Re: [PATCH bpf-next v3 1/2] bpf: Do not include stack ptr register in precision backtracking bookkeeping

[Eduard Zingerman <eddyz87@gmail.com>]

Yonghong Song <yonghong.song@linux.dev> writes:

[...]

> Let us say that we remove the code
>
> +    if (!src_reg) {
> +        if (linked_regs)
> +            return push_insn_history(env, state, 0, linked_regs);
> +        return 0;
> +    }
>
> The code should still work. But we might end up with more unnecessary
> jump history entries. For example,
>    dreg->type == PTR_TO_STACK, sreg is faked (i.e., BPF_SRC(insn->code) == BPF_K)
>    and linked_regs = 0
>
> In this particular, we will still generate a jump table entry which
> is not used in backtrack_insn().

If linked_regs set is not empty the push_insn_history() is necessary
even if src_reg is fake. Linked registers are handled at entry and at
exit from backtrack_insn() outside of instruction pattern match if-else
block.