> On 11 Apr 2019, at 04:13, Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx> wrote: > > On Wed, 10 Apr 2019 20:50:20 +0100, Jiong Wang wrote: >> @@ -2169,6 +2188,12 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn >> value_regno); >> if (reg_type_may_be_null(reg_type)) >> regs[value_regno].id = ++env->id_gen; >> + /* A load of ctx field could have different >> + * actual load size with the one encoded in the >> + * insn. When the dst is PTR, it is for sure not >> + * a sub-register. >> + */ >> + regs[value_regno].subreg_def = DEF_NOT_SUBREG; > > Can't the rewrite generate a 32bit load? E.g. reading skb->len will be > LDX | W, we still gotta clear the top bits in that case, no? No. for 32-bit load, I think the type won’t be ptr type. This override is gated by !SCALAR_VALUE. Regards, Jiong > > I can't find the explanation of this case with a quick scan of the code > and cover letter.. > >> } >> regs[value_regno].type = reg_type; >> } >
