[PATCH v9 3/3] selftests/ptrace: add a test case for PTRACE_SET_SYSCALL_INFO syscall skipping

[Renzo Davoli <renzo@cs.unibo.it>]

Check whether PTRACE_SET_SYSCALL_INFO syscall skipping semantics
implemented in the kernel matches userspace expectations.

Signed-off-by: Renzo Davoli <renzo@cs.unibo.it>
[v8: also check an error return value, and that the syscall cannot be
 changed anymore once skipped while its return value still can;
 checkpatch fixes]
Assisted-by: claude-opus-5-5
---
 .../selftests/ptrace/set_syscall_info.c       | 230 +++++++++++++++++-
 1 file changed, 229 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/ptrace/set_syscall_info.c b/tools/testing/selftests/ptrace/set_syscall_info.c
index 1cc411a41cd6..e4fc2779ee5a 100644
--- a/tools/testing/selftests/ptrace/set_syscall_info.c
+++ b/tools/testing/selftests/ptrace/set_syscall_info.c
@@ -11,9 +11,15 @@
 #include <err.h>
 #include <fcntl.h>
 #include <signal.h>
+#include <stdlib.h>
+#include <stddef.h>
 #include <asm/unistd.h>
+#include <sys/prctl.h>
 #include <linux/types.h>
 #include <linux/ptrace.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/prctl.h>
 
 #if defined(_MIPS_SIM) && _MIPS_SIM == _MIPS_SIM_NABI32
 /*
@@ -36,6 +42,7 @@ struct si_exit {
 
 static unsigned int ptrace_stop;
 static pid_t tracee_pid;
+static pid_t tracer_pid;
 
 static int
 kill_tracee(pid_t pid)
@@ -64,6 +71,25 @@ sys_ptrace(int request, pid_t pid, unsigned long addr, unsigned long data)
 		       ptrace_stop, ##__VA_ARGS__);		\
 	} while (0)
 
+static int sys_seccomp(unsigned int operation, unsigned int flags, void *args)
+{
+	return syscall(__NR_seccomp, operation, flags, args);
+}
+
+static struct sock_filter seccomp_filter[] = {
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, offsetof(struct seccomp_data, nr)),
+
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_restart_syscall, 0, 1),
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRACE),
+};
+
+static struct sock_fprog seccomp_prog = {
+	.filter = seccomp_filter,
+	.len = ARRAY_SIZE(seccomp_filter)
+};
+
 static void
 check_psi_entry(struct __test_metadata *_metadata,
 		const struct ptrace_syscall_info *info,
@@ -128,7 +154,6 @@ check_psi_exit(struct __test_metadata *_metadata,
 
 TEST(set_syscall_info)
 {
-	const pid_t tracer_pid = getpid();
 	const kernel_ulong_t dummy[] = {
 		(kernel_ulong_t) 0xdad0bef0bad0fed0ULL,
 		(kernel_ulong_t) 0xdad1bef1bad1fed1ULL,
@@ -138,6 +163,7 @@ TEST(set_syscall_info)
 		(kernel_ulong_t) 0xdad5bef5bad5fed5ULL,
 	};
 	int splice_in[2], splice_out[2];
+	tracer_pid = getpid();
 
 	ASSERT_EQ(0, pipe(splice_in));
 	ASSERT_EQ(0, pipe(splice_out));
@@ -516,4 +542,206 @@ TEST(set_syscall_info)
 	ASSERT_EQ(ptrace_stop, ARRAY_SIZE(si) * 2);
 }
 
