objtool: Add more robust signal error handling, detect and warn about stack overflows

When the kernel build fails due to an objtool segfault, the error
message is a bit obtuse and confusing:

  make[5]: *** [scripts/Makefile.build:503: drivers/scsi/qla2xxx/qla2xxx.o] Error 139
                                                                            ^^^^^^^^^
  make[5]: *** Deleting file 'drivers/scsi/qla2xxx/qla2xxx.o'
  make[4]: *** [scripts/Makefile.build:556: drivers/scsi/qla2xxx] Error 2
  make[3]: *** [scripts/Makefile.build:556: drivers/scsi] Error 2
  make[2]: *** [scripts/Makefile.build:556: drivers] Error 2
  make[1]: *** [/home/jpoimboe/git/linux/Makefile:2013: .] Error 2
  make: *** [Makefile:248: __sub-make] Error 2

Add a signal handler to objtool which prints an error message like if
the local stack has overflown (for which there's a chance as objtool
makes heavy use of recursion):

  drivers/scsi/qla2xxx/qla2xxx.o: error: SIGSEGV: objtool stack overflow!

or:

  drivers/scsi/qla2xxx/qla2xxx.o: error: SIGSEGV: objtool crash!

Also, re-raise the signal so the core dump still gets triggered.

[ mingo: Applied a build fix, added more comments and prettified the code. ]

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: David Laight <david.laight.linux@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/mi4tihk4dbncn7belrhp6ooudhpw4vdggerktu5333w3gqf3uf@vqlhc3y667mg

tools/objtool/Build

@@ -18,6 +18,7 @@ objtool-y += libstring.o
 objtool-y += libctype.o
 objtool-y += str_error_r.o
 objtool-y += librbtree.o
+objtool-y += signal.o
 
 $(OUTPUT)libstring.o: ../lib/string.c FORCE
 	$(call rule_mkdir)

tools/objtool/include/objtool/objtool.h

@@ -41,6 +41,8 @@ struct objtool_file {
 
 char *top_level_dir(const char *file);
 
+int init_signal_handler(void);
+
 struct objtool_file *objtool_open_read(const char *_objname);
 
 int objtool_pv_add(struct objtool_file *file, int idx, struct symbol *func);

tools/objtool/objtool.c

@@ -104,11 +104,13 @@ char *top_level_dir(const char *file)
 	return str;
 }
 
-
 int main(int argc, const char **argv)
 {
 	static const char *UNUSED = "OBJTOOL_NOT_IMPLEMENTED";
 
+	if (init_signal_handler())
+		return -1;
+
 	/* libsubcmd init */
 	exec_cmd_init("objtool", UNUSED, UNUSED, UNUSED);
 	pager_init(UNUSED);

tools/objtool/signal.c

@@ -0,0 +1,135 @@
+/*
+ * signal.c: Register a sigaltstack for objtool, to be able to
+ *	     run a signal handler on a separate stack even if
+ *	     the main process stack has overflown. Print out
+ *	     stack overflow errors when this happens.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <unistd.h>
+#include <sys/resource.h>
+#include <string.h>
+
+#include <objtool/objtool.h>
+#include <objtool/warn.h>
+
+static unsigned long stack_limit;
+
+static bool is_stack_overflow(void *fault_addr)
+{
+	unsigned long fault = (unsigned long)fault_addr;
+
+	/* Check if fault is in the guard page just below the limit. */
+	return fault < stack_limit && fault >= stack_limit - 4096;
+}
+
+static void signal_handler(int sig_num, siginfo_t *info, void *context)
+{
+	struct sigaction sa_dfl = {0};
+	const char *sig_name;
+	char msg[256];
+	int msg_len;
+
+	switch (sig_num) {
+	case SIGSEGV:	sig_name = "SIGSEGV";		break;
+	case SIGBUS:	sig_name = "SIGBUS";		break;
+	case SIGILL:	sig_name = "SIGILL";		break;
+	case SIGABRT:	sig_name = "SIGABRT";		break;
+	default:	sig_name = "Unknown signal";	break;
+	}
+
+	if (is_stack_overflow(info->si_addr)) {
+		msg_len = snprintf(msg, sizeof(msg),
+				   "%s: error: %s: objtool stack overflow!\n",
+				   objname, sig_name);
+	} else {
+		msg_len = snprintf(msg, sizeof(msg),
+				   "%s: error: %s: objtool crash!\n",
+				   objname, sig_name);
+	}
+
+	msg_len = write(STDERR_FILENO, msg, msg_len);
+
+	/* Re-raise the signal to trigger the core dump */
+	sa_dfl.sa_handler = SIG_DFL;
+	sigaction(sig_num, &sa_dfl, NULL);
+	raise(sig_num);
+}
+
+static int read_stack_limit(void)
+{
+	unsigned long stack_start, stack_end;
+	struct rlimit rlim;
+	char line[256];
+	int ret = 0;
+	FILE *fp;
+
+	if (getrlimit(RLIMIT_STACK, &rlim)) {
+		ERROR_GLIBC("getrlimit");
+		return -1;
+	}
+
+	fp = fopen("/proc/self/maps", "r");
+	if (!fp) {
+		ERROR_GLIBC("fopen");
+		return -1;
+	}
+
+	while (fgets(line, sizeof(line), fp)) {
+		if (strstr(line, "[stack]")) {
+			if (sscanf(line, "%lx-%lx", &stack_start, &stack_end) != 2) {
+				ERROR_GLIBC("sscanf");
+				ret = -1;
+				goto done;
+			}
+			stack_limit = stack_end - rlim.rlim_cur;
+			goto done;
+		}
+	}
+
+	ret = -1;
+	ERROR("/proc/self/maps: can't find [stack]");
+
+done:
+	fclose(fp);
+
+	return ret;
+}
+
+int init_signal_handler(void)
+{
+	int signals[] = {SIGSEGV, SIGBUS, SIGILL, SIGABRT};
+	struct sigaction sa;
+	stack_t ss;
+
+	if (read_stack_limit())
+		return -1;
+
+	ss.ss_sp = malloc(SIGSTKSZ);
+	if (!ss.ss_sp) {
+		ERROR_GLIBC("malloc");
+		return -1;
+	}
+	ss.ss_size = SIGSTKSZ;
+	ss.ss_flags = 0;
+
+	if (sigaltstack(&ss, NULL) == -1) {
+		ERROR_GLIBC("sigaltstack");
+		return -1;
+	}
+
+	sa.sa_sigaction = signal_handler;
+	sigemptyset(&sa.sa_mask);
+
+	sa.sa_flags = SA_ONSTACK | SA_SIGINFO;
+
+	for (int i = 0; i < ARRAY_SIZE(signals); i++) {
+		if (sigaction(signals[i], &sa, NULL) == -1) {
+			ERROR_GLIBC("sigaction");
+			return -1;
+		}
+	}
+
+	return 0;
+}