[LTP] [PATCH v3 3/3] ptrace: add test for /proc/pid/mem writes under ptrace

[Jan Polensky <japo@linux.ibm.com>]

Add ptrace13 to verify that a tracer can write to tracee memory via
/proc/pid/mem when CONFIG_PROC_MEM_FORCE_PTRACE=y is active.

The test uses PTRACE_SEIZE to attach to a child process and writes to
its read-only memory pages via /proc/pid/mem. This validates that:
- FOLL_FORCE is allowed when actively ptracing
- Parent can modify child's read-only memory
- Write-stop-continue cycle works correctly (100 iterations)

Test validates kernel commit 41e8149c8892 ("proc: add config & param to
block forcing mem writes").

Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 runtest/syscalls                            |   1 +
 testcases/kernel/syscalls/ptrace/.gitignore |   1 +
 testcases/kernel/syscalls/ptrace/ptrace13.c | 204 ++++++++++++++++++++
 3 files changed, 206 insertions(+)
 create mode 100644 testcases/kernel/syscalls/ptrace/ptrace13.c

diff --git a/runtest/syscalls b/runtest/syscalls
index 9df4684cca41..476c69d4c5f0 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -1177,6 +1177,7 @@ ptrace10 ptrace10
 ptrace11 ptrace11
 
