Re: [PATCH] mm: kfence: Fix false positives on big endian

[Michael Ellerman]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> On Fri, 5 May 2023 16:02:17 +0000 David Laight <David.Laight@xxxxxxxxxx> wrote:
>
>> From: Michael Ellerman
>> > Sent: 05 May 2023 04:51
>> > 
>> > Since commit 1ba3cbf3ec3b ("mm: kfence: improve the performance of
>> > __kfence_alloc() and __kfence_free()"), kfence reports failures in
>> > random places at boot on big endian machines.
>> > 
>> > The problem is that the new KFENCE_CANARY_PATTERN_U64 encodes the
>> > address of each byte in its value, so it needs to be byte swapped on big
>> > endian machines.
>> > 
>> > The compiler is smart enough to do the le64_to_cpu() at compile time, so
>> > there is no runtime overhead.
>> > 
>> > Fixes: 1ba3cbf3ec3b ("mm: kfence: improve the performance of __kfence_alloc() and __kfence_free()")
>> > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> > ---
>> >  mm/kfence/kfence.h | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > 
>> > diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
>> > index 2aafc46a4aaf..392fb273e7bd 100644
>> > --- a/mm/kfence/kfence.h
>> > +++ b/mm/kfence/kfence.h
>> > @@ -29,7 +29,7 @@
>> >   * canary of every 8 bytes is the same. 64-bit memory can be filled and checked
>> >   * at a time instead of byte by byte to improve performance.
>> >   */
>> > -#define KFENCE_CANARY_PATTERN_U64 ((u64)0xaaaaaaaaaaaaaaaa ^ (u64)(0x0706050403020100))
>> > +#define KFENCE_CANARY_PATTERN_U64 ((u64)0xaaaaaaaaaaaaaaaa ^ (u64)(le64_to_cpu(0x0706050403020100)))
>> 
>> What at the (u64) casts for?
>> The constants should probably have a ul (or ull) suffix.
>> 
>
> I tried that, didn't fix the sparse warnings described at
> https://lkml.kernel.org/r/202305132244.DwzBUcUd-lkp@xxxxxxxxx.
>
> Michael, have you looked into this?

I haven't sorry, been chasing other bugs.

> I'll merge it upstream - I guess we can live with the warnings for a while.

Thanks, yeah spurious WARNs are more of a pain than some sparse warnings.

Maybe using le64_to_cpu() is too fancy, could just do it with an ifdef? eg.

diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
index 392fb273e7bd..510355a5382b 100644
--- a/mm/kfence/kfence.h
+++ b/mm/kfence/kfence.h
@@ -29,7 +29,11 @@
  * canary of every 8 bytes is the same. 64-bit memory can be filled and checked
  * at a time instead of byte by byte to improve performance.
  */
-#define KFENCE_CANARY_PATTERN_U64 ((u64)0xaaaaaaaaaaaaaaaa ^ (u64)(le64_to_cpu(0x0706050403020100)))
+#ifdef __LITTLE_ENDIAN__
+#define KFENCE_CANARY_PATTERN_U64 (0xaaaaaaaaaaaaaaaaULL ^ 0x0706050403020100ULL)
+#else
+#define KFENCE_CANARY_PATTERN_U64 (0xaaaaaaaaaaaaaaaaULL ^ 0x0001020304050607ULL)
+#endif
 
 /* Maximum stack depth for reports. */
 #define KFENCE_STACK_DEPTH 64


cheers