Support using llvm-libunwind

This commit adds support for using llvm-libunwind for kernels built
using llvm and clang. The two differences are that the largest register
index is given by _LIBUNWIND_HIGHEST_DWARF_REGISTER, we need to check
whether the register is a floating point register and the prototype
for unw_regname takes the unwind cursor as the first argument.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Sebastian Pauka <me@spauka.se>
Closes #17230
(cherry picked from commit 1b4826b9a2)

config/user-libunwind.m4

@@ -34,6 +34,22 @@ AC_DEFUN([ZFS_AC_CONFIG_USER_LIBUNWIND], [
 			], [
 				AC_MSG_RESULT([no])
 			])
+			dnl LLVM includes it's own libunwind library, which
+			dnl defines the highest numbered register in a different
+			dnl way, and has an incompatible unw_resname function.
+			AC_MSG_CHECKING([whether libunwind is llvm libunwind])
+			AC_COMPILE_IFELSE([
+				AC_LANG_PROGRAM([
+					#include <libunwind.h>
+					#if !defined(_LIBUNWIND_HIGHEST_DWARF_REGISTER)
+					#error "_LIBUNWIND_HIGHEST_DWARF_REGISTER is not defined"
+					#endif
+				], [])], [
+					AC_MSG_RESULT([yes])
+					AC_DEFINE(IS_LIBUNWIND_LLVM, 1, [libunwind is llvm libunwind])
+				], [
+					AC_MSG_RESULT([no])
+			])
 			AX_RESTORE_FLAGS
 		], [
 			AS_IF([test "x$with_libunwind" = "xyes"], [

lib/libspl/backtrace.c

@@ -41,9 +41,26 @@
 	do { ssize_t r __maybe_unused = write(fd, s, n); } while (0)
 #define	spl_bt_write(fd, s)		spl_bt_write_n(fd, s, sizeof (s)-1)
 
-#if defined(HAVE_LIBUNWIND)
+#ifdef HAVE_LIBUNWIND
+/*
+ * libunwind-gcc and libunwind-llvm both list registers using an enum,
+ * unw_regnum_t, however they indicate the highest numbered register for
+ * a given architecture in different ways. We can check which one is defined
+ * and mark which libunwind is in use
+ */
+#ifdef IS_LIBUNWIND_LLVM
+#include <libunwind.h>
+#define	LAST_REG_INDEX _LIBUNWIND_HIGHEST_DWARF_REGISTER
+#else
+/*
+ * Need to define UNW_LOCAL_ONLY before importing libunwind.h
+ * if using libgcc libunwind.
+ */
 #define	UNW_LOCAL_ONLY
 #include <libunwind.h>
+#define	LAST_REG_INDEX UNW_TDEP_LAST_REG
+#endif
+
 
 /*
  * Convert `v` to ASCII hex characters. The bottom `n` nybbles (4-bits ie one
@@ -102,14 +119,13 @@ libspl_backtrace(int fd)
 	unw_init_local(&cp, &uc);
 
 	/*
-	 * libunwind's list of possible registers for this architecture is an
+	 * Iterate over all registers for the architecture. We've figured
-	 * enum, unw_regnum_t. UNW_TDEP_LAST_REG is the highest-numbered
+	 * out the highest number above, however, not all register numbers in
-	 * register in that list, however, not all register numbers in this
+	 * this range are defined by the architecture, and not all defined
-	 * range are defined by the architecture, and not all defined registers
+	 * registers will be present on every implementation of that
-	 * will be present on every implementation of that architecture.
+	 * architecture. Moreover, libunwind provides nice names for most, but
-	 * Moreover, libunwind provides nice names for most, but not all
+	 * not all registers, but these are hardcoded; a name being available
-	 * registers, but these are hardcoded; a name being available does not
+	 * does not mean that register is available.
-	 * mean that register is available.
 	 *
 	 * So, we have to pull this all together here. We try to get the value
 	 * of every possible register. If we get a value for it, then the
@@ -120,26 +136,42 @@ libspl_backtrace(int fd)
 	 * thing.
 	 */
 	uint_t cols = 0;
-	for (uint_t regnum = 0; regnum <= UNW_TDEP_LAST_REG; regnum++) {
+	for (uint_t regnum = 0; regnum <= LAST_REG_INDEX; regnum++) {
 		/*
 		 * Get the value. Any error probably means the register
-		 * doesn't exist, and we skip it.
+		 * doesn't exist, and we skip it. LLVM libunwind iterates over
+		 * fp registers in the same list, however they have to be
+		 * accessed using unw_get_fpreg instead. Here, we just ignore
+		 * them.
 		 */
+#ifdef IS_LIBUNWIND_LLVM
+		if (unw_is_fpreg(&cp, regnum) ||
+		    unw_get_reg(&cp, regnum, &v) < 0)
+			continue;
+#else
 		if (unw_get_reg(&cp, regnum, &v) < 0)
 			continue;
+#endif
 
 		/*
-		 * Register name. If libunwind doesn't have a name for it,
+		 * Register name. If GCC libunwind doesn't have a name for it,
 		 * it will return "???". As a shortcut, we just treat '?'
-		 * is an alternate end-of-string character.
+		 * is an alternate end-of-string character. LLVM libunwind will
+		 * return the string 'unknown register', which we detect by
+		 * checking if the register name is longer than 5 characters.
 		 */
+#ifdef IS_LIBUNWIND_LLVM
+		const char *name = unw_regname(&cp, regnum);
+#else
 		const char *name = unw_regname(regnum);
+#endif
 		for (n = 0; name[n] != '\0' && name[n] != '?'; n++) {}
-		if (n == 0) {
+		if (n == 0 || n > 5) {
 			/*
-			 * No valid name, so make one of the form "?xx", where
+			 * No valid name, or likely llvm_libunwind returned
-			 * "xx" is the two-char hex of libunwind's register
+			 * unknown_register, so make one of the form "?xx",
-			 * number.
+			 * where "xx" is the two-char hex of libunwind's
+			 * register number.
 			 */
 			buf[0] = '?';
 			n = spl_bt_u64_to_hex_str(regnum, 2,