Re: [PATCH v1] slab: support for compiler-assisted type-based slab cache partitioning

[Marco Elver <elver@google.com>]

On Thu, 2 Apr 2026 at 15:33, Dan Carpenter <error27@gmail.com> wrote:
>
> Hi Marco,
>
> kernel test robot noticed the following build warnings:
>
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Marco-Elver/slab-support-for-compiler-assisted-type-based-slab-cache-partitioning/20260401-035608
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20260331111240.153913-1-elver%40google.com
> patch subject: [PATCH v1] slab: support for compiler-assisted type-based slab cache partitioning
> config: um-randconfig-r072-20260401
> compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
> smatch: v0.5.0-9004-gb810ac53
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Reported-by: Dan Carpenter <error27@gmail.com>
> | Closes: https://lore.kernel.org/r/202604020400.jEq32K95-lkp@intel.com/
>
> New smatch warnings:
> drivers/misc/lkdtm/heap.c:118 lkdtm_READ_AFTER_FREE() warn: potential pointer math issue ('base' is a 32 bit pointer)
> drivers/misc/lkdtm/heap.c:169 lkdtm_KFENCE_READ_AFTER_FREE() warn: potential pointer math issue ('base' is a 32 bit pointer)

How is this related to the patch I sent? Did the <linux/slab.h> change
force rechecking of all these files and it found latent issues?

> vim +118 drivers/misc/lkdtm/heap.c
>
> 73f62e60d80c2d drivers/misc/lkdtm/heap.c Kees Cook    2022-03-03   92  static void lkdtm_READ_AFTER_FREE(void)
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26   93  {
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26   94   int *base, *val, saw;
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26   95   size_t len = 1024;
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26   96   /*
> e12145cf1c3a80 drivers/misc/lkdtm/heap.c Kees Cook    2020-06-25   97    * The slub allocator will use the either the first word or
> e12145cf1c3a80 drivers/misc/lkdtm/heap.c Kees Cook    2020-06-25   98    * the middle of the allocation to store the free pointer,
> e12145cf1c3a80 drivers/misc/lkdtm/heap.c Kees Cook    2020-06-25   99    * depending on configurations. Store in the second word to
> e12145cf1c3a80 drivers/misc/lkdtm/heap.c Kees Cook    2020-06-25  100    * avoid running into the freelist.
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  101    */
> e12145cf1c3a80 drivers/misc/lkdtm/heap.c Kees Cook    2020-06-25  102   size_t offset = sizeof(*base);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  103
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  104   base = kmalloc(len, GFP_KERNEL);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  105   if (!base) {
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  106           pr_info("Unable to allocate base memory.\n");
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  107           return;
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  108   }
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  109
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  110   val = kmalloc(len, GFP_KERNEL);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  111   if (!val) {
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  112           pr_info("Unable to allocate val memory.\n");
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  113           kfree(base);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  114           return;
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  115   }
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  116
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  117   *val = 0x12345678;
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26 @118   base[offset] = *val;
>
> This doesn't really matter, but the comment says we are writing to the
> second word.  The base[] array holds 256 integers, so:
>
>         base[sizeof(int)] = *val;
>
> would be the third word, right?  A word is unsigned long, right?  All
> of a sudden I am unsure.  :P

???

So this is clearly a minor defect in this existing code (comment wrong
or code might want to match what the comment said), but "slab: support
for compiler-assisted type-based slab cache partitioning" didn't touch
that.

> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  119   pr_info("Value in memory before free: %x\n", base[offset]);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  120
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  121   kfree(base);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  122
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  123   pr_info("Attempting bad read from freed memory\n");
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  124   saw = base[offset];
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  125   if (saw != *val) {
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  126           /* Good! Poisoning happened, so declare a win. */
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  127           pr_info("Memory correctly poisoned (%x)\n", saw);
> 5b777131bd8005 drivers/misc/lkdtm/heap.c Kees Cook    2021-06-23  128   } else {
> 5b777131bd8005 drivers/misc/lkdtm/heap.c Kees Cook    2021-06-23  129           pr_err("FAIL: Memory was not poisoned!\n");
> 5b777131bd8005 drivers/misc/lkdtm/heap.c Kees Cook    2021-06-23  130           pr_expected_config_param(CONFIG_INIT_ON_FREE_DEFAULT_ON, "init_on_free");
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  131   }
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  132
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  133   kfree(val);
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  134  }
> ffc514f3fcac4a drivers/misc/lkdtm_heap.c Kees Cook    2016-06-26  135
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  136  static void lkdtm_KFENCE_READ_AFTER_FREE(void)
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  137  {
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  138   int *base, val, saw;
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  139   unsigned long timeout, resched_after;
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  140   size_t len = 1024;
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  141   /*
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  142    * The slub allocator will use the either the first word or
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  143    * the middle of the allocation to store the free pointer,
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  144    * depending on configurations. Store in the second word to
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  145    * avoid running into the freelist.
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  146    */
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  147   size_t offset = sizeof(*base);
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  148
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  149   /*
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  150    * 100x the sample interval should be more than enough to ensure we get
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  151    * a KFENCE allocation eventually.
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  152    */
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  153   timeout = jiffies + msecs_to_jiffies(100 * kfence_sample_interval);
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  154   /*
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  155    * Especially for non-preemption kernels, ensure the allocation-gate
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  156    * timer can catch up: after @resched_after, every failed allocation
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  157    * attempt yields, to ensure the allocation-gate timer is scheduled.
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  158    */
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  159   resched_after = jiffies + msecs_to_jiffies(kfence_sample_interval);
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  160   do {
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  161           base = kmalloc(len, GFP_KERNEL);
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  162           if (!base) {
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  163                   pr_err("FAIL: Unable to allocate kfence memory!\n");
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  164                   return;
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  165           }
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  166
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  167           if (is_kfence_address(base)) {
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29  168                   val = 0x12345678;
> aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 @169                   base[offset] = val;
>                                                                                         ^^^^^^^^^^^^^^^^^^
> Same here.

Unrelated to "slab: support for compiler-assisted type-based slab
cache partitioning".