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SubjectRe: a question about IP checksum helper for arm64
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Hi Bo,

On 06/07/18 17:27, Bo Yan wrote:
> 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

What's your config and reproducer here? I've had UBSan enabled a few
times since that patch went in and never noticed anything. I've just
tried it with 4.18-rc3 and indeed don't see anything from just booting
the machine and making some network traffic. It does indeed fire if I
also turn on CONFIG_UBSAN_ALIGNMENT, but then it's almost lost among a
million other warnings for all manner of types - that's to be expected
since, as the help text says, "Enabling this option on architectures
that support unaligned accesses may produce a lot of false positives."

>
> 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.

My regular toolchain is currently Linaro 7.2.1-2017.11, but I also tried
the last GCC 6 I had installed (6.3.1-2017.05), and for both at -O2 I
see LDP emitted as expected for most of the identifiable int128 accesses
(both in a standalone test harness and a quick survey of kernel code via
'aarch64-linux-gnu-objdump -S net/ipv4/*.o'). Of course, there may well
be places where the compiler gets clever enough to elide all or part of
that load where data is already held in registers - I've not audited
*that* closely - but the whole point of having a pure C implementation
is that it can be aggressively inlined more than inline asm ever could.

> 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.

Yet AArch64 GCC definitely does support __uint128_t, or this code
wouldn't even build ;)

Robin.

>
> Thanks
>
> Bo

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