Re: [PATCH] munmap sealed memory cause memory to split (bug)

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Another thing about etiquette - sending a barely coherent _failing_ test
with basically zero explanation as a... patch is NOT how to interact with
the upstream community.

The sensible, respectful and workable way of doing this is to send
something like a [DISCUSSION] or something and say 'hey guys I think I
found a bug' with an explanation and a test patch attached.

A lot of your problems with the community could be resolved by being more
polite, respectful and taking a step back and breathing and _communicating_
with us who are here to try to help fix problems.

We are all _extremely_ busy, I am ill today also, so taking the time to try
to explain problems patiently instead of firing off barely documented
patches is far more likely to get you good results.

Also you fail to actually say what the problem is, what fails, where yoru
test fails etc.

Anyway, let's try to decode (please take this as input as to how you should
try to communicate these things):


So we start with a VMA like this:

012345678901
xxxxxxxxxxxx

We then seal the middle, starting at offset 4:

012345678901
xxxx****xxxx

This sets the VM_SEALED flag in the middle and splits VMAs resulting in 3
VMAs.

We then attempt to unmap 4 pages from offset 2, but this fails, as
expected.

012345678901
xxxx****xxxx
  |--| fail

We then attempt to unmap 4 pages from offset 6, but this fails, as
expected.

012345678901
xxxx****xxxx
      |--| fail

At each stage we should observe 4 VMAs.

Are you suggesting there is a larger unexpected split? Where? Under what
circumstances?

Let's figure out if there's a problem here _together_.

On Thu, Oct 17, 2024 at 02:26:27AM +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@google.com>
>
> It appears there is a regression on the latest mm,
> when munmap sealed memory, it can cause unexpected VMA split.
> E.g. repro use this test.
> ---
>  tools/testing/selftests/mm/mseal_test.c | 76 +++++++++++++++++++++++++
>  1 file changed, 76 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/mseal_test.c b/tools/testing/selftests/mm/mseal_test.c
> index fa74dbe4a684..0af33e13b606 100644
> --- a/tools/testing/selftests/mm/mseal_test.c
> +++ b/tools/testing/selftests/mm/mseal_test.c
> @@ -1969,6 +1969,79 @@ static void test_madvise_filebacked_was_writable(bool seal)
>  	REPORT_TEST_PASS();
>  }
>
> +static void test_munmap_free_multiple_ranges_with_split(bool seal)
> +{
> +	void *ptr;
> +	unsigned long page_size = getpagesize();
> +	unsigned long size = 12 * page_size;
> +	int ret;
> +	int prot;
> +
> +	setup_single_address(size, &ptr);
> +	FAIL_TEST_IF_FALSE(ptr != (void *)-1);
> +
> +	/* seal the middle 4 page */
> +	if (seal) {
> +		ret = sys_mseal(ptr + 4 * page_size, 4 * page_size);
> +		FAIL_TEST_IF_FALSE(!ret);
> +
> +		size = get_vma_size(ptr, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +
> +		size = get_vma_size(ptr +  4 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +
> +		size = get_vma_size(ptr +  8 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +	}
> +
> +	/* munmap 4  pages from the third page */
> +	ret = sys_munmap(ptr + 2 * page_size, 4 * page_size);
> +	if (seal) {
> +		FAIL_TEST_IF_FALSE(ret);
> +		FAIL_TEST_IF_FALSE(errno == EPERM);
> +
> +		size = get_vma_size(ptr, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +
> +		size = get_vma_size(ptr +  4 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +
> +		size = get_vma_size(ptr +  8 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +	} else
> +		FAIL_TEST_IF_FALSE(!ret);
> +
> +	/* munmap 4 pages from the sealed page */
> +	ret = sys_munmap(ptr + 6 * page_size, 4 * page_size);
> +	if (seal) {
> +		FAIL_TEST_IF_FALSE(ret);
> +		FAIL_TEST_IF_FALSE(errno == EPERM);
> +
> +		size = get_vma_size(ptr + 4 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);

This is repeated below, presumably you mean to do size = get_vma_size(ptr,
&prot) here?

> +
> +		size = get_vma_size(ptr +  4 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +
> +		size = get_vma_size(ptr +  8 * page_size, &prot);
> +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> +		FAIL_TEST_IF_FALSE(prot == 4);
> +	} else
> +		FAIL_TEST_IF_FALSE(!ret);
> +
> +	REPORT_TEST_PASS();
> +}
> +
> +
>  int main(int argc, char **argv)
>  {
>  	bool test_seal = seal_support();
> @@ -2099,5 +2172,8 @@ int main(int argc, char **argv)
>  	test_madvise_filebacked_was_writable(false);
>  	test_madvise_filebacked_was_writable(true);
>
> +	test_munmap_free_multiple_ranges_with_split(false);
> +	test_munmap_free_multiple_ranges_with_split(true);
> +
>  	ksft_finished();
>  }
> --
> 2.47.0.rc1.288.g06298d1525-goog
>