Re: [PATCH bpf-next v3 1/4] bpf: Adapt 32-bit return value kfunc for 32-bit ARM when zext extension

[Alexei Starovoitov]

On Sat, Nov 26, 2022 at 05:45:27PM +0800, Yang Jihong wrote:
> For ARM32 architecture, if data width of kfunc return value is 32 bits,
> need to do explicit zero extension for high 32-bit, insn_def_regno should
> return dst_reg for BPF_JMP type of BPF_PSEUDO_KFUNC_CALL. Otherwise,
> opt_subreg_zext_lo32_rnd_hi32 returns -EFAULT, resulting in BPF failure.
> 
> Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> ---
>  kernel/bpf/verifier.c | 44 ++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 264b3dc714cc..193ea927aa69 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1927,6 +1927,21 @@ find_kfunc_desc(const struct bpf_prog *prog, u32 func_id, u16 offset)
>  		       sizeof(tab->descs[0]), kfunc_desc_cmp_by_id_off);
>  }
>  
> +static int kfunc_desc_cmp_by_imm(const void *a, const void *b);
> +
> +static const struct bpf_kfunc_desc *
> +find_kfunc_desc_by_imm(const struct bpf_prog *prog, s32 imm)
> +{
> +	struct bpf_kfunc_desc desc = {
> +		.imm = imm,
> +	};
> +	struct bpf_kfunc_desc_tab *tab;
> +
> +	tab = prog->aux->kfunc_tab;
> +	return bsearch(&desc, tab->descs, tab->nr_descs,
> +		       sizeof(tab->descs[0]), kfunc_desc_cmp_by_imm);
> +}
> +
>  static struct btf *__find_kfunc_desc_btf(struct bpf_verifier_env *env,
>  					 s16 offset)
>  {
> @@ -2342,6 +2357,13 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  			 */
>  			if (insn->src_reg == BPF_PSEUDO_CALL)
>  				return false;
> +
> +			/* Kfunc call will reach here because of insn_has_def32,
> +			 * conservatively return TRUE.
> +			 */
> +			if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL)
> +				return true;
> +
>  			/* Helper call will reach here because of arg type
>  			 * check, conservatively return TRUE.
>  			 */
> @@ -2405,10 +2427,26 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  }
>  
>  /* Return the regno defined by the insn, or -1. */
> -static int insn_def_regno(const struct bpf_insn *insn)
> +static int insn_def_regno(struct bpf_verifier_env *env, const struct bpf_insn *insn)
>  {
>  	switch (BPF_CLASS(insn->code)) {
>  	case BPF_JMP:
> +		if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {
> +			const struct bpf_kfunc_desc *desc;
> +
> +			/* The value of desc cannot be NULL */
> +			desc = find_kfunc_desc_by_imm(env->prog, insn->imm);
> +
> +			/* A kfunc can return void.
> +			 * The btf type of the kfunc's return value needs
> +			 * to be checked against "void" first
> +			 */
> +			if (desc->func_model.ret_size == 0)
> +				return -1;
> +			else
> +				return insn->dst_reg;
> +		}
> +		fallthrough;

I cannot make any sense of this patch.
insn->dst_reg above is 0.
The kfunc call doesn't define a register from insn_def_regno() pov.

Are you hacking insn_def_regno() to return 0 so that
if (WARN_ON(load_reg == -1)) {
  verbose(env, "verifier bug. zext_dst is set, but no reg is defined\n");
  return -EFAULT;
}
in opt_subreg_zext_lo32_rnd_hi32() doesn't trigger ?

But this verifier message should have been a hint that you need
to analyze why zext_dst is set on this kfunc call.
Maybe it shouldn't ?
Did you analyze the logic of mark_btf_func_reg_size() ?

Before producing any patches please understand the logic fully.
Your commit log
"insn_def_regno should
 return dst_reg for BPF_JMP type of BPF_PSEUDO_KFUNC_CALL."

Makes no sense to me, since dst_reg is unused in JMP insn.
There is no concept of a src or dst register in a JMP insn.

32-bit x86 supports calling kfuncs. See emit_kfunc_call().
And we don't have this "verifier bug. zext_dst is set" issue there, right?
But what you're saying in the commit log:
"if data width of kfunc return value is 32 bits"
should have been applicable to x86-32 as well.
So please start with a test that demonstrates the issue on x86-32 and
then we can discuss the way to fix it.

The patch 2 sort-of makes sense.

For patch 3 pls add new test funcs to bpf_testmod.
We will move all of them from net/bpf/test_run.c to bpf_testmod eventually.