netdev - Re: [PATCH v2 bpf-next] libbpf: support CO-RE relocations for LDX/ST/STX instructions

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Date:   Mon, 23 Dec 2019 18:10:36 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andriin@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>
CC:     "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next] libbpf: support CO-RE relocations for
 LDX/ST/STX instructions



On 12/23/19 10:03 AM, Andrii Nakryiko wrote:
> Clang patch [0] enables emitting relocatable generic ALU/ALU64 instructions
> (i.e, shifts and arithmetic operations), as well as generic load/store
> instructions. The former ones are already supported by libbpf as is. This
> patch adds further support for load/store instructions. Relocatable field
> offset is encoded in BPF instruction's 16-bit offset section and are adjusted
> by libbpf based on target kernel BTF.
> 
> These Clang changes and corresponding libbpf changes allow for more succinct
> generated BPF code by encoding relocatable field reads as a single
> ST/LDX/STX instruction. It also enables relocatable access to BPF context.
> Previously, if context struct (e.g., __sk_buff) was accessed with CO-RE
> relocations (e.g., due to preserve_access_index attribute), it would be
> rejected by BPF verifier due to modified context pointer dereference. With
> Clang patch, such context accesses are both relocatable and have a fixed
> offset from the point of view of BPF verifier.
> 
>    [0] https://reviews.llvm.org/D71790
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>
Acked-by: Yonghong Song <yhs@...com>