libspl/backtrace: dump registers in libunwind backtraces

lib/libspl/backtrace.c

@@ -25,30 +25,43 @@
 
 #include <sys/backtrace.h>
 #include <sys/types.h>
+#include <sys/debug.h>
 #include <unistd.h>
 
 /*
- * libspl_backtrace() must be safe to call from inside a signal hander. This
- * mostly means it must not allocate, and so we can't use things like printf.
+ * Output helpers. libspl_backtrace() must not block, must be thread-safe and
+ * must be safe to call from a signal handler. At least, that means not having
+ * printf, so we end up having to call write() directly on the fd. That's
+ * awkward, as we always have to pass through a length, and some systems will
+ * complain if we don't consume the return. So we have some macros to make
+ * things a little more palatable.
  */
+#define	spl_bt_write_n(fd, s, n) \
+	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))
 
 #if defined(HAVE_LIBUNWIND)
 #define	UNW_LOCAL_ONLY
 #include <libunwind.h>
 
+/*
+ * Convert `v` to ASCII hex characters. The bottom `n` nybbles (4-bits ie one
+ * hex digit) will be written, up to `buflen`. The buffer will not be
+ * null-terminated. Returns the number of digits written.
+ */
 static size_t
-libspl_u64_to_hex_str(uint64_t v, size_t digits, char *buf, size_t buflen)
+spl_bt_u64_to_hex_str(uint64_t v, size_t n, char *buf, size_t buflen)
 {
 	static const char hexdigits[] = {
 	    '0', '1', '2', '3', '4', '5', '6', '7',
 	    '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'
 	};
 
 	size_t pos = 0;
-	boolean_t want = (digits == 0);
+	boolean_t want = (n == 0);
 	for (int i = 15; i >= 0; i--) {
 		const uint64_t d = v >> (i * 4) & 0xf;
-		if (!want && (d != 0 || digits > i))
+		if (!want && (d != 0 || n > i))
 			want = B_TRUE;
 		if (want) {
 			buf[pos++] = hexdigits[d];
@@ -62,56 +75,194 @@ libspl_u64_to_hex_str(uint64_t v, size_t digits, char *buf, size_t buflen)
 void
 libspl_backtrace(int fd)
 {
-	ssize_t ret __attribute__((unused));
 	unw_context_t uc;
 	unw_cursor_t cp;
-	unw_word_t loc;
+	unw_word_t v;
 	char buf[128];
 	size_t n;
+	int err;
 
-	ret = write(fd, "Call trace:\n", 12);
+	/* Snapshot the current frame and state. */
 	unw_getcontext(&uc);
+
+	/*
+	 * TODO: walk back to the frame that tripped the assertion / the place
+	 *       where the signal was recieved.
+	 */
+
+	/*
+	 * Register dump. We're going to loop over all the registers in the
+	 * top frame, and show them, with names, in a nice three-column
+	 * layout, which keeps us within 80 columns.
+	 */
+	spl_bt_write(fd, "Registers:\n");
+
+	/* Initialise a frame cursor, starting at the current frame */
 	unw_init_local(&cp, &uc);
-	while (unw_step(&cp) > 0) {
-		unw_get_reg(&cp, UNW_REG_IP, &loc);
-		ret = write(fd, "  [0x", 5);
-		n = libspl_u64_to_hex_str(loc, 10, buf, sizeof (buf));
-		ret = write(fd, buf, n);
-		ret = write(fd, "] ", 2);
-		unw_get_proc_name(&cp, buf, sizeof (buf), &loc);
-		for (n = 0; n < sizeof (buf) && buf[n] != '\0'; n++) {}
-		ret = write(fd, buf, n);
-		ret = write(fd, "+0x", 3);
-		n = libspl_u64_to_hex_str(loc, 2, buf, sizeof (buf));
-		ret = write(fd, buf, n);
+
+	/*
+	 * libunwind's list of possible registers for this architecture is an
+	 * enum, unw_regnum_t. UNW_TDEP_LAST_REG is the highest-numbered
+	 * register in that list, however, not all register numbers in this
+	 * range are defined by the architecture, and not all defined registers
+	 * will be present on every implementation of that architecture.
+	 * Moreover, libunwind provides nice names for most, but not all
+	 * registers, but these are hardcoded; a name being available does not
+	 * 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
+	 * register must exist, and so we get its name. If libunwind has no
+	 * name for it, we synthesize something. These cases should be rare,
+	 * and they're usually for uninteresting or niche registers, so it
+	 * shouldn't really matter. We can see the value, and that's the main
+	 * thing.
+	 */
+	uint_t cols = 0;
+	for (uint_t regnum = 0; regnum <= UNW_TDEP_LAST_REG; regnum++) {
+		/*
+		 * Get the value. Any error probably means the register
+		 * doesn't exist, and we skip it.
+		 */
+		if (unw_get_reg(&cp, regnum, &v) < 0)
+			continue;
+
+		/*
+		 * Register name. If libunwind doesn't have a name for it,
+		 * it will return "???". As a shortcut, we just treat '?'
+		 * is an alternate end-of-string character.
+		 */
+		const char *name = unw_regname(regnum);
+		for (n = 0; name[n] != '\0' && name[n] != '?'; n++) {}
+		if (n == 0) {
+			/*
+			 * No valid name, so make one of the form "?xx", where
+			 * "xx" is the two-char hex of libunwind's register
+			 * number.
+			 */
+			buf[0] = '?';
+			n = spl_bt_u64_to_hex_str(regnum, 2,
+			    &buf[1], sizeof (buf)-1) + 1;
+			name = buf;
+		}
+
+		/*
+		 * Two spaces of padding before each column, plus extra
+		 * spaces to align register names shorter than three chars.
+		 */
+		spl_bt_write_n(fd, "      ", 5-MIN(n, 3));
+
+		/* Register name and column punctuation */
+		spl_bt_write_n(fd, name, n);
+		spl_bt_write(fd, ": 0x");
+
+		/*
+		 * Convert register value (from unw_get_reg()) to hex. We're
+		 * assuming that all registers are 64-bits wide, which is
+		 * probably fine for any general-purpose registers on any
+		 * machine currently in use. A more generic way would be to
+		 * look at the width of unw_word_t, but that would also
+		 * complicate the column code a bit. This is fine.
+		 */
+		n = spl_bt_u64_to_hex_str(v, 16, buf, sizeof (buf));
+		spl_bt_write_n(fd, buf, n);
+
+		/* Every third column, emit a newline */
+		if (!(++cols % 3))
+			spl_bt_write(fd, "\n");
+	}
+
+	/* If we finished before the third column, emit a newline. */
+	if (cols % 3)
+		spl_bt_write(fd, "\n");
+
+	/* Now the main event, the backtrace. */
+	spl_bt_write(fd, "Call trace:\n");
+
+	/* Reset the cursor to the top again. */
+	unw_init_local(&cp, &uc);
+
+	do {
+		/*
+		 * Getting the IP should never fail; libunwind handles it
+		 * specially, because its used a lot internally. Still, no
+		 * point being silly about it, as the last thing we want is
+		 * our crash handler to crash. So if it ever does fail, we'll
+		 * show an error line, but keep going to the next frame.
+		 */
+		if (unw_get_reg(&cp, UNW_REG_IP, &v) < 0) {
+			spl_bt_write(fd, "  [couldn't get IP register; "
+			    "corrupt frame?]");
+			continue;
+		}
+
+		/* IP & punctuation */
+		n = spl_bt_u64_to_hex_str(v, 16, buf, sizeof (buf));
+		spl_bt_write(fd, "  [0x");
+		spl_bt_write_n(fd, buf, n);
+		spl_bt_write(fd, "] ");
+
+		/*
+		 * Function ("procedure") name for the current frame. `v`
+		 * receives the offset from the named function to the IP, which
+		 * we show as a "+offset" suffix.
+		 *
+		 * If libunwind can't determine the name, we just show "???"
+		 * instead. We've already displayed the IP above; that will
+		 * have to do.
+		 *
+		 * unw_get_proc_name() will return ENOMEM if the buffer is too
+		 * small, instead truncating the name. So we treat that as a
+		 * success and use whatever is in the buffer.
+		 */
+		err = unw_get_proc_name(&cp, buf, sizeof (buf), &v);
+		if (err == 0 || err == -UNW_ENOMEM) {
+			for (n = 0; n < sizeof (buf) && buf[n] != '\0'; n++) {}
+			spl_bt_write_n(fd, buf, n);
+
+			/* Offset from proc name */
+			spl_bt_write(fd, "+0x");
+			n = spl_bt_u64_to_hex_str(v, 2, buf, sizeof (buf));
+			spl_bt_write_n(fd, buf, n);
+		} else
+			spl_bt_write(fd, "???");
+
 #ifdef HAVE_LIBUNWIND_ELF
-		ret = write(fd, " (in ", 5);
-		unw_get_elf_filename(&cp, buf, sizeof (buf), &loc);
-		for (n = 0; n < sizeof (buf) && buf[n] != '\0'; n++) {}
-		ret = write(fd, buf, n);
-		ret = write(fd, " +0x", 4);
-		n = libspl_u64_to_hex_str(loc, 2, buf, sizeof (buf));
-		ret = write(fd, buf, n);
-		ret = write(fd, ")", 1);
+		/*
+		 * Newer libunwind has unw_get_elf_filename(), which gets
+		 * the name of the ELF object that the frame was executing in.
+		 * Like `unw_get_proc_name()`, `v` recieves the offset within
+		 * the file, and UNW_ENOMEM indicates that a truncate filename
+		 * was left in the buffer.
+		 */
+		err = unw_get_elf_filename(&cp, buf, sizeof (buf), &v);
+		if (err == 0 || err == -UNW_ENOMEM) {
+			for (n = 0; n < sizeof (buf) && buf[n] != '\0'; n++) {}
+			spl_bt_write(fd, " (in ");
+			spl_bt_write_n(fd, buf, n);
+
+			/* Offset within file */
+			spl_bt_write(fd, " +0x");
+			n = spl_bt_u64_to_hex_str(v, 2, buf, sizeof (buf));
+			spl_bt_write_n(fd, buf, n);
+			spl_bt_write(fd, ")");
+		}
 #endif
-		ret = write(fd, "\n", 1);
-	}
+		spl_bt_write(fd, "\n");
+	} while (unw_step(&cp) > 0);
 }
 #elif defined(HAVE_BACKTRACE)
 #include <execinfo.h>
 
 void
 libspl_backtrace(int fd)
 {
-	ssize_t ret __attribute__((unused));
 	void *btptrs[64];
 	size_t nptrs = backtrace(btptrs, 64);
-	ret = write(fd, "Call trace:\n", 12);
+	spl_bt_write(fd, "Call trace:\n");
 	backtrace_symbols_fd(btptrs, nptrs, fd);
 }
 #else
-#include <sys/debug.h>
-
 void
 libspl_backtrace(int fd __maybe_unused)
 {