[PATCH] powerpc/kasan: require memintrinsic prefix support for KASAN
Christophe Leroy (CS GROUP)
chleroy at kernel.org
Sat Sep 12 11:53:16 PDT 2026
Le 12/09/2026 à 19:35, Christophe Leroy (CS GROUP) a écrit :
> Hi Mukesh,
>
> Le 11/09/2026 à 19:39, Mukesh Kumar Chaurasiya a écrit :
>>
>> [...]
>>>> diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/
>>>> cputable.c
>>>> index 6f6801da9dc1..44115f904c2c 100644
>>>> --- a/arch/powerpc/kernel/cputable.c
>>>> +++ b/arch/powerpc/kernel/cputable.c
>>>> @@ -36,8 +36,8 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
>>>> t = PTRRELOC(t);
>>>> /*
>>>> - * use memcpy() instead of *t = *s so that GCC replaces it
>>>> - * by __memcpy() when KASAN is active
>>>> + * use memcpy() instead of *t = *s so that the compiler
>>>> replaces it
>>>> + * by __asan_memcpy() when KASAN is active
>>>> */
>>>
>>> Does the initial problem still exist with the new __asan_memcpy()
>>> approach ?
>>> If not the comment should be removed.
>>>
>> Hey Christophe,
>>
>> Thanks for pointing it out, i took a deeper look into this, here's my
>> understanding on it.
>>
>> On PowerPC during very early boot the kernel is loaded by the
>> bootloader/firmware at some physical address, but the kernel was linked
>> expecting it to run at KERNELBASE(virtual address like
>> 0xc000000000000000). The MMU mapping that makes that virtual address
>> valid hasn't been set up yet. So far for a window of early boot, code is
>> executing at the physical load address while all symbol addresses in the
>> binary refer to the virtual linked address. reloc_offset() computes the
>> gap between these two and PTRRELOC applies it to any pointer.
>>
>> So PTRRELOC(&the_cpu_spec) gives the physical address where the struct
>> actually lives in memory right now, not where the linker thinks it lives.
>>
>> Why *t = *s would be wrong?
>>
>> In set_cur_cpu_spec:
>>
>> struct cpu_spec *t = &the_cpu_spec; // linked (virtual) address
>> t = PTRRELOC(t); // physical address — where it
>> actually is
>> memcpy(t, s, sizeof(*t)); // copy into the right place
>>
>> If you wrote *t = *s instead, the compiler generates a struct assignment.
>> For a large struct like cpu_spec, GCC is free to implement that however
>> it likes — including emitting a call to memcpy(). But crucially, a
>> compiler-generated memcpy call resolves through the GOT/PLT or direct
>> symbol — which points to the linked virtual address of memcpy, not the
>> physical address. At this point in boot, calling through the wrong
>> address would jump to garbage or an unmapped page.
>>
>> memcpy(t, s, sizeof(*t)) written explicitly is different: t is already
>> the corrected physical address, s points into the cpu_specs table which
>> has also been PTRRELOC'd. The explicit call goes through the normal
>> early-boot call mechanism which is safe.
>>
>> The original comment said:
>>
>> "use memcpy() instead of *t = *s so that GCC replaces it by __memcpy()
>> when KASAN is active"
>>
>> This was added because under the old KASAN scheme
>> (!CC_HAS_KASAN_MEMINTRINSIC_PREFIX), KASAN overrode the memset/memcpy
>> linker symbols globally with C wrappers that called kasan_check_range().
>> If the compiler turned *t = *s into an implicit memcpy(), that would hit
>> the KASAN wrapper — calling kasan_check_range() at a point in early boot
>> where the KASAN shadow isn't mapped yet, causing a crash.
>>
>> Writing memcpy(t, s, sizeof(*t)) explicitly made GCC emit __memcpy()
>> (the raw assembly alias exposed by _GLOBAL_KASAN) instead of the
>> KASAN-wrapped memcpy(), bypassing the shadow check.
>>
>> That was the secondary reason. The primary reason that t is a
>> PTRRELOC-adjusted physical pointer and the copy must go through it
>> correctly was never stated.
>>
>> So the KASAN comment is not required but i think we still need to state
>> why memcpy is required. For PTRRELOC adjustment, comment should reflect
>> that.
>>
>> I'll update the comment and commit message and send out a new version.
>
> Explanation based on kernel v5.10
>
> The problem was not linked to PTRRELOC, the t = PTRRELOC(t) followed by
> *t = *s works well in term of adressing, regardless of whether
> CONFIG_KASAN is enabled or not.
>
> The problem is that with *t = *s, gcc emits a call to memcpy(). When
> CONFIG_KASAN is enabled, memcpy() is instrumented. But we don't want
> cputable.o instrumented as we have KASAN_SANITIZE_cputable.o := n in
> Makefile.
>
> In asm/string.h we have:
>
> #if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
> /*
> * For files that are not instrumented (e.g. mm/slub.c) we
> * should use not instrumented version of mem* functions.
> */
> #define memcpy(dst, src, len) __memcpy(dst, src, len)
> #define memmove(dst, src, len) __memmove(dst, src, len)
> #define memset(s, c, n) __memset(s, c, n)
>
> Because in non-instrumented files like cputable.o we want memcpy() to be
> replaced at buildtime by __memcpy() to skip KASAN instrumentation. But
> this is resolved by pre-processing, and pre-processor doesn't know that
> the compiler will emit a call to memcpy().
>
> By replacing *t = *s by the memcpy(), the pre-processor replaces
> memcpy() by __memcpy() when CONFIG_KASAN is enabled.
>
> See the difference:
>
> This is v5.10
>
> 00000000 <set_cur_cpu_spec>:
> 0: 94 21 ff e0 stwu r1,-32(r1)
> 4: 7c 69 1b 78 mr r9,r3
> 8: bf c1 00 18 stmw r30,24(r1)
> c: 3f e0 00 00 lis r31,0
> e: R_PPC_ADDR16_HA .data..read_mostly
> 10: 3b ff 00 00 addi r31,r31,0
> 12: R_PPC_ADDR16_LO .data..read_mostly
> 14: 7c 08 02 a6 mflr r0
> 18: 7d 3e 4b 78 mr r30,r9
> 1c: 7f e3 fb 78 mr r3,r31
> 20: 90 01 00 24 stw r0,36(r1)
> 24: 48 00 00 01 bl 24 <set_cur_cpu_spec+0x24>
> 24: R_PPC_REL24 add_reloc_offset
> 28: 7f c4 f3 78 mr r4,r30
> 2c: 38 a0 00 58 li r5,88
> 30: 48 00 00 01 bl 30 <set_cur_cpu_spec+0x30>
> 30: R_PPC_REL24 __memcpy
> 34: 38 7f 00 58 addi r3,r31,88
> 38: 48 00 00 01 bl 38 <set_cur_cpu_spec+0x38>
> 38: R_PPC_REL24 add_reloc_offset
> 3c: 93 e3 00 00 stw r31,0(r3)
> 40: 80 01 00 24 lwz r0,36(r1)
> 44: 83 c1 00 18 lwz r30,24(r1)
> 48: 83 e1 00 1c lwz r31,28(r1)
> 4c: 7c 08 03 a6 mtlr r0
> 50: 38 21 00 20 addi r1,r1,32
> 54: 4e 80 00 20 blr
>
> This is v5.10 with commit adcf59187e270 reverted:
>
> 00000000 <set_cur_cpu_spec>:
> 0: 94 21 ff e0 stwu r1,-32(r1)
> 4: 7c 69 1b 78 mr r9,r3
> 8: bf c1 00 18 stmw r30,24(r1)
> c: 3f e0 00 00 lis r31,0
> e: R_PPC_ADDR16_HA .data..read_mostly
> 10: 3b ff 00 00 addi r31,r31,0
> 12: R_PPC_ADDR16_LO .data..read_mostly
> 14: 7c 08 02 a6 mflr r0
> 18: 7d 3e 4b 78 mr r30,r9
> 1c: 7f e3 fb 78 mr r3,r31
> 20: 90 01 00 24 stw r0,36(r1)
> 24: 48 00 00 01 bl 24 <set_cur_cpu_spec+0x24>
> 24: R_PPC_REL24 add_reloc_offset
> 28: 7f c4 f3 78 mr r4,r30
> 2c: 38 a0 00 58 li r5,88
> 30: 48 00 00 01 bl 30 <set_cur_cpu_spec+0x30>
> 30: R_PPC_REL24 memcpy
> 34: 38 7f 00 58 addi r3,r31,88
> 38: 48 00 00 01 bl 38 <set_cur_cpu_spec+0x38>
> 38: R_PPC_REL24 add_reloc_offset
> 3c: 93 e3 00 00 stw r31,0(r3)
> 40: 80 01 00 24 lwz r0,36(r1)
> 44: 83 c1 00 18 lwz r30,24(r1)
> 48: 83 e1 00 1c lwz r31,28(r1)
> 4c: 7c 08 03 a6 mtlr r0
> 50: 38 21 00 20 addi r1,r1,32
> 54: 4e 80 00 20 blr
>
> So my question is ? Do we still have this issue nowadays ?
I now did the same test with v7.2 without and with adcf59187e270
reverted. I both cases I get memcpy().
Christophe
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