Re: a question about IP checksum helper for arm64

[Robin Murphy <robin.murphy@arm.com>]

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