netdev - RE: [PATCH bpf-next 1/3] bpf: Propagate scalar ranges through register assignments.

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Message-ID: <5f7e52ce81308_1a83120890@john-XPS-13-9370.notmuch>
Date:   Wed, 07 Oct 2020 16:44:14 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        davem@...emloft.net
Cc:     daniel@...earbox.net, john.fastabend@...il.com,
        netdev@...r.kernel.org, bpf@...r.kernel.org, kernel-team@...com
Subject: RE: [PATCH bpf-next 1/3] bpf: Propagate scalar ranges through
 register assignments.

Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> 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@...nel.org>
> ---
>  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 = &regs[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;
>  }