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SubjectRe: [PATCH ipsec-next v1 6/7] bpf: selftests: test_tunnel: Disable CO-RE relocations
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On 11/26/23 3:14 PM, Eduard Zingerman wrote:
> On Sat, 2023-11-25 at 20:22 -0800, Yonghong Song wrote:
> [...]
>> --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
>> +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
>> @@ -6,7 +6,10 @@
>> * modify it under the terms of version 2 of the GNU General Public
>> * License as published by the Free Software Foundation.
>> */
>> -#define BPF_NO_PRESERVE_ACCESS_INDEX
>> +#if __has_attribute(preserve_static_offset)
>> +struct __attribute__((preserve_static_offset)) erspan_md2;
>> +struct __attribute__((preserve_static_offset)) erspan_metadata;
>> +#endif
>> #include "vmlinux.h"
> [...]
>> int bpf_skb_get_fou_encap(struct __sk_buff *skb_ctx,
>> @@ -174,9 +177,13 @@ int erspan_set_tunnel(struct __sk_buff *skb)
>> __u8 hwid = 7;
>>
>> md.version = 2;
>> +#if __has_attribute(preserve_static_offset)
>> md.u.md2.dir = direction;
>> md.u.md2.hwid = hwid & 0xf;
>> md.u.md2.hwid_upper = (hwid >> 4) & 0x3;
>> +#else
>> + /* Change bit-field store to byte(s)-level stores. */
>> +#endif
>> #endif
>>
>> ret = bpf_skb_set_tunnel_opt(skb, &md, sizeof(md));
>>
>> ====
>>
>> Eduard, could you double check whether this is a valid use case
>> to solve this kind of issue with preserve_static_offset attribute?
> Tbh I'm not sure. This test passes with preserve_static_offset
> because it suppresses preserve_access_index. In general clang
> translates bitfield access to a set of IR statements like:
>
> C:
> struct foo {
> unsigned _;
> unsigned a:1;
> ...
> };
> ... foo->a ...
>
> IR:
> %a = getelementptr inbounds %struct.foo, ptr %0, i32 0, i32 1
> %bf.load = load i8, ptr %a, align 4
> %bf.clear = and i8 %bf.load, 1
> %bf.cast = zext i8 %bf.clear to i32
>
> With preserve_static_offset the getelementptr+load are replaced by a
> single statement which is preserved as-is till code generation,
> thus load with align 4 is preserved.
>
> On the other hand, I'm not sure that clang guarantees that load or
> stores used for bitfield access would be always aligned according to
> verifier expectations.

I think it should be true. The frontend does alignment analysis based on
types and (packed vs. unpacked) and assign each load/store with proper
alignment (like 'align 4' in the above). 'align 4' truely means
the load itself is 4-byte aligned. Otherwise, it will be very confusing
for arch's which do not support unaligned memory access (e.g. BPF).

>
> I think we should check if there are some clang knobs that prevent
> generation of unaligned memory access. I'll take a look.


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