selftests: bpf: mmap question

* selftests: bpf: mmap question
@ 2020-07-23 11:02 Yauheni Kaliuta
  2020-07-28  5:15 ` Andrii Nakryiko
  0 siblings, 1 reply; 4+ messages in thread
From: Yauheni Kaliuta @ 2020-07-23 11:02 UTC (permalink / raw)
  To: bpf; +Cc: Andrii Nakryiko, Jiri Olsa, Jiri Benc

Hi!

I have a question about the part of the test:

	/* check some more advanced mmap() manipulations */

	/* map all but last page: pages 1-3 mapped */
	tmp1 = mmap(NULL, 3 * page_size, PROT_READ, MAP_SHARED,
			  data_map_fd, 0);
	if (CHECK(tmp1 == MAP_FAILED, "adv_mmap1", "errno %d\n", errno))
		goto cleanup;

	/* unmap second page: pages 1, 3 mapped */
	err = munmap(tmp1 + page_size, page_size);
	if (CHECK(err, "adv_mmap2", "errno %d\n", errno)) {
		munmap(tmp1, map_sz);
		goto cleanup;
	}

	/* map page 2 back */
	tmp2 = mmap(tmp1 + page_size, page_size, PROT_READ,
		    MAP_SHARED | MAP_FIXED, data_map_fd, 0);
	if (CHECK(tmp2 == MAP_FAILED, "adv_mmap3", "errno %d\n", errno)) {
		munmap(tmp1, page_size);
		munmap(tmp1 + 2*page_size, page_size);
		goto cleanup;
	}
	CHECK(tmp1 + page_size != tmp2, "adv_mmap4",
	      "tmp1: %p, tmp2: %p\n", tmp1, tmp2);

	/* re-map all 4 pages */
	tmp2 = mmap(tmp1, 4 * page_size, PROT_READ, MAP_SHARED | MAP_FIXED,
		    data_map_fd, 0);
	if (CHECK(tmp2 == MAP_FAILED, "adv_mmap5", "errno %d\n", errno)) {
		munmap(tmp1, 3 * page_size); /* unmap page 1 */
		goto cleanup;
	}
	CHECK(tmp1 != tmp2, "adv_mmap6", "tmp1: %p, tmp2: %p\n", tmp1, tmp2);

In my configuration the first mapping

	/* map all but last page: pages 1-3 mapped */
	tmp1 = mmap(NULL, 3 * page_size, PROT_READ, MAP_SHARED,
			  data_map_fd, 0);

maps the area to the 3 pages right before the TLS page.
I find it's pretty ok.

But then the 4 page mapping

	/* re-map all 4 pages */
	tmp2 = mmap(tmp1, 4 * page_size, PROT_READ, MAP_SHARED | MAP_FIXED,
		    data_map_fd, 0);

since it has MAP_FIXED flag, unmaps TLS and maps the former TLS
address BPF map.

Which is again exactly the behaviour of MAP_FIXED, but it breaks
the test.

Using MAP_FIXED_NOREPLACE fails the check:

CHECK(tmp1 != tmp2, "adv_mmap6", "tmp1: %p, tmp2: %p\n", tmp1, tmp2);

as expected.

Should the test be modified to be a bit more relaxed? Since the
kernel behaviour looks correct or I'm missing something?

PS: BTW, the previous data_map mapping left unmmaped. Is it expected?

-- 
WBR,
Yauheni Kaliuta

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