From: Oliver Glitta glittao@gmail.com
Add documentation for a KUnit test for SLUB debugging functionality.
Signed-off-by: Oliver Glitta glittao@gmail.com --- 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. + +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 +option SLUB_KUNIT_TEST on. It is tristate option so it can also +be built as a module. This option depends on SLUB_DEBUG and +KUNIT options. By default it is on with all kunit tests. + +Error counting + +To get number of errors discovered in slub is used test API 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 +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. + +KASAN option + +Only 2 out of 5 tests are runnig with KASAN option is on. +The other three tests deliberately modifies non-allocated objects. And KASAN +does not detect some errors in the same way as SLUB_DEBUG. So, these tests +does not run when KASAN option is on. + +TESTS + +1. test_clobber_zone + + SLUB cache with SLUB_REDZONE flag can detects writings after object. This + 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