linux-kernel - a question about IP checksum helper for arm64

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Date:   Fri, 6 Jul 2018 09:27:42 -0700
From:   Bo Yan <byan@...dia.com>
To:     <robin.murphy@....com>
CC:     <linux-kernel@...r.kernel.org>, <luke.starrett@...adcom.com>
Subject: a question about IP checksum helper for arm64

Hi Robin, Luke,

Recently I bumped into an error when running GCC undefined behavior 
sanitizer:

UBSAN: Undefined behaviour in 
kernel-4.9/arch/arm64/include/asm/checksum.h:34:6
        load of misaligned address ffffffc198c8b254 for type 'const 
__int128 unsigned'
        which requires 16 byte alignment

The relevant code:

         tmp = *(const __uint128_t *)iph;
         iph += 16;
         ihl -= 4;
         tmp += ((tmp >> 64) | (tmp << 64));
         sum = tmp >> 64;
         do {
                 sum += *(const u32 *)iph;
                 iph += 4;
         } while (--ihl);

But, I checked the generated disassembly, it doesn't look like anything 
special is generated taking advantage of that.

I'm using Linaro GCC 6.4-2017.08, expecting ldp instructions to be 
emitted, but don't see it.

There were some prior discussions about GCC behavior, like this thread: 
https://patchwork.kernel.org/patch/9081911/ , in which you talked about 
the difference between GCC4 and GCC5.3. It looks to me this is regressed 
in Linaro GCC6.4 build.

I have not checked newer GCC versions.

Will it be more stable to just do this with inline assembly instead of 
relying on __uint128_t data type?

GCC documentation says __int128 is supported for targets which have an 
integer mode wide enough to hold 128 bits. aarch64 doesn't have such an 
integer mode.

Thanks

Bo