+TEST(set_syscall_info_seccomp)
+{
+	/*
+	 * Check for seccomp filter support without installing a filter:
+	 * a NULL filter fails with EFAULT if SECCOMP_SET_MODE_FILTER is
+	 * supported, and with EINVAL or ENOSYS otherwise.
+	 */
+	ASSERT_EQ(-1, sys_seccomp(SECCOMP_SET_MODE_FILTER, 0, NULL));
+	if (errno != EFAULT)
+		SKIP(return, "SECCOMP_SET_MODE_FILTER: %m");
+
+	tracer_pid = getpid();
+	tracee_pid = fork();
+
+	ASSERT_LE(0, tracee_pid) {
+		TH_LOG("fork: %m");
+	}
+
+	/* tracee */
+	if (tracee_pid == 0) {
+		tracee_pid = getpid();
+		ASSERT_EQ(0, sys_ptrace(PTRACE_TRACEME, 0, 0, 0)) {
+			TH_LOG("PTRACE_TRACEME: %m");
+		}
+		ASSERT_EQ(0, kill(tracee_pid, SIGSTOP)) {
+			/* cannot happen */
+			TH_LOG("kill SIGSTOP: %m");
+		}
+
+		ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+			TH_LOG("prctl: %m");
+		}
+		ASSERT_EQ(0, sys_seccomp(SECCOMP_SET_MODE_FILTER, 0,
+					(void *) &seccomp_prog)) {
+			TH_LOG("seccomp: %m");
+		}
+
+		/*
+		 * From now on every syscall stops the tracee, so do not use
+		 * ASSERT_*() here: it would write() its message before
+		 * _exit(1) and the tracer would report a syscall nr mismatch
+		 * instead of the test case that failed.
+		 */
+
+		/* run getpid unmodified */
+		if (getpid() != tracee_pid)
+			_exit(1);
+
+		/* run getppid instead of getpid */
+		if (getpid() != tracer_pid)
+			_exit(1);
+
+		/* skip getpid and return 42 */
+		if (getpid() != 42)
+			_exit(1);
+
+		/* skip getpid and fail with ESRCH */
+		errno = 0;
+		if (syscall(__NR_getpid) != -1 || errno != ESRCH)
+			_exit(1);
+
+		_exit(0);
+	}
+
+	int status;
+
+	/* tracer */
+	ASSERT_LE(0, waitpid(-1, &status, 0)) {
+		LOG_KILL_TRACEE("waitpid: %m");
+	}
+
+	ASSERT_EQ(0, sys_ptrace(PTRACE_SETOPTIONS, tracee_pid, 0,
+			(PTRACE_O_TRACESECCOMP | PTRACE_O_TRACESYSGOOD)))
+		LOG_KILL_TRACEE("PTRACE_SETOPTIONS: %m");
+
+	ASSERT_EQ(0, sys_ptrace(PTRACE_CONT, tracee_pid, 0, 0)) {
+		LOG_KILL_TRACEE("PTRACE_CONT: %m");
+	}
+
+	static const char * const testname[] = {
+		"unknown error",
+		"getpid seccomp unchanged",
+		"getpid seccomp nr change: getppid",
+		"getpid seccomp skip+set retvalue",
+		"getpid seccomp skip+set error"
+	};
+
+	static const unsigned int expected_nr[] = {
+		__NR_getpid,
+		__NR_getpid,
+		__NR_getpid,
+		__NR_getpid,
+		__NR_exit_group
+	};
+
+	for (ptrace_stop = 0; ; ++ptrace_stop) {
+		ASSERT_EQ(tracee_pid, wait(&status)) {
+			/* cannot happen */
+			LOG_KILL_TRACEE("wait: %m");
+		}
+
+		if (WIFEXITED(status)) {
+			tracee_pid = 0; /* the tracee is no more */
+			ASSERT_EQ(0, WEXITSTATUS(status)) {
+				LOG_KILL_TRACEE("unexpected exit status %u",
+						WEXITSTATUS(status));
+			}
+			break;
+		}
+		ASSERT_FALSE(WIFSIGNALED(status)) {
+			tracee_pid = 0; /* the tracee is no more */
+			LOG_KILL_TRACEE("unexpected signal %u",
+					WTERMSIG(status));
+		}
+		ASSERT_TRUE(WIFSTOPPED(status)) {
+			LOG_KILL_TRACEE("unexpected wait status %#x", status);
+		}
+
+		ASSERT_EQ(status >> 8, SIGTRAP | (PTRACE_EVENT_SECCOMP << 8)) {
+			LOG_KILL_TRACEE("unexpected stop, wait status %#x", status);
+		}
+
+		struct ptrace_syscall_info info = {
+			.op = 0xff  /* invalid PTRACE_SYSCALL_INFO_* op */
+		};
+		size_t info_size = sizeof(info);
+
+		ASSERT_LT(0, sys_ptrace(PTRACE_GET_SYSCALL_INFO, tracee_pid,
+					info_size, (uintptr_t) &info)) {
+			LOG_KILL_TRACEE("PTRACE_GET_SYSCALL_INFO: %m");
+		}
+		ASSERT_EQ(PTRACE_SYSCALL_INFO_SECCOMP, info.op) {
+			LOG_KILL_TRACEE("entry op mismatch: %m");
+		}
+		ASSERT_TRUE(info.arch) {
+			LOG_KILL_TRACEE("entry arch mismatch: %m");
+		}
+		ASSERT_TRUE(info.instruction_pointer) {
+			LOG_KILL_TRACEE("entry instruction_pointer mismatch: %m");
+		}
+		ASSERT_TRUE(info.stack_pointer) {
+			LOG_KILL_TRACEE("entry stack_pointer mismatch: %m");
+		}
+
+		ASSERT_LT(ptrace_stop, ARRAY_SIZE(expected_nr)) {
+			LOG_KILL_TRACEE("ptrace stop overflow");
+		}
+		ASSERT_FALSE(info.seccomp.nr == __NR_exit_group && info.seccomp.args[0] == 1) {
+			LOG_KILL_TRACEE("tracee error: %s",
+					testname[ptrace_stop]);
+		}
+		ASSERT_EQ(info.seccomp.nr, expected_nr[ptrace_stop]) {
+			LOG_KILL_TRACEE("syscall nr mismatch");
+		}
+		const struct ptrace_syscall_info seccomp_info = info;
+
+		switch (ptrace_stop) {
+		case 0:
+		case 4:
+			break;
+		case 1:
+			info.seccomp.nr = __NR_getppid;
+			break;
+		case 2:
+			info.op = PTRACE_SYSCALL_INFO_EXIT;
+			info.exit.rval = 41;
+			info.exit.is_error = 0;
+			ASSERT_EQ(0, sys_ptrace(PTRACE_SET_SYSCALL_INFO,
+						tracee_pid, info_size,
+						(uintptr_t) &info)) {
+				LOG_KILL_TRACEE("PTRACE_SET_SYSCALL_INFO: %m");
+			}
+			/* the syscall cannot be changed anymore once skipped */
+			ASSERT_EQ(-1, sys_ptrace(PTRACE_SET_SYSCALL_INFO,
+						 tracee_pid, info_size,
+						 (uintptr_t) &seccomp_info)) {
+				LOG_KILL_TRACEE("PTRACE_SET_SYSCALL_INFO after skip succeeded");
+			}
+			ASSERT_EQ(EINVAL, errno) {
+				LOG_KILL_TRACEE("PTRACE_SET_SYSCALL_INFO after skip: %m");
+			}
+			/* but its return value can */
+			info.exit.rval = 42;
+			break;
+		case 3:
+			info.op = PTRACE_SYSCALL_INFO_EXIT;
+			info.exit.rval = -ESRCH;
+			info.exit.is_error = 1;
+			break;
+		}
+
+		ASSERT_EQ(0, sys_ptrace(PTRACE_SET_SYSCALL_INFO, tracee_pid,
+					info_size, (uintptr_t) &info)) {
+			LOG_KILL_TRACEE("PTRACE_SET_SYSCALL_INFO: %m");
+		}
+
+		ASSERT_EQ(0, sys_ptrace(PTRACE_CONT, tracee_pid, 0, 0)) {
+			LOG_KILL_TRACEE("PTRACE_CONT: %m");
+		}
+	}
+}
+
 TEST_HARNESS_MAIN
-- 
2.55.0