On Tue, Feb 22, 2022 at 12:23:49PM IST, Alexei Starovoitov wrote:
> On Sun, Feb 20, 2022 at 07:18:02PM +0530, Kumar Kartikeya Dwivedi wrote:
> > static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regno,
> > int off, int bpf_size, enum bpf_access_type t,
> > - int value_regno, bool strict_alignment_once)
> > + int value_regno, bool strict_alignment_once,
> > + struct bpf_reg_state *atomic_load_reg)
>
> No new side effects please.
> value_regno is not pretty already.
> At least its known ugliness that we need to clean up one day.
>
> > static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn)
> > {
> > + struct bpf_reg_state atomic_load_reg;
> > int load_reg;
> > int err;
> >
> > + __mark_reg_unknown(env, &atomic_load_reg);
> > +
> > switch (insn->imm) {
> > case BPF_ADD:
> > case BPF_ADD | BPF_FETCH:
> > @@ -4813,6 +4894,7 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> > else
> > load_reg = insn->src_reg;
> >
> > + atomic_load_reg = *reg_state(env, load_reg);
> > /* check and record load of old value */
> > err = check_reg_arg(env, load_reg, DST_OP);
> > if (err)
> > @@ -4825,20 +4907,21 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> > }
> >
> > /* Check whether we can read the memory, with second call for fetch
> > - * case to simulate the register fill.
> > + * case to simulate the register fill, which also triggers checks
> > + * for manipulation of BTF ID pointers embedded in BPF maps.
> > */
> > err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> > - BPF_SIZE(insn->code), BPF_READ, -1, true);
> > + BPF_SIZE(insn->code), BPF_READ, -1, true, NULL);
> > if (!err && load_reg >= 0)
> > err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> > BPF_SIZE(insn->code), BPF_READ, load_reg,
> > - true);
> > + true, load_reg >= 0 ? &atomic_load_reg : NULL);
>
> Special xchg logic should be down outside of check_mem_access()
> instead of hidden by layers of calls.
Right, it's ugly, but if we don't capture the reg state before that
check_reg_arg(env, load_reg, DST_OP), it's not possible to see the actual
PTR_TO_BTF_ID being moved into the map, since check_reg_arg will do a
mark_reg_unknown for value_regno. Any other ideas on what I can do?
37086bfdc737 ("bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH")
changed the order of check_mem_access and DST_OP check_reg_arg.
--
Kartikeya
