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

Tvrtko Ursulin
2017-07-31T09:41:10Z

On 28/07/2017 13:08, Chris Wilson wrote:
> 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

Stripping away constness wouldn't work with a simpler:

uint8_t *us = (uint8_t *)s?

> +
> + 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);

Inconsistent whitespace in the above three lines. First and second need
a tidy.

> + }
> + 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")
>

Apart from the little details, and ptrace API which I haven't looked in
detail (if it works it works), looks simple enough.

Regards,

Tvrtko