Re: a question about IP checksum helper for arm64

[Bo Yan <byan@nvidia.com>]

Hi Robin,

That UBSAN error prompted me to check the generated instructions. The 
error by itself doesn't make sense to me because there is no requirement 
for 128b alignment on ldp/stp.

With 4.18-rc3, when I build for the default "defconfig" in 
arch/arm64/configs/, I see the disassembled code shows two ldr instead 
of a ldp.

One example is in function "ip_rcv" (/net/ipv4/ip_input.c). After 
disassembling vmlinux, I see following:

	if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
ffff0000089bcdb8:	38776b05 	ldrb	w5, [x24,x23]
static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
{
	__uint128_t tmp;
	u64 sum;

	tmp = *(const __uint128_t *)iph;
ffff0000089bcdbc:	f9400481 	ldr	x1, [x4,#8]
ffff0000089bcdc0:	f8776b00 	ldr	x0, [x24,x23]
ffff0000089bcdc4:	12000ca5 	and	w5, w5, #0xf
ffff0000089bcdc8:	510014a3 	sub	w3, w5, #0x5

This is done with "make ARCH=arm64 CROSS_COMPILE=... defconfig", so the 
default optimization level is -O2.

I tried the same test as you did: aarch64-linux-gnu-objdump -S -d *.o 
in net/ipv4. The result is inconsistent. In some instances, I do see ldp 
instruction being generated, in some other cases, it's two ldr. For 
example, in inet_gro_receive and ip_mc_check_igmp, it's compiled as I 
expected. For ip_rcv, it's not.

So it looks like this is not very consistent, but it also looks like in 
majority of cases it generates the ldp instructions.



On 07/09/2018 04:54 AM, Robin Murphy wrote:
> 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