[Intel-gfx] [PATCH igt 02/10] igt/gem_mmap_gtt: Simulate gdb inspecting a GTT mmap using ptrace()

[Chris Wilson]

gdb uses ptrace() to peek and poke bytes of the target's address space.
The kernel must implement an vm_ops->access() handler or else gdb will
be unable to inspect the pointer and report it as out-of-bounds. Worse
than useless as it causes immediate suspicion of the valid GTT pointer.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 tests/gem_mmap_gtt.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/tests/gem_mmap_gtt.c b/tests/gem_mmap_gtt.c
index 4ff5e7f1..61c08406 100644
--- a/tests/gem_mmap_gtt.c
+++ b/tests/gem_mmap_gtt.c
@@ -36,6 +36,8 @@
 #include <errno.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>
+#include <sys/ptrace.h>
+#include <sys/wait.h>
 #include "drm.h"
 
 #include "igt.h"
@@ -310,6 +312,81 @@ test_write_gtt(int fd)
 	munmap(src, OBJECT_SIZE);
 }
 
+static void *memchr_inv(const void *s, int c, size_t n)
+{
+	const uint8_t *us = s;
+	const uint8_t uc = c;
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-qual"
+	while (n--) {
+		if (*us != uc)
+			return (void *) us;
+		us++;
+	}
+#pragma GCC diagnostic pop
+
+	return NULL;
+}
+
+static void
+test_ptrace(int fd)
+{
+	long AA, CC;
+	long *gtt, *copy;
+	uint32_t bo;
+	pid_t pid;
+
+	memset(&AA, 0xaa, sizeof(AA));
+	memset(&CC, 0x55, sizeof(CC));
+
+	copy = malloc(OBJECT_SIZE);
+	memset(copy, AA, OBJECT_SIZE);
+
+	bo = gem_create(fd, OBJECT_SIZE);
+	gtt = mmap_bo(fd, bo);
+	memset(gtt, CC, OBJECT_SIZE);
+	gem_close(fd, bo);
+
+	igt_assert(!memchr_inv(gtt, CC, OBJECT_SIZE));
+	igt_assert(!memchr_inv(copy, AA, OBJECT_SIZE));
+
+	switch ((pid = fork())) {
+		case -1:
+			igt_assert(pid != -1);
+			break;
+
+		case 0:
+			ptrace(PTRACE_TRACEME, 0, NULL, NULL);
+			raise(SIGSTOP);
+			raise(SIGKILL);
+			exit(0);
+			break;
+
+		default:
+			/* Wait for the child to ready themselves */
+			wait(NULL);
+
+			ptrace(PTRACE_ATTACH, pid, NULL, NULL);
+			for (int i = 0; i < OBJECT_SIZE/sizeof(long); i++) {
+				copy[i] = ptrace(PTRACE_PEEKDATA, pid, gtt+i, NULL);
+				ptrace(PTRACE_POKEDATA, pid, gtt + i, AA);
+			}
+			ptrace(PTRACE_DETACH, pid, NULL, NULL);
+
+			/* Wakeup the child for it to exit */
+			kill(SIGCONT, pid);
+			break;
+	}
+
+	/* The contents of the two buffers should now be swapped */
+	igt_assert(!memchr_inv(gtt, AA, OBJECT_SIZE));
+	igt_assert(!memchr_inv(copy, CC, OBJECT_SIZE));
+
+	munmap(gtt, OBJECT_SIZE);
+	free(copy);
+}
+
 static void
 test_coherency(int fd)
 {
@@ -809,6 +886,8 @@ igt_main
 		test_write(fd);
 	igt_subtest("basic-write-gtt")
 		test_write_gtt(fd);
+	igt_subtest("ptrace")
+		test_ptrace(fd);
 	igt_subtest("coherency")
 		test_coherency(fd);
 	igt_subtest("clflush")
-- 
2.13.3