backtrace: rework for signal safety

Mostly, try a lot harder to not allocate anything.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #16181

cmd/zdb/zdb.c

@@ -845,7 +845,7 @@ static void sig_handler(int signo)
 {
 	struct sigaction action;
 
-	libspl_backtrace();
+	libspl_backtrace(STDERR_FILENO);
 	dump_debug_buffer();
 
 	/*

cmd/ztest.c

@@ -623,7 +623,7 @@ static void sig_handler(int signo)
 {
 	struct sigaction action;
 
-	libspl_backtrace();
+	libspl_backtrace(STDERR_FILENO);
 	dump_debug_buffer();
 
 	/*

lib/libspl/assert.c

@@ -103,7 +103,7 @@ libspl_assertf(const char *file, const char *func, int line,
 	    getpid(), libspl_getprogname(),
 	    libspl_gettid(), tname);
 
-	libspl_backtrace();
+	libspl_backtrace(STDERR_FILENO);
 
 #if !__has_feature(attribute_analyzer_noreturn) && !defined(__COVERITY__)
 	if (libspl_assert_ok) {

lib/libspl/backtrace.c

@@ -24,57 +24,94 @@
  */
 
 #include <sys/backtrace.h>
-#include <stdio.h>
+#include <sys/types.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.
+ */
 
 #if defined(HAVE_LIBUNWIND)
 #define	UNW_LOCAL_ONLY
 #include <libunwind.h>
 
+static size_t
+libspl_u64_to_hex_str(uint64_t v, size_t digits, 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);
+	for (int i = 15; i >= 0; i--) {
+		const uint64_t d = v >> (i * 4) & 0xf;
+		if (!want && (d != 0 || digits > i))
+			want = B_TRUE;
+		if (want) {
+			buf[pos++] = hexdigits[d];
+			if (pos == buflen)
+				break;
+		}
+	}
+	return (pos);
+}
+
 void
-libspl_backtrace(void)
+libspl_backtrace(int fd)
 {
+	ssize_t ret __attribute__((unused));
 	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
+	unw_word_t loc;
+	char buf[128];
+	size_t n;
 
-	fprintf(stderr, "Call trace:\n");
+	ret = write(fd, "Call trace:\n", 12);
 	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);
+		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);
 #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);
+		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);
 #endif
+		ret = write(fd, "\n", 1);
 	}
 }
 #elif defined(HAVE_BACKTRACE)
 #include <execinfo.h>
 
 void
-libspl_backtrace(void)
+libspl_backtrace(int fd)
 {
-	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);
+	ssize_t ret __attribute__((unused));
+	void *btptrs[64];
+	size_t nptrs = backtrace(btptrs, 64);
+	ret = write(fd, "Call trace:\n", 12);
+	backtrace_symbols_fd(btptrs, nptrs, fd);
 }
 #else
 void
-libspl_backtrace(void)
+libspl_backtrace(int fd __maybe_unused)
 {
 }
 #endif
-

lib/libspl/include/sys/backtrace.h

@@ -27,6 +27,6 @@
 #ifndef _LIBSPL_SYS_BACKTRACE_H
 #define	_LIBSPL_SYS_BACKTRACE_H
 
-void libspl_backtrace(void);
+void libspl_backtrace(int fd);
 
 #endif