Messages in this thread | | | From | Bo Yan <> | Subject | a question about IP checksum helper for arm64 | Date | Fri, 6 Jul 2018 09:27:42 -0700 |
| |
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
| |