Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> The llvm register allocator may use two different registers representing the
> same virtual register. In such case the following pattern can be observed:
> 1047: (bf) r9 = r6
> 1048: (a5) if r6 < 0x1000 goto pc+1
> 1050: ...
> 1051: (a5) if r9 < 0x2 goto pc+66
> 1052: ...
> 1053: (bf) r2 = r9 /* r2 needs to have upper and lower bounds */
>
> In order to track this information without backtracking allocate ID
> for scalars in a similar way as it's done for find_good_pkt_pointers().
>
> When the verifier encounters r9 = r6 assignment it will assign the same ID
> to both registers. Later if either register range is narrowed via conditional
> jump propagate the register state into the other register.
>
> Clear register ID in adjust_reg_min_max_vals() for any alu instruction.
Do we also need to clear the register ID on reg0 for CALL ops into a
helper?
Looks like check_helper_call might mark reg0 as a scalar, but I don't
see where it would clear the reg->id? Did I miss it. Either way maybe
a comment here would help make it obvious how CALLs are handled?
Thanks,
John
>
> Newly allocated register ID is ignored for scalars in regsafe() and doesn't
> affect state pruning. mark_reg_unknown() also clears the ID.
>
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 38 +++++++++++++++++++
> .../testing/selftests/bpf/prog_tests/align.c | 16 ++++----
> .../bpf/verifier/direct_packet_access.c | 2 +-
> 3 files changed, 47 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 01120acab09a..09e17b483b0b 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6432,6 +6432,8 @@ static int adjust_reg_min_max_vals(struct bpf_verifier_env *env,
> src_reg = NULL;
> if (dst_reg->type != SCALAR_VALUE)
> ptr_reg = dst_reg;
> + else
> + dst_reg->id = 0;
> if (BPF_SRC(insn->code) == BPF_X) {
> src_reg = ®s[insn->src_reg];
> if (src_reg->type != SCALAR_VALUE) {
> @@ -6565,6 +6567,8 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> /* case: R1 = R2
> * copy register state to dest reg
> */
> + if (src_reg->type == SCALAR_VALUE)
> + src_reg->id = ++env->id_gen;
> *dst_reg = *src_reg;
> dst_reg->live |= REG_LIVE_WRITTEN;
> dst_reg->subreg_def = DEF_NOT_SUBREG;
> @@ -7365,6 +7369,30 @@ static bool try_match_pkt_pointers(const struct bpf_insn *insn,
> return true;
> }
