init: add a segv debugging aid, disabled by default

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

1 files changed, 51 insertions, 0 deletions

diff --git a/init/init.c b/init/init.c
index c540faa70..724894698 100644
--- 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