libspl/assert: use libunwind for backtrace when available

libunwind seems to do a better job of resolving a symbols than
backtrace(), and is also useful on platforms that don't have backtrace()
(eg musl). If it's available, use it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16140

config/user-libunwind.m4

@@ -0,0 +1,44 @@
+dnl
+dnl Checks for libunwind, which usually does a better job than backtrace() when
+dnl resolving symbols in the stack backtrace. Newer versions have support for
+dnl getting info about the object file the function came from, so we look for
+dnl that too and use it if found.
+dnl
+AC_DEFUN([ZFS_AC_CONFIG_USER_LIBUNWIND], [
+	AC_ARG_WITH([libunwind],
+	    AS_HELP_STRING([--with-libunwind],
+		[use libunwind for backtraces in userspace assertions]),
+	    [],
+	    [with_libunwind=auto])
+
+	AS_IF([test "x$with_libunwind" != "xno"], [
+		ZFS_AC_FIND_SYSTEM_LIBRARY(LIBUNWIND, [libunwind], [libunwind.h], [], [unwind], [], [
+			dnl unw_get_elf_filename() is sometimes a macro, other
+			dnl times a proper symbol, so we can't just do a link
+			dnl check; we need to include the header properly.
+			AX_SAVE_FLAGS
+			CFLAGS="$CFLAGS $LIBUNWIND_CFLAGS"
+			LIBS="$LIBS $LIBUNWIND_LIBS"
+			AC_MSG_CHECKING([for unw_get_elf_filename in libunwind])
+			AC_LINK_IFELSE([
+				AC_LANG_PROGRAM([
+					#define UNW_LOCAL_ONLY
+					#include <libunwind.h>
+				], [
+					unw_get_elf_filename(0, 0, 0, 0);
+				])
+			], [
+				AC_MSG_RESULT([yes])
+				AC_DEFINE(HAVE_LIBUNWIND_ELF, 1,
+				    [libunwind has unw_get_elf_filename])
+			], [
+				AC_MSG_RESULT([no])
+			])
+			AX_RESTORE_FLAGS
+		], [
+			AS_IF([test "x$with_libunwind" = "xyes"], [
+				AC_MSG_FAILURE([--with-libunwind was given, but libunwind is not available, try installing libunwind-devel])
+			])
+		])
+	])
+])

config/user.m4

@@ -27,6 +27,7 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_CLOCK_GETTIME
 	ZFS_AC_CONFIG_USER_PAM
 	ZFS_AC_CONFIG_USER_BACKTRACE
+	ZFS_AC_CONFIG_USER_LIBUNWIND
 	ZFS_AC_CONFIG_USER_RUNSTATEDIR
 	ZFS_AC_CONFIG_USER_MAKEDEV_IN_SYSMACROS
 	ZFS_AC_CONFIG_USER_MAKEDEV_IN_MKDEV

lib/libspl/Makefile.am

@@ -1,6 +1,6 @@
 include $(srcdir)/%D%/include/Makefile.am
 
-libspl_assert_la_CFLAGS = $(AM_CFLAGS) $(LIBRARY_CFLAGS)
+libspl_assert_la_CFLAGS = $(AM_CFLAGS) $(LIBRARY_CFLAGS) $(LIBUNWIND_CFLAGS)
 libspl_la_CFLAGS        = $(libspl_assert_la_CFLAGS)
 
 noinst_LTLIBRARIES += libspl_assert.la libspl.la
@@ -44,4 +44,4 @@ libspl_la_LIBADD = \
 
 libspl_la_LIBADD += $(LIBATOMIC_LIBS) $(LIBCLOCK_GETTIME)
 
-libspl_assert_la_LIBADD = $(BACKTRACE_LIBS)
+libspl_assert_la_LIBADD = $(BACKTRACE_LIBS) $(LIBUNWIND_LIBS)

lib/libspl/assert.c

@@ -49,7 +49,38 @@
 	pthread_getname_np(pthread_self(), buf, len);
 #endif
 
-#if defined(HAVE_BACKTRACE)
+#if defined(HAVE_LIBUNWIND)
+#define	UNW_LOCAL_ONLY
+#include <libunwind.h>
+
+static inline void
+libspl_dump_backtrace(void)
+{
+	unw_context_t uc;
+	unw_cursor_t cp;
+	unw_word_t ip, off;
+	char funcname[128];
+#ifdef HAVE_LIBUNWIND_ELF
+	char objname[128];
+	unw_word_t objoff;
+#endif
+
+	fprintf(stderr, "Call trace:\n");
+	unw_getcontext(&uc);
+	unw_init_local(&cp, &uc);
+	while (unw_step(&cp) > 0) {
+		unw_get_reg(&cp, UNW_REG_IP, &ip);
+		unw_get_proc_name(&cp, funcname, sizeof (funcname), &off);
+#ifdef HAVE_LIBUNWIND_ELF
+		unw_get_elf_filename(&cp, objname, sizeof (objname), &objoff);
+		fprintf(stderr, "  [0x%08lx] %s+0x%2lx (in %s +0x%2lx)\n",
+		    ip, funcname, off, objname, objoff);
+#else
+		fprintf(stderr, "  [0x%08lx] %s+0x%2lx\n", ip, funcname, off);
+#endif
+	}
+}
+#elif defined(HAVE_BACKTRACE)
 #include <execinfo.h>
 
 static inline void