Getting the saved instruction pointer address from a signal handler

Question

My question is somewhat different from others that have asked about fault addresses. I'm trying to implement a horrible hack to determine, from a signal handler, whether the signal interrupted a syscall or ordinary user code by inspecting the code at the saved instruction pointer and comparing it against the possible syscall entry instructions for the host architecture it's running on. This is part of implementing correct POSIX thread cancellation that does not suffer from the race condition and resource leak described in my old question:

How are POSIX cancellation points supposed to behave?

If this approach is unreliable or otherwise wrong, I'd also like to hear reasons.

Avitzur

See also https://mcmap.net/q/455087/-how-to-determine-if-code-is-running-in-signal-handler-context/371250 – Cusick

And the comment here lxr.free-electrons.com/source/arch/x86/kernel/… – Cusick

I don't need to determine if I'm in a signal handler; this code will only be run from a signal handler. What I need to determine is the address of the next instruction that will be executed when the signal handler returns. I agree it's an ugly hack, but I can't find any other way to perform a cancellation at a syscall without a race condition where a resource allocated by the syscall might be leaked. – Avitzur

I think I just need to be able to inspect the struct sigcontext structure, but I don't know how to access it from the signal handler... – Avitzur

Looks like I just have to use the third void * argument to the SA_SIGINFO handler and cast it to ucontext_t *, then poke around in there... – Avitzur

Answer 1

/* sigsegv.c */
/**
 * This source file is used to print out a stack-trace when your program
 * segfaults. It is relatively reliable and spot-on accurate.
 *
 * This code is in the public domain. Use it as you see fit, some credit
 * would be appreciated, but is not a prerequisite for usage. Feedback
 * on it's use would encourage further development and maintenance.
 *
 * Due to a bug in gcc-4.x.x you currently have to compile as C++ if you want
 * demangling to work.
 *
 * Please note that it's been ported into my ULS library, thus the check for
 * HAS_ULSLIB and the use of the sigsegv_outp macro based on that define.
 *
 * Author: Jaco Kroon <[email protected]>
 *
 * Copyright (C) 2005 - 2010 Jaco Kroon
 */
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif

/* Bug in gcc prevents from using CPP_DEMANGLE in pure "C" */
#if !defined(__cplusplus) && !defined(NO_CPP_DEMANGLE)
#define NO_CPP_DEMANGLE
#endif

#include <memory.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <ucontext.h>
#include <dlfcn.h>
#ifndef NO_CPP_DEMANGLE
#include <cxxabi.h>
#ifdef __cplusplus
using __cxxabiv1::__cxa_demangle;
#endif
#endif

#ifdef HAS_ULSLIB
#include "uls/logger.h"
#define sigsegv_outp(x)         sigsegv_outp(,gx)
#else
#define sigsegv_outp(x, ...)    fprintf(stderr, x "\n", ##__VA_ARGS__)
#endif

#if defined(REG_RIP)
# define SIGSEGV_STACK_IA64
# define REGFORMAT "%016lx"
#elif defined(REG_EIP)
# define SIGSEGV_STACK_X86
# define REGFORMAT "%08x"
#else
# define SIGSEGV_STACK_GENERIC
# define REGFORMAT "%x"
#endif

static void signal_segv(int signum, siginfo_t* info, void*ptr) {
    static const char *si_codes[3] = {"", "SEGV_MAPERR", "SEGV_ACCERR"};

    int i, f = 0;
    ucontext_t *ucontext = (ucontext_t*)ptr;
    Dl_info dlinfo;
    void **bp = 0;
    void *ip = 0;

    sigsegv_outp("Segmentation Fault!");
    sigsegv_outp("info.si_signo = %d", signum);
    sigsegv_outp("info.si_errno = %d", info->si_errno);
    sigsegv_outp("info.si_code  = %d (%s)", info->si_code, si_codes[info->si_code]);
    sigsegv_outp("info.si_addr  = %p", info->si_addr);
    for(i = 0; i < NGREG; i++)
        sigsegv_outp("reg[%02d]       = 0x" REGFORMAT, i, ucontext->uc_mcontext.gregs[i]);

#ifndef SIGSEGV_NOSTACK
#if defined(SIGSEGV_STACK_IA64) || defined(SIGSEGV_STACK_X86)
#if defined(SIGSEGV_STACK_IA64)
    ip = (void*)ucontext->uc_mcontext.gregs[REG_RIP];
    bp = (void**)ucontext->uc_mcontext.gregs[REG_RBP];
#elif defined(SIGSEGV_STACK_X86)
    ip = (void*)ucontext->uc_mcontext.gregs[REG_EIP];
    bp = (void**)ucontext->uc_mcontext.gregs[REG_EBP];
#endif

    sigsegv_outp("Stack trace:");
    while(bp && ip) {
        if(!dladdr(ip, &dlinfo))
            break;

        const char *symname = dlinfo.dli_sname;

#ifndef NO_CPP_DEMANGLE
        int status;
        char * tmp = __cxa_demangle(symname, NULL, 0, &status);

        if (status == 0 && tmp)
            symname = tmp;
#endif

        sigsegv_outp("% 2d: %p <%s+%lu> (%s)",
                 ++f,
                 ip,
                 symname,
                 (unsigned long)ip - (unsigned long)dlinfo.dli_saddr,
                 dlinfo.dli_fname);

#ifndef NO_CPP_DEMANGLE
        if (tmp)
            free(tmp);
#endif

        if(dlinfo.dli_sname && !strcmp(dlinfo.dli_sname, "main"))
            break;

        ip = bp[1];
        bp = (void**)bp[0];
    }
#else
    sigsegv_outp("Stack trace (non-dedicated):");
    sz = backtrace(bt, 20);
    strings = backtrace_symbols(bt, sz);
    for(i = 0; i < sz; ++i)
        sigsegv_outp("%s", strings[i]);
#endif
    sigsegv_outp("End of stack trace.");
#else
    sigsegv_outp("Not printing stack strace.");
#endif
    _exit (-1);
}

static void __attribute__((constructor)) setup_sigsegv() {
    struct sigaction action;
    memset(&action, 0, sizeof(action));
    action.sa_sigaction = signal_segv;
    action.sa_flags = SA_SIGINFO;
    if(sigaction(SIGSEGV, &action, NULL) < 0)
        perror("sigaction");
}

$ g++ -fPIC -shared -o libsigsegv.so -ldl sigsegv

$ export LD_PRELOAD=/path/to/libsigsegv.so

I found this code on a LUG. Couldn't get to the page to point the URL here, so pasted the whole code. This code prints a small stack trace when SIGSEGV occurs. Not sure if there is some other way that does not use ucontext_t.

Steenbok

+1 and accepted; this code shows how to use the third argument to the SA_SIGINFO signal handler to get at the necessary information. – Avitzur