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169c31761c
This patch adds a .gen_epilogue to the bpf_verifier_ops. It is similar to the existing .gen_prologue. Instead of allowing a subsystem to run code at the beginning of a bpf prog, it allows the subsystem to run code just before the bpf prog exit. One of the use case is to allow the upcoming bpf qdisc to ensure that the skb->dev is the same as the qdisc->dev_queue->dev. The bpf qdisc struct_ops implementation could either fix it up or drop the skb. Another use case could be in bpf_tcp_ca.c to enforce snd_cwnd has sane value (e.g. non zero). The epilogue can do the useful thing (like checking skb->dev) if it can access the bpf prog's ctx. Unlike prologue, r1 may not hold the ctx pointer. This patch saves the r1 in the stack if the .gen_epilogue has returned some instructions in the "epilogue_buf". The existing .gen_prologue is done in convert_ctx_accesses(). The new .gen_epilogue is done in the convert_ctx_accesses() also. When it sees the (BPF_JMP | BPF_EXIT) instruction, it will be patched with the earlier generated "epilogue_buf". The epilogue patching is only done for the main prog. Only one epilogue will be patched to the main program. When the bpf prog has multiple BPF_EXIT instructions, a BPF_JA is used to goto the earlier patched epilogue. Majority of the archs support (BPF_JMP32 | BPF_JA): x86, arm, s390, risv64, loongarch, powerpc and arc. This patch keeps it simple and always use (BPF_JMP32 | BPF_JA). A new macro BPF_JMP32_A is added to generate the (BPF_JMP32 | BPF_JA) insn. Acked-by: Eduard Zingerman <eddyz87@gmail.com> Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org> Link: https://lore.kernel.org/r/20240829210833.388152-4-martin.lau@linux.dev Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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