[Bug 1035] __rte_raw_cksum() crash with misaligned pointer

[Emil Berg]

I've been sketching on an efficient solution to this. What about something along the way below? I've run it with the combinations of:
even buf, even len
even buf, odd len
odd buf, even len
odd buf, odd len

and it seems to give the same results as the older version of __rte_raw_cksum, before 21.03. I ran it without optimizations and such to ensure the compiler didn't insert vector instructions and such so the results were comparable.

static inline uint32_t
__rte_raw_cksum_newest(const void *buf, size_t len, uint32_t sum)
{
	const uint8_t *end = buf + len;

	uint32_t sum_even = 0;
	for (const uint8_t *p = buf + 1; p < end; p += 2) {
		sum_even += *p;
	}
	sum += sum_even << 8;

	uint32_t sum_odd = 0;
	for (const uint8_t *p = buf; p < end; p += 2) {
		sum_odd += *p;
	}
	sum += sum_odd;

	return sum;
}

/Emil

-----Original Message-----
From: Emil Berg 
Sent: den 16 juni 2022 07:45
To: Morten Brørup <mb at smartsharesystems.com>; bugzilla at dpdk.org
Cc: dev at dpdk.org
Subject: RE: [Bug 1035] __rte_raw_cksum() crash with misaligned pointer

Hi!

We want the B option, i.e. the 6 bytes option. Perhaps adding alignment detection to __rte_raw_cksum() is a good idea.

A minor comment but I think buf & 1 won't work since buf isn't an integral type, but something along that way.

I'm starting to think about an efficient way to do this.

Thank you!

-----Original Message-----
From: Morten Brørup <mb at smartsharesystems.com>
Sent: den 15 juni 2022 16:41
To: Emil Berg <emil.berg at ericsson.com>; bugzilla at dpdk.org
Cc: dev at dpdk.org
Subject: RE: [Bug 1035] __rte_raw_cksum() crash with misaligned pointer

> From: bugzilla at dpdk.org [mailto:bugzilla at dpdk.org]
> Sent: Wednesday, 15 June 2022 09.16
> 
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-45444
> 5555731-2e92ae6bf759c0c5&q=1&e=b3fc70af-5d37-4ffb-b34d-9a51927f5f6d&u=
> https%3A%2F%2Fbugs.dpdk.org%2Fshow_bug.cgi%3Fid%3D1035
> 
>             Bug ID: 1035
>            Summary: __rte_raw_cksum() crash with misaligned pointer
>            Product: DPDK
>            Version: 21.11
>           Hardware: All
>                 OS: All
>             Status: UNCONFIRMED
>           Severity: normal
>           Priority: Normal
>          Component: ethdev
>           Assignee: dev at dpdk.org
>           Reporter: emil.berg at ericsson.com
>   Target Milestone: ---
> 
> See rte_raw_cksum() in rte_ip.h, which is part of the public API. See 
> also the subfunction __rte_raw_cksum().
> 
> _rte_raw_cksum assumes that the buffer over which the checksum is 
> calculated is an even address (divisible by two). See for example this 
> stack overflow
> post:
> https://stackoverflow.com/questions/46790550/c-undefined-behavior-
> strict-aliasing-rule-or-incorrect-alignment
> 
> The post explains that there is undefined behavior in C11 when 
> "conversion between two pointer types produces a result that is 
> incorrectly aligned". When the buf argument starts on an odd address 
> we thus have undefined behavior, since a pointer is cast from void* to 
> uint16_t*.
> 
> In most cases (at least on x86) that isn't a problem, but with higher 
> optimization levels it may break due to vector instructions. This new 
> function seems to be easier to optimize by the compiler, resulting in 
> a crash when the buf argument is odd. Please note that the undefined 
> behavior is present in earlier versions of dpdk as well.
> 
> Now you're probably thinking: "Just align your buffers". The problem 
> is that we have a packet buffer which is aligned. The checksum is 
> calculated on a subset of that aligned packet buffer, and that 
> sometimes lies on odd addresses.
> 
> The question remains if this is an issue with dpdk or not.

I can imagine other systems doing what you describe too. So it needs to be addressed.

Off the top of my head, an easy fix would be updating __rte_raw_cksum() like this:

static inline uint32_t
__rte_raw_cksum(const void *buf, size_t len, uint32_t sum) {
	if (likely((buf & 1) == 0)) {
		/* The buffer is 16 bit aligned. */
		Keep the existing, optimized implementation here.
	} else {
		/* The buffer is not 16 bit aligned. */
		Add a new odd-buf tolerant implementation here.
	}
}

However, I'm not sure that it covers your scenario!

The checksum is 16 bit wide, so if you calculate the checksum of e.g. 4 bytes of memory starting at offset 1 in a 6 byte packet buffer, the memory block can be treated as either 4 or 6 bytes relative to the data covered by the checksum, i.e.:

A: XX [01 02] [03 04] XX --> cksum = [04 06]

B: [XX 01] [02 03] [04 XX] --> cksum = [06 04]

Which one do you need?

Perhaps an additional function is required to support your use case, and the documentation for rte_raw_cksum() and __rte_raw_cksum() needs to reflect that the buffer must be 16 bit aligned.

Or the rte_raw_cksum() function can be modified to support an odd buffer pointer as outlined above, with documentation added about alignment of the running checksum.