[PATCH bpf-next v3 07/11] bpf: Fix a false rejection caused by AND operation

[Yonghong Song]

On 4/20/24 1:33 AM, Xu Kuohai wrote:
> On 4/20/2024 7:00 AM, Eduard Zingerman wrote:
>> On Thu, 2024-04-11 at 20:27 +0800, Xu Kuohai wrote:
>>> From: Xu Kuohai <xukuohai at huawei.com>
>>>
>>> With lsm return value check, the no-alu32 version 
>>> test_libbpf_get_fd_by_id_opts
>>> is rejected by the verifier, and the log says:
>>>
>>>    0: R1=ctx() R10=fp0
>>>    ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) 
>>> @ test_libbpf_get_fd_by_id_opts.c:27
>>>    0: (b7) r0 = 0                        ; R0_w=0
>>>    1: (79) r2 = *(u64 *)(r1 +0)
>>>    func 'bpf_lsm_bpf_map' arg0 has btf_id 916 type STRUCT 'bpf_map'
>>>    2: R1=ctx() R2_w=trusted_ptr_bpf_map()
>>>    ; if (map != (struct bpf_map *)&data_input) @ 
>>> test_libbpf_get_fd_by_id_opts.c:29
>>>    2: (18) r3 = 0xffff9742c0951a00       ; 
>>> R3_w=map_ptr(map=data_input,ks=4,vs=4)
>>>    4: (5d) if r2 != r3 goto pc+4         ; 
>>> R2_w=trusted_ptr_bpf_map() R3_w=map_ptr(map=data_input,ks=4,vs=4)
>>>    ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) 
>>> @ test_libbpf_get_fd_by_id_opts.c:27
>>>    5: (79) r0 = *(u64 *)(r1 +8)          ; R0_w=scalar() R1=ctx()
>>>    ; if (fmode & FMODE_WRITE) @ test_libbpf_get_fd_by_id_opts.c:32
>>>    6: (67) r0 <<= 62                     ; 
>>> R0_w=scalar(smax=0x4000000000000000,umax=0xc000000000000000,smin32=0,smax32=umax32=0,var_off=(0x0; 
>>> 0xc000000000000000))
>>>    7: (c7) r0 s>>= 63                    ; 
>>> R0_w=scalar(smin=smin32=-1,smax=smax32=0)
>>>    ;  @ test_libbpf_get_fd_by_id_opts.c:0
>>>    8: (57) r0 &= -13                     ; 
>>> R0_w=scalar(smax=0x7ffffffffffffff3,umax=0xfffffffffffffff3,smax32=0x7ffffff3,umax32=0xfffffff3,var_off=(0x0; 
>>> 0xfffffffffffffff3))
>>>    ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) 
>>> @ test_libbpf_get_fd_by_id_opts.c:27
>>>    9: (95) exit
>>>
>>> And here is the C code of the prog.
>>>
>>> SEC("lsm/bpf_map")
>>> int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode)
>>> {
>>>     if (map != (struct bpf_map *)&data_input)
>>>         return 0;
>>>
>>>     if (fmode & FMODE_WRITE)
>>>         return -EACCES;
>>>
>>>     return 0;
>>> }
>>>
>>> It is clear that the prog can only return either 0 or -EACCESS, and 
>>> both
>>> values are legal.
>>>
>>> So why is it rejected by the verifier?
>>>
>>> The verifier log shows that the second if and return value setting
>>> statements in the prog is optimized to bitwise operations "r0 s>>= 63"
>>> and "r0 &= -13". The verifier correctly deduces that the the value of
>>> r0 is in the range [-1, 0] after verifing instruction "r0 s>>= 63".
>>> But when the verifier proceeds to