selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.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;
+}