ftrace: Synchronize the initialization of ftrace_opsgrafted

There's some internal state that ftrace_ops needs to have set, but since
it can be declared outside of the ftrace.c code, it calls
ftrace_ops_init() on the ops in every global function. The issue is that
if two tasks call it on the same ops at the same time it is possible to
have the initialization of one corrupt the initialization of the other
call.

Create a ops_mutex to use to synchronize every initialization of the
ftrace_ops. The mutex is taken within checking the ftrace_ops flag that
states it was initializied but the flag is checked again after the mutex
has been taken. Checking first outside the mutex allows it to shortcut
having to take the mutex. But then the check needs to be done again after
the mute is taken in case of races.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260902095501.6b59af20@gandalf.local.home
Fixes: f04f24fb7e48d ("ftrace, kprobes: Fix a deadlock on ftrace_regex_lock")
Reported-by: sashiko-bot@kernel.org
Close: https://lore.kernel.org/all/20260829025528.49A831F000E9@smtp.kernel.org/
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 135 insertions, 0 deletions

diff --git a/tools/objtool/signal.c b/tools/objtool/signal.c
new file mode 100644
index 000000000..af5c65c0f
--- /dev/null
+++ b/tools/objtool/signal.c
@@ -0,0 +1,135 @@
+/*
+ * signal.c: Register a sigaltstack for objtool, to be able to
+ *	     run a signal handler on a separate stack even if
+ *	     the main process stack has overflown. Print out
+ *	     stack overflow errors when this happens.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <unistd.h>
+#include <sys/resource.h>
+#include <string.h>
+
+#include <objtool/objtool.h>
+#include <objtool/warn.h>
+
+static unsigned long stack_limit;
+
+static bool is_stack_overflow(void *fault_addr)
+{
+	unsigned long fault = (unsigned long)fault_addr;
+
+	/* Check if fault is in the guard page just below the limit. */
+	return fault < stack_limit && fault >= stack_limit - 4096;
+}
+
+static void signal_handler(int sig_num, siginfo_t *info, void *context)
+{
+	struct sigaction sa_dfl = {0};
+	const char *sig_name;
+	char msg[256];
+	int msg_len;
+
+	switch (sig_num) {
+	case SIGSEGV:	sig_name = "SIGSEGV";		break;
+	case SIGBUS:	sig_name = "SIGBUS";		break;
+	case SIGILL:	sig_name = "SIGILL";		break;
+	case SIGABRT:	sig_name = "SIGABRT";		break;
+	default:	sig_name = "Unknown signal";	break;
+	}
+
+	if (is_stack_overflow(info->si_addr)) {
+		msg_len = snprintf(msg, sizeof(msg),
+				   "%s: error: %s: objtool stack overflow!\n",
+				   objname, sig_name);
+	} else {
+		msg_len = snprintf(msg, sizeof(msg),
+				   "%s: error: %s: objtool crash!\n",
+				   objname, sig_name);
+	}
+
+	msg_len = write(STDERR_FILENO, msg, msg_len);
+
+	/* Re-raise the signal to trigger the core dump */
+	sa_dfl.sa_handler = SIG_DFL;
+	sigaction(sig_num, &sa_dfl, NULL);
+	raise(sig_num);
+}
+
+static int read_stack_limit(void)
+{
+	unsigned long stack_start, stack_end;
+	struct rlimit rlim;
+	char line[256];
+	int ret = 0;
+	FILE *fp;
+
+	if (getrlimit(RLIMIT_STACK, &rlim)) {
+		ERROR_GLIBC("getrlimit");
+		return -1;
+	}
+
+	fp = fopen("/proc/self/maps", "r");
+	if (!fp) {
+		ERROR_GLIBC("fopen");
+		return -1;
+	}
+
+	while (fgets(line, sizeof(line), fp)) {
+		if (strstr(line, "[stack]")) {
+			if (sscanf(line, "%lx-%lx", &stack_start, &stack_end) != 2) {
+				ERROR_GLIBC("sscanf");
+				ret = -1;
+				goto done;
+			}
+			stack_limit = stack_end - rlim.rlim_cur;
+			goto done;
+		}
+	}
+
+	ret = -1;
+	ERROR("/proc/self/maps: can't find [stack]");
+
+done:
+	fclose(fp);
+
+	return ret;
+}
+
+int init_signal_handler(void)
+{
+	int signals[] = {SIGSEGV, SIGBUS, SIGILL, SIGABRT};
+	struct sigaction sa;
+	stack_t ss;
+
+	if (read_stack_limit())
+		return -1;
+
+	ss.ss_sp = malloc(SIGSTKSZ);
+	if (!ss.ss_sp) {
+		ERROR_GLIBC("malloc");
+		return -1;
+	}
+	ss.ss_size = SIGSTKSZ;
+	ss.ss_flags = 0;
+
+	if (sigaltstack(&ss, NULL) == -1) {
+		ERROR_GLIBC("sigaltstack");
+		return -1;
+	}
+
+	sa.sa_sigaction = signal_handler;
+	sigemptyset(&sa.sa_mask);
+
+	sa.sa_flags = SA_ONSTACK | SA_SIGINFO;
+
+	for (int i = 0; i < ARRAY_SIZE(signals); i++) {
+		if (sigaction(signals[i], &sa, NULL) == -1) {
+			ERROR_GLIBC("sigaction");
+			return -1;
+		}
+	}
+
+	return 0;
+}