Re: [PATCH] docs: add documentation for SLUB cache kunit tests

Daniel Latypov
2021-06-08T19:35:44Z

On Tue, Jun 8, 2021 at 1:47 AM <glittao@gmail.com> wrote:
>
> From: Oliver Glitta <glittao@gmail.com>
>
> Add documentation for a KUnit test for SLUB debugging functionality.
>
> Signed-off-by: Oliver Glitta <glittao@gmail.com>

I'm not in the intended audience for this documentation, so I've
mainly just gone and pointed out some small spelling mistakes and some
formatting suggestions.

> ---
> Documentation/vm/slub.rst | 104 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 104 insertions(+)
>
> diff --git a/Documentation/vm/slub.rst b/Documentation/vm/slub.rst
> index 03f294a638bd..ca82fc1649ee 100644
> --- a/Documentation/vm/slub.rst
> +++ b/Documentation/vm/slub.rst
> @@ -384,5 +384,109 @@ c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the
> 40,60`` range will plot only samples collected between 40th and
> 60th seconds).
>
> +KUnit tests for SLUB debugging functionality
> +============================================
> +
> +These KUnit tests are used to test some of the SLUB debugging
> +functionalities.
> +
> +KUnit tests are used for unit testing in Linux kernel and easy to run,
> +so it is probably the best choice for this type of tests.

Note: if we think KUnit is worth explaining here, perhaps it's also
worth linking to KUnit documentation?

Something like
"KUnit (Documentation/dev-tools/kunit/index.rst) ..."

should work, or "See KUnit documentation (...) for more details", etc.

> +
> +There are tests, which corrupt redzone, the free objects and the freelist.
> +Tests are corrupting specific bytes in cache and checking if validation
> +finds expected number of bugs. Bug reports are silenced.
> +
> +Config option
> +
> +In order to built and then run this tests you need to switch

nit: "to build"
nit: "run these tests"

> +option SLUB_KUNIT_TEST on. It is tristate option so it can also

minor nit: s/is tristate/is a tristate

> +be built as a module. This option depends on SLUB_DEBUG and
> +KUNIT options. By default it is on with all kunit tests.

I assume this last sentence is about CONFIG_KUNIT_ALL_TESTS.
I don't think we need to document that here.

It could also mislead users as selecting CONFIG_KUNIT_ALL_TESTS=y will
not be sufficient to make the test run on its own.

> +
> +Error counting
> +
> +To get number of errors discovered in slub is used test API kunit_resource.

Nit: "To get the number"

Nit: can we use ``kunit_resource`` to format code-related terms?

Note: we can link to the API reference by including "test API
(Documentation/dev-tools/kunit/api/test.rst)"

Not sure if you saw, but this is that page:
https://www.kernel.org/doc/html/latest/dev-tools/kunit/api/test.html
Coincidentally enough, kunit_resource is the first thing shown for now :)

Note: it'd be cool if we could link to a specific section, but I
haven't checked if that's actually possible with automarkup.py, e.g.
like https://www.kernel.org/doc/html/latest/dev-tools/kunit/api/test.html#c.kunit_resource

> +In test_init the reference to the integer variable slab_errors is added
> +to the resource of this tests.
> +
> +During slub cache checking always when bug should be reported or fixed function

Nit: "when a bug"

> +slab_add_kunit_errors() is called. This function find resource to kunit test
> +and increment value of data in founded resource, which is slab_errors
> +variable.
> +
> +Silence bug reports
> +
> +The function slab_add_kunit_errors() is returning bool, which is true if there is kunit test
> +with correct kunit_resource running, to silence bug reports, so they are not printed.
> +We do not want to correct errors we only want to know they occurred, so these reports
> +are unnnecessary.

nit: "unnecessary"

> +
> +KASAN option
> +
> +Only 2 out of 5 tests are runnig with KASAN option is on.

nit: "running"

> +The other three tests deliberately modifies non-allocated objects. And KASAN

nit: "other three tests ... modify"

> +does not detect some errors in the same way as SLUB_DEBUG. So, these tests
> +does not run when KASAN option is on.
> +
> +TESTS

I'm not the intended target for this doc, so feel free to ignore this
comment entirely.

Hmm, I'm not really a fan of the docs listing out each test case and
what they do.
I feel like this will get out of sync and you now are expected to read
another file to understand what the tests do.
I can see the benefit of a high-level explanation, especially if we
started having more tests scattered across multiple files, but I think
we should wait until that time comes.

So I'd personally prefer it if we converted these sections into
comments on the test cases themselves.

> +
> +1. test_clobber_zone
> +
> + SLUB cache with SLUB_REDZONE flag can detects writings after object. This

nit: is there a missing word after "after object.", perhaps "deallocation"?
Perhaps then: "can detect writes after objection deallocation", or
more simply "can detect write-after-free errors" ?

> + functionality is tested here on allocated memory.
> +
> + First, there is allocated memory with SLAB_REDZONE and then the first byte
> + after allocated space is modified. Validation founds 2 errors, because of
> + the bug and the fix of the memory.
> +
> +
> +2. test_next_pointer
> +
> + SLUB have list of free objects and the address of the next free object
> + is always saved in free object at offset specified in variable offset
> + in struct kmem_cache. This test try to corrupt this freelist and
> + then correct it.
> +
> + First, there is allocated and freed memory to get a pointer to free object.
> + After that, the pointer to next free object is corrupted. The first validation finds
> + 3 errors. One for corrupted freechain, the second for the wrong count of objects
> + in use and the third for fixing the issue. This fix only set number of objects
> + in use to a number of all objects minus 1, because the first free object
> + was corrupted.
> +
> + Then the free pointer is fixed to his previous value. The second validation finds
> + 2 errors. One for the wrong count of objects in use and one for fixing this error.
> +
> + Last validation is used to check if all errors were corrected so no error
> + is found.
> +
> +3. test_first_word
> +
> + SLUB cache with SLAB_POISON flag can detect poisoning free objects. This
> + functionality is tested in this test. The test tries to corrupt
> + the first byte in freed memory.
> +
> + First of all, memory is allocated and freed to get a pointer to a free object
> + and then the first byte is corrupted. After that, validation finds 2 errors,
> + one for the bug and the other one for the fix of the memory.
> +
> +4. test_clobber_50th_byte
> +
> + In this test SLAB_POISON functionality is tested. The test tries to
> + corrupt the 50th byte in freed memory.
> +
> + First, pointer to a free memory is acquired by allocating and freeing memory.
> + Then 50th byte is corrupted and validation finds 2 errors for the bug and
> + the fix of the memory.
> +
> +5. test_clobber_redzone_free
> +
> + This test tests redzone functionality of SLUB cache on a freed object.
> +
> + First, it gets pointer to the free object with allocating and freeing and
> + then corrupts the first byte after the freed object. Validation finds
> + 2 errors for the bug and the fix of the memory.
> +
> Christoph Lameter, May 30, 2007
> Sergey Senozhatsky, October 23, 2015
> --
> 2.31.1.272.g89b43f80a5
>
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