[PATCH] powerpc/kasan: require memintrinsic prefix support for KASAN
Christophe Leroy (CS GROUP)
chleroy at kernel.org
Sat Sep 12 10:35:32 PDT 2026
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 ?
Christophe
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