Hi folks,
I have a probably louzy question regarding sigaction() behaviour when an
alternate signal stack is used: it seems that I can not get the user
stack reference in the ucontext_t stack context ; ie. the uc_stack
member contains reference of the alternate signal stack, not the stack
that was used before the crash.
Is this is a normal behaviour ? Is there a way to retrieve the original
user's stack inside the signal callback ?
The example given below demonstrates the issue:
top of stack==0x7fffff3d7000, alternative_stack==0x501010
SEGV==0x7fffff3d6ff8; sp==0x501010; current stack is the alternate stack
It is obvious that the SEGV was a stack overflow: the si_addr address is
just on the page below the stack limit.
/* gcc -g [ -D_REENTRANT ] stacktest.c [ -lpthread ] -o stacktest */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/resource.h>
#include <sys/ucontext.h>
#ifdef _REENTRANT
#include <pthread.h>
#endif
/* the alternative stack reference */
static stack_t ss;
/* this function does nasty things */
static void overflow(void) { overflow(); }
/* test entry point */
static void* threadEntry(void* parg) {
struct rlimit rlim;
/* setup alternative strack for the current thread */
ss.ss_flags = 0;
ss.ss_size = SIGSTKSZ;
ss.ss_sp = malloc(ss.ss_size);
if (ss.ss_sp == NULL) {
abort();
}
if (sigaltstack(&ss, NULL) == -1) {
abort();
}
/* print current stack limit */
if (getrlimit(RLIMIT_STACK, &rlim) == 0) {
const unsigned long page_size = (unsigned long) sysconf(_SC_PAGE_SIZE);
const unsigned long stack_bottom =
(((unsigned long)&rlim-rlim.rlim_cur+page_size-1)/page_size)*page_size;
printf("bottom of stack==%p, alternative_stack==%p\n", (void*)stack_bottom,
(void*)ss.ss_sp);
}
/* do something very nasty */
overflow();
/* we may not reach this point */
return NULL;
}
/* SEGV handler */
static void saHandler(int code, siginfo_t *si, void *sc_) {
void *kenny = (void*) &code;
ucontext_t * const sc = (ucontext_t*) sc_;
printf("SEGV==%p; sp==%p; current stack is the %s\n", (void*)si->si_addr,
(void*)((ucontext_t*)sc_)->uc_stack.ss_sp,
( kenny >= ss.ss_sp && kenny < ss.ss_sp + SIGSTKSZ )
? "alternate stack" : "original stack");
abort();
}
/* main entry point */
int main(void) {
/* catch SEGV with SA_ONSTACK enabled */
struct sigaction s;
memset(&s, 0, sizeof(s));
sigemptyset(&s.sa_mask);
s.sa_flags = SA_SIGINFO | SA_ONSTACK;
s.sa_sigaction = saHandler;
if(sigaction (SIGSEGV, &s, NULL)) {
abort();
}
#ifdef _REENTRANT
/* threaded version */
{
pthread_t t;
pthread_create(&t, NULL, threadEntry, NULL);
pause(); /* wait (almost) endlessly */
}
#else
/* single threaded version */
(void) threadEntry(NULL);
#endif
/* not reached */
abort();
return 0;
}
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