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Date: Mon, 10 Dec 2018 09:45:30 +0000
From: Jiong Wang <jiong.wang@...ronome.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, ecree@...arflare.com,
netdev@...r.kernel.org, oss-drivers@...ronome.com
Subject: Re: [PATCH v2 bpf-next] bpf: relax verifier restriction on BPF_MOV |
BPF_ALU
On 09/12/2018 22:17, Alexei Starovoitov wrote:
> On Fri, Dec 07, 2018 at 12:16:18PM -0500, Jiong Wang wrote:
>> Currently, the destination register is marked as unknown for 32-bit
>> sub-register move (BPF_MOV | BPF_ALU) whenever the source register type is
>> SCALAR_VALUE.
>>
>> This is too conservative that some valid cases will be rejected.
>> Especially, this may turn a constant scalar value into unknown value that
>> could break some assumptions of verifier.
>>
>> For example, test_l4lb_noinline.c has the following C code:
>>
>> struct real_definition *dst
>>
>> 1: if (!get_packet_dst(&dst, &pckt, vip_info, is_ipv6))
>> 2: return TC_ACT_SHOT;
>> 3:
>> 4: if (dst->flags & F_IPV6) {
>>
>> get_packet_dst is responsible for initializing "dst" into valid pointer and
>> return true (1), otherwise return false (0). The compiled instruction
>> sequence using alu32 will be:
>>
>> 412: (54) (u32) r7 &= (u32) 1
>> 413: (bc) (u32) r0 = (u32) r7
>> 414: (95) exit
>>
>> insn 413, a BPF_MOV | BPF_ALU, however will turn r0 into unknown value even
>> r7 contains SCALAR_VALUE 1.
>>
>> This causes trouble when verifier is walking the code path that hasn't
>> initialized "dst" inside get_packet_dst, for which case 0 is returned and
>> we would then expect verifier concluding line 1 in the above C code pass
>> the "if" check, therefore would skip fall through path starting at line 4.
>> Now, because r0 returned from callee has became unknown value, so verifier
>> won't skip analyzing path starting at line 4 and "dst->flags" requires
>> dereferencing the pointer "dst" which actually hasn't be initialized for
>> this path.
>>
>> This patch relaxed the code marking sub-register move destination. For a
>> SCALAR_VALUE, it is safe to just copy the value from source then truncate
>> it into 32-bit.
>>
>> A unit test also included to demonstrate this issue. This test will fail
>> before this patch.
>>
>> This relaxation could let verifier skipping more paths for conditional
>> comparison against immediate. It also let verifier recording a more
>> accurate/strict value for one register at one state, if this state end up
>> with going through exit without rejection and it is used for state
>> comparison later, then it is possible an inaccurate/permissive value is
>> better. So the real impact on verifier processed insn number is complex.
>> But in all, without this fix, valid program could be rejected.
>>
>> From real benchmarking on kernel selftests and Cilium bpf tests, there is
>> no impact on processed instruction number when tests ares compiled with
>> default compilation options. There is slightly improvements when they are
>> compiled with -mattr=+alu32 after this patch.
>>
>> Also, test_xdp_noinline/-mattr=+alu32 now passed verification. It is
>> rejected before this fix.
>>
>> Insn processed before/after this patch:
>>
>> default -mattr=+alu32
>>
>> Kernel selftest
>> ===
>> test_xdp.o 371/371 369/369
>> test_l4lb.o 6345/6345 5623/5623
>> test_xdp_noinline.o 2971/2971 rejected/2727
>> test_tcp_estates.o 429/429 430/430
>>
>> Cilium bpf
>> ===
>> bpf_lb-DLB_L3.o: 2085/2085 1685/1687
>> bpf_lb-DLB_L4.o: 2287/2287 1986/1982
>> bpf_lb-DUNKNOWN.o: 690/690 622/622
>> bpf_lxc.o: 95033/95033 N/A
>> bpf_netdev.o: 7245/7245 N/A
>> bpf_overlay.o: 2898/2898 3085/2947
>>
>> NOTE:
>> - bpf_lxc.o and bpf_netdev.o compiled by -mattr=+alu32 are rejected by
>> verifier due to another issue inside verifier on supporting alu32
>> binary.
>> - Each cilium bpf program could generate several processed insn number,
>> above number is sum of them.
>>
>> v1->v2:
>> - Restrict the change on SCALAR_VALUE.
>> - Update benchmark numbers on Cilium bpf tests.
>>
>> Signed-off-by: Jiong Wang <jiong.wang@...ronome.com>
>> ---
>> kernel/bpf/verifier.c | 16 ++++++++++++----
>> tools/testing/selftests/bpf/test_verifier.c | 13 +++++++++++++
>> 2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 9584438..777720a 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -3583,12 +3583,15 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
>> return err;
>>
>> if (BPF_SRC(insn->code) == BPF_X) {
>> + struct bpf_reg_state *src_reg = regs + insn->src_reg;
>> + struct bpf_reg_state *dst_reg = regs + insn->dst_reg;
>> +
>> if (BPF_CLASS(insn->code) == BPF_ALU64) {
>> /* case: R1 = R2
>> * copy register state to dest reg
>> */
>> - regs[insn->dst_reg] = regs[insn->src_reg];
>> - regs[insn->dst_reg].live |= REG_LIVE_WRITTEN;
>> + *dst_reg = *src_reg;
>> + dst_reg->live |= REG_LIVE_WRITTEN;
>> } else {
>> /* R1 = (u32) R2 */
>> if (is_pointer_value(env, insn->src_reg)) {
>> @@ -3596,9 +3599,14 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
>> "R%d partial copy of pointer\n",
>> insn->src_reg);
>> return -EACCES;
>> + } else if (src_reg->type == SCALAR_VALUE) {
>> + *dst_reg = *src_reg;
>> + dst_reg->live |= REG_LIVE_WRITTEN;
>> + } else {
>> + mark_reg_unknown(env, regs,
>> + insn->dst_reg);
> shouldn't we do dst_reg->live |= REG_LIVE_WRITTEN here as well ?
> Not a new issue, but probably should fix in the same patch?
check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK) is called before this
chunk of code, it has set REG_LIVE_WRITTEN in dst_reg->live, it just
doesn't update range info. So, if we don't do full register copy, then
I think there is no need to re-set REG_LIVE_WRITTEN, make sense?
Regards,
Jiong
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