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

[Yang Jihong]

On 2022/11/28 9:57, Alexei Starovoitov wrote:
> 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() ?
make r0 zext is not caused by mark_btf_func_reg_size.

This problem occurs when running the kfunc_call_test_ref_btf_id test 
case in the 32-bit ARM environment.
The bpf prog is as follows:
int kfunc_call_test_ref_btf_id(struct __sk_buff *skb)
{
struct prog_test_ref_kfunc *pt;
unsigned long s = 0;
int ret = 0;

pt = bpf_kfunc_call_test_acquire(&s);
if (pt) {
     // here, do_check clears the upper 32bits of r0 through:
     // check_alu_op
     //   ->check_reg_arg
     //    ->mark_insn_zext
if (pt->a != 42 || pt->b != 108)
ret = -1;
bpf_kfunc_call_test_release(pt);
}
return ret;
}

> 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.
> .