libspl/assert: dump backtrace in assert

Adds a check for the backtrace() function. If available, uses it to show
a stack backtrace in the assertion output.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Sponsored-by: https://despairlabs.com/sponsor/
Closes openzfs#16140

Author: robn

config/user-backtrace.m4

@@ -0,0 +1,14 @@
+dnl
+dnl backtrace(), for userspace assertions. glibc has this directly in libc.
+dnl FreeBSD and (sometimes) musl have it in a separate -lexecinfo. It's assumed
+dnl that this will also get the companion function backtrace_symbols().
+dnl
+AC_DEFUN([ZFS_AC_CONFIG_USER_BACKTRACE], [
+	AX_SAVE_FLAGS
+	LIBS=""
+	AC_SEARCH_LIBS([backtrace], [execinfo], [
+		AC_DEFINE(HAVE_BACKTRACE, 1, [backtrace() is available])
+		AC_SUBST([BACKTRACE_LIBS], ["$LIBS"])
+	])
+	AX_RESTORE_FLAGS
+])

config/user.m4

@@ -26,6 +26,7 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_AIO_H
 	ZFS_AC_CONFIG_USER_CLOCK_GETTIME
 	ZFS_AC_CONFIG_USER_PAM
+	ZFS_AC_CONFIG_USER_BACKTRACE
 	ZFS_AC_CONFIG_USER_RUNSTATEDIR
 	ZFS_AC_CONFIG_USER_MAKEDEV_IN_SYSMACROS
 	ZFS_AC_CONFIG_USER_MAKEDEV_IN_MKDEV

lib/libspl/Makefile.am

@@ -43,3 +43,5 @@ libspl_la_LIBADD = \
 	libspl_assert.la
 
 libspl_la_LIBADD += $(LIBATOMIC_LIBS) $(LIBCLOCK_GETTIME)
+
+libspl_assert_la_LIBADD = $(BACKTRACE_LIBS)

lib/libspl/assert.c

@@ -49,6 +49,24 @@
 	pthread_getname_np(pthread_self(), buf, len);
 #endif
 
+#if defined(HAVE_BACKTRACE)
+#include <execinfo.h>
+
+static inline void
+libspl_dump_backtrace(void)
+{
+	void *btptrs[100];
+	size_t nptrs = backtrace(btptrs, 100);
+	char **bt = backtrace_symbols(btptrs, nptrs);
+	fprintf(stderr, "Call trace:\n");
+	for (size_t i = 0; i < nptrs; i++)
+		fprintf(stderr, "  %s\n", bt[i]);
+	free(bt);
+}
+#else
+#define	libspl_dump_backtrace()
+#endif
+
 static boolean_t libspl_assert_ok = B_FALSE;
 
 void
@@ -83,6 +101,8 @@ libspl_assertf(const char *file, const char *func, int line,
 	    getpid(), libspl_getprogname(),
 	    libspl_gettid(), tname);
 
+	libspl_dump_backtrace();
+
 #if !__has_feature(attribute_analyzer_noreturn) && !defined(__COVERITY__)
 	if (libspl_assert_ok) {
 		pthread_mutex_unlock(&assert_lock);