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

Chris Wilson
2017-07-28T12:14:17Z

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@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