init: add a segv debugging aid, disabled by default

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/init/init.c b/init/init.c
index c540faa707941660ffbe6b646f9e8e4a9757b3a5..7248946988333a8c543a942b6bda9abbac3a1f4a 100644 (file)
--- a/init/init.c
+++ b/init/init.c
@@ -108,6 +108,8 @@
 //config:        Note that on Linux, init attempts to detect serial terminal and
 //config:        sets TERM to "vt102" if one is found.
 
+#define DEBUG_SEGV_HANDLER 0
+
 #include "libbb.h"
 #include <syslog.h>
 #include <paths.h>
@@ -118,6 +120,15 @@
 #endif
 #include "reboot.h" /* reboot() constants */
 
+#if DEBUG_SEGV_HANDLER
+# undef _GNU_SOURCE
+# define _GNU_SOURCE 1
+# undef __USE_GNU
+# define __USE_GNU 1
+# include <execinfo.h>
+# include <sys/ucontext.h>
+#endif
+
 /* Used only for sanitizing purposes in set_sane_term() below. On systems where
  * the baud rate is stored in a separate field, we can safely disable them. */
 #ifndef CBAUD
@@ -957,6 +968,33 @@ static int check_delayed_sigs(void)
        }
 }
 
+#if DEBUG_SEGV_HANDLER
+static
+void handle_sigsegv(int sig, siginfo_t *info, void *ucontext)
+{
+       long ip;
+       ucontext_t *uc;
+
+       uc = ucontext;
+       ip = uc->uc_mcontext.gregs[REG_EIP];
+       fdprintf(2, "signal:%d address:0x%lx ip:0x%lx\n",
+                       sig,
+                       /* this is void*, but using %p would print "(null)"
+                        * even for ptrs which are not exactly 0, but, say, 0x123:
+                        */
+                       (long)info->si_addr,
+                       ip);
+       {
+               /* glibc extension */
+               void *array[50];
+               int size;
+               size = backtrace(array, 50);
+               backtrace_symbols_fd(array, size, 2);
+       }
+       for (;;) sleep(9999);
+}
+#endif
+
 int init_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int init_main(int argc UNUSED_PARAM, char **argv)
 {
@@ -964,6 +1002,19 @@ int init_main(int argc UNUSED_PARAM, char **argv)
                return kill(1, SIGHUP);
        }
 
+#if DEBUG_SEGV_HANDLER
+       {
+               struct sigaction sa;
+               memset(&sa, 0, sizeof(sa));
+               sa.sa_sigaction = handle_sigsegv;
+               sa.sa_flags = SA_SIGINFO;
+               sigaction(SIGSEGV, &sa, NULL);
+               sigaction(SIGILL, &sa, NULL);
+               sigaction(SIGFPE, &sa, NULL);
+               sigaction(SIGBUS, &sa, NULL);
+       }
+#endif
+
        if (!DEBUG_INIT) {
                /* Expect to be invoked as init with PID=1 or be invoked as linuxrc */
                if (getpid() != 1