Re: [PATCH bpf-next 1/3] bpf: Support inlining bpf_kptr_xchg() helper

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2023-12-19 at 21:56 +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
[...]
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9456ee0ad129..7814c4f7576e 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -19668,6 +19668,23 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  			continue;
>  		}
>  
> +		/* Implement bpf_kptr_xchg inline */
> +		if (prog->jit_requested && BITS_PER_LONG == 64 &&
> +		    insn->imm == BPF_FUNC_kptr_xchg &&
> +		    bpf_jit_supports_ptr_xchg()) {
> +			insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_2);
> +			insn_buf[1] = BPF_ATOMIC_OP(BPF_DW, BPF_XCHG, BPF_REG_1, BPF_REG_0, 0);
> +			cnt = 2;
> +
> +			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> +			if (!new_prog)
> +				return -ENOMEM;
> +
> +			delta    += cnt - 1;
> +			env->prog = prog = new_prog;
> +			insn      = new_prog->insnsi + i + delta;
> +			continue;
> +		}
>  patch_call_imm:
>  		fn = env->ops->get_func_proto(insn->imm, env->prog);
>  		/* all functions that have prototype and verifier allowed

Hi Hou,

I have a suggestion about testing this rewrite.
It is possible to use function get_xlated_program() from
tools/testing/selftests/bpf/prog_tests/ctx_rewrite.c,
to obtain a BPF disassembly for the program after
do_misc_fixups() are applied.

So, it shouldn't be difficult to:
- prepare a dummy program in progs/ that uses bpf_kptr_xchg();
- prepare a new test_* function in prog_tests/ that:
  - loads that dummy program;
  - queries it's disassembly using get_xlated_program();
  - compares it with expected template.

I know that do_misc_fixups() are usually not tested this way,
but that does not mean they shouldn't, wdyt?

Thanks,
Eduard