deref_L1_i\n\t"
		".type deref_L1_i, STT_FUNC\n\t"
		"\n\t"
		"mov $1, %1\n\t"
		"deref_L1_i:\n\t"
		"mov (%2), %0\n\t"
		"mov $0, %1\n\t"
		"deref_L2_i:\n\t"
		"\n\t"
		".data\n\t"
		".align 2\n\t"
		"\n\t"
		"deref_asmsz_i:\n\t"
		".int  deref_L2_i - deref_L1_i\n\t"
		"\n\t"
		"\n\t"
		".text\n\t"
		""
		: "=r" (res), "=&r" (failed)
		: "r" (p)
		);
	*p_failed = failed;
	return res;
}

static void insthan() {
	struct sigaction acts;
	sigemptyset(&acts.sa_mask);
	acts.sa_flags=SA_SIGINFO;
	acts.sa_sigaction = segvhan;
	sigaction(SIGSEGV,&acts,NULL);
}

#else
/* fallback to long jump */

#define METHOD "Long Jump"

static sigjmp_buf env;

static void segvhan(int signo)
{
	siglongjmp(env, 1);
}

int deref_i(int *p, int *failed)
{
	int result = 0;

	*failed = 1;
	if (!sigsetjmp(env, 1)) {
		result = *p;
		*failed = 0;
	}

	return result;
}

static void insthan() {
	signal(SIGSEGV, segvhan);
}

#endif /* long jump */

static void check(int *p) {
	int res, failed;

	res = deref_i(p, &failed);

	if (failed) {
		fprintf(stderr, "error reading memory at %p\n", p);
	} else {
		printf("int value at %p is %d\n", p, res);
		fflush(stdout);
	}
}

#endif /* signals */

int main(int argc, char* argv[])
{
	int i = 42;
	printf("recover: %s\n", METHOD);
	fflush(stdout);
	insthan();
	check(NULL);
	check((int*)0x4);
	check(&i);

	/* this should terminate */
	*(volatile int*)0 = 0;
	return 0;
}