 ptrace12 ptrace12
+ptrace13 ptrace13
 pwrite01 pwrite01
 pwrite02 pwrite02
 pwrite03 pwrite03
diff --git a/testcases/kernel/syscalls/ptrace/.gitignore b/testcases/kernel/syscalls/ptrace/.gitignore
index 8631219312d5..72f9cef98e22 100644
--- a/testcases/kernel/syscalls/ptrace/.gitignore
+++ b/testcases/kernel/syscalls/ptrace/.gitignore
@@ -10,3 +10,4 @@
 /ptrace10
 /ptrace11
 /ptrace12
+/ptrace13
diff --git a/testcases/kernel/syscalls/ptrace/ptrace13.c b/testcases/kernel/syscalls/ptrace/ptrace13.c
new file mode 100644
index 000000000000..a052fab81cb6
--- /dev/null
+++ b/testcases/kernel/syscalls/ptrace/ptrace13.c
@@ -0,0 +1,204 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2025 SUSE LLC <japo@suse.cz>
+ */
+
+/*\
+ * Verify that a parent process can write to a traced child's memory
+ * via /proc/pid/mem when the child is in a stopped state.
+ *
+ * This test validates the ptrace-based memory write mechanism that
+ * becomes mandatory when CONFIG_PROC_MEM_FORCE_PTRACE=y is active.
+ *
+ * Test flow:
+ *
+ * 1. Parent forks a child process
+ * 2. Child signals readiness via checkpoint
+ * 3. Parent attaches with PTRACE_SEIZE
+ * 4. Child self-stops with raise(SIGSTOP)
+ * 5. Parent writes to child's memory via /proc/pid/mem
+ * 6. Parent continues child with PTRACE_CONT
+ * 7. Child verifies the write took effect
+ * 8. Repeat for multiple iterations
+ *
+ * The test uses checkpoint synchronization to prevent race conditions
+ * during ptrace attachment.
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/ptrace.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "tst_test.h"
+#include "tst_checkpoint.h"
+
+#define TEST_ITERATIONS 100
+
+struct shared_state {
+	int *test_ptr;
+	int expected_val;
+};
+
+static struct shared_state *shared;
+static pid_t tracee_pid;
+
+static void tracee_main(void)
+{
+	int i;
+
+	shared->test_ptr = SAFE_MMAP(NULL, sizeof(int), PROT_READ | PROT_WRITE,
+				     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	*shared->test_ptr = 0;
+
+	/* Force parent write through /proc/pid/mem to require FOLL_FORCE */
+	SAFE_MPROTECT((void *)shared->test_ptr, sizeof(int), PROT_READ);
+
+	TST_CHECKPOINT_WAKE(0);
+	TST_CHECKPOINT_WAIT(1);
+	raise(SIGSTOP);
+
+	for (i = 0; i < TEST_ITERATIONS; i++) {
+		if (*shared->test_ptr != shared->expected_val) {
+			tst_res(TFAIL,
+				"Iteration %d: expected 0x%x (written in iteration %d), got 0x%x",
+				i, shared->expected_val, i > 0 ? i-1 : 0, *shared->test_ptr);
+			exit(1);
+		}
+
+		raise(SIGSTOP);
+	}
+
+	while (1)
+		pause();
+}
+
+static void setup(void)
+{
+	/* Allocate shared memory for parent-child communication */
+	shared = SAFE_MMAP(NULL, sizeof(*shared), PROT_READ | PROT_WRITE,
+			   MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+	memset(shared, 0, sizeof(*shared));
+}
+
+static void run(void)
+{
+	char path[64];
+	int memfd;
+	int status;
+	int i;
+
+	tracee_pid = SAFE_FORK();
+	if (!tracee_pid) {
+		tracee_main();
+		exit(0);
+	}
+
+	TST_CHECKPOINT_WAIT(0);
+	SAFE_PTRACE(PTRACE_SEIZE, tracee_pid, NULL, NULL);
+	TST_CHECKPOINT_WAKE(1);
+	SAFE_WAITPID(tracee_pid, &status, 0);
+
+	if (WIFEXITED(status)) {
+		tst_brk(TBROK,
+			"Tracee exited unexpectedly before first SIGSTOP: %s",
+			tst_strstatus(status));
+	}
+
+	if (WIFSIGNALED(status)) {
+		tst_brk(TBROK,
+			"Tracee was killed before first SIGSTOP: %s",
+			tst_strstatus(status));
+	}
+
+	if (!WIFSTOPPED(status) || WSTOPSIG(status) != SIGSTOP) {
+		tst_brk(TBROK,
+			"Tracee did not self-stop with SIGSTOP: %s",
+			tst_strstatus(status));
+	}
+
+	snprintf(path, sizeof(path), "/proc/%d/mem", tracee_pid);
+	memfd = SAFE_OPEN(path, O_RDWR);
+
+	/*
+	 * Write-stop-continue cycle: tracee is already stopped from initial raise(SIGSTOP).
+	 * Pattern: write → continue → tracee verifies → tracee stops → repeat
+	 */
+	for (i = 0; i < TEST_ITERATIONS; i++) {
+		int write_val = 0xdead0000 | i;
+
+		shared->expected_val = write_val;
+		SAFE_LSEEK(memfd, (off_t)shared->test_ptr, SEEK_SET);
+		SAFE_WRITE(SAFE_WRITE_ALL, memfd, &write_val, sizeof(write_val));
+		SAFE_PTRACE(PTRACE_CONT, tracee_pid, NULL, NULL);
+		SAFE_WAITPID(tracee_pid, &status, 0);
+
+		if (WIFEXITED(status)) {
+			if (WEXITSTATUS(status) == 0) {
+				tst_brk(TBROK,
+					"Tracee exited unexpectedly at iteration %d: %s",
+					i, tst_strstatus(status));
+			}
+			/* Tracee reported failure and exited */
+			SAFE_CLOSE(memfd);
+			tracee_pid = 0;
+			return;
+		}
+
+		if (WIFSIGNALED(status)) {
+			tst_brk(TBROK,
+				"Tracee was killed at iteration %d: %s",
+				i, tst_strstatus(status));
+		}
+
+		if (!WIFSTOPPED(status) || WSTOPSIG(status) != SIGSTOP) {
+			tst_brk(TBROK,
+				"Tracee did not stop correctly at iteration %d: %s",
+				i, tst_strstatus(status));
+		}
+	}
+
+	SAFE_CLOSE(memfd);
+
+	/* Test passed - all writes were successful and tracee verified them */
+	tst_res(TPASS,
+		"Successfully wrote to tracee memory via /proc/pid/mem "
+		"for %d iterations", TEST_ITERATIONS);
+
+	/* Detach and terminate tracee */
+	SAFE_PTRACE(PTRACE_DETACH, tracee_pid, NULL, NULL);
+	SAFE_KILL(tracee_pid, SIGKILL);
+	SAFE_WAITPID(tracee_pid, &status, 0);
+	tracee_pid = 0;
+}
+
+static void cleanup(void)
+{
+	if (tracee_pid > 0) {
+		/* Kill tracee if still alive (e.g., test aborted) */
+		if (kill(tracee_pid, 0) == 0) {
+			SAFE_KILL(tracee_pid, SIGKILL);
+			tst_reap_children();
+		}
+		tracee_pid = 0;
+	}
+
+	if (shared)
+		SAFE_MUNMAP(shared, sizeof(*shared));
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.setup = setup,
+	.cleanup = cleanup,
+	.forks_child = 1,
+	.needs_checkpoints = 1,
+	.tags = (const struct tst_tag[]) {
+		{"linux-git", "41e8149c8892"},
+		{}
+	}
+};
-- 
2.55.0


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