gistfile1.txt

linux:
             NULL pointer: si_signo=11, si_status=0, si_code=1, si_pid=0, si_uid=0, si_addr=0x0
         Unmapped address: si_signo=11, si_status=0, si_code=1, si_pid=131072, si_uid=0, si_addr=0x20000
         Readonly address: si_signo=11, si_status=0, si_code=2, si_pid=1896829136, si_uid=21917, si_addr=0x559d710f50d0
    Non-canonical address: si_signo=11, si_status=0, si_code=128, si_pid=0, si_uid=0, si_addr=0x0
   kill(getpid(),SIGSEGV): si_signo=11, si_status=0, si_code=0, si_pid=900131, si_uid=1000, si_addr=0x3e8000dbc23
          subprocess kill: si_signo=11, si_status=0, si_code=0, si_pid=900132, si_uid=1000, si_addr=0x3e8000dbc24

(on linux, si_pid/si_uid overlap si_addr, so you must use si_code to determine which is valid)



macos:
             NULL pointer: si_signo=11, si_status=0, si_code=1, si_pid=0, si_uid=0, si_addr=0x0
         Unmapped address: si_signo=11, si_status=0, si_code=1, si_pid=0, si_uid=0, si_addr=0x20000
         Readonly address: si_signo=10, si_status=0, si_code=2, si_pid=0, si_uid=0, si_addr=0x1071bfc80
    Non-canonical address: si_signo=11, si_status=0, si_code=0, si_pid=0, si_uid=0, si_addr=0x0
   kill(getpid(),SIGSEGV): si_signo=11, si_status=0, si_code=0, si_pid=0, si_uid=0, si_addr=0x0
          subprocess kill: si_signo=11, si_status=0, si_code=0, si_pid=0, si_uid=0, si_addr=0x0

(note that kill() and noncanonical addresses general indistinguishable results)

sigdump.c

// unusually, this program doesn't work *unless* compiled with -O2

#define _GNU_SOURCE
#include <stddef.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/wait.h>

#if defined(__x86_64__) && defined(__linux__)
#define PC(ctx) \
  (((ucontext_t*)(ctx))->uc_mcontext.gregs[REG_RIP])
#else
  #include <sys/ucontext.h>
  #include <AvailabilityMacros.h>

  #define CONTEXT_STATE(context) \
    (((ucontext_t *)(context))->uc_mcontext->__ss)
  #define PC(ctx) \
     (CONTEXT_STATE(ctx).__rip)
//#error "unknown PC reg"
#endif

const char* msg;
siginfo_t infos[100];
const char* msgs[100];
int ninfos = 0;
void* volatile segfault_before;
void* volatile segfault_after;

static void handler(int sig, siginfo_t* info, void* uctx) {
  msgs[ninfos] = msg;
  infos[ninfos] = *info;
  ninfos++;
  if (PC(uctx) == (long long int)segfault_before)
    PC(uctx) = (long long int)segfault_after;
}

__attribute__((noinline,optimize("O2"))) void deref(char* ptr) {
  segfault_before = &&before;
  segfault_after = &&after;
  goto *segfault_before;
 before:
  *ptr = 1;
 after:
  return;
}

int main() {
  struct sigaction act;
  int i;
  act.sa_sigaction = &handler;
  sigemptyset(&act.sa_mask);
  act.sa_flags = SA_SIGINFO;
  sigaction(SIGSEGV, &act, NULL);

  msg = "NULL pointer";
  deref(NULL);
  msg = "Unmapped address";
  deref((void*)0x20000);
  msg = "Readonly address";
  deref((void*)&main);
  msg = "Non-canonical address";
  deref((void*)0x1010101010101010);
  msg = "kill(getpid(),SIGSEGV)";
  kill(getpid(), SIGSEGV);
  msg = "subprocess kill";
  if (fork()==0){kill(getppid(),SIGSEGV);_exit(0);}
  wait(NULL);

  for (i = 0; i<ninfos; i++){
    siginfo_t s = infos[i];
    printf("%25s: si_signo=%d, si_status=%d, si_code=%d, si_pid=%ld, si_uid=%ld, si_addr=0x%lx\n", msgs[i],
           s.si_signo, s.si_status, s.si_code, (long)s.si_pid, (long)s.si_uid, (unsigned long)s.si_addr);
  }
}