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

From: Dan Carpenter

Date: Thu Apr 02 2026 - 09:41:07 EST

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@xxxxxxxxx>
| Reported-by: Dan Carpenter <error27@xxxxxxxxx>
| Closes: https://lore.kernel.org/r/202604020400.jEq32K95-lkp@xxxxxxxxx/

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)

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

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.

aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 170 pr_info("Value in memory before free: %x\n", base[offset]);
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 171
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 172 kfree(base);
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 173
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 174 pr_info("Attempting bad read from freed memory\n");
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 175 saw = base[offset];
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 176 if (saw != val) {
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 177 /* Good! Poisoning happened, so declare a win. */
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 178 pr_info("Memory correctly poisoned (%x)\n", saw);
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 179 } else {
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 180 pr_err("FAIL: Memory was not poisoned!\n");
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 181 pr_expected_config_param(CONFIG_INIT_ON_FREE_DEFAULT_ON, "init_on_free");
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 182 }
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 183 return;
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 184 }
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 185
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 186 kfree(base);
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 187 if (time_after(jiffies, resched_after))
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 188 cond_resched();
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 189 } while (time_before(jiffies, timeout));
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 190
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 191 pr_err("FAIL: kfence memory never allocated!\n");
aabf7c37dfbce3 drivers/misc/lkdtm/heap.c Stephen Boyd 2023-11-29 192 }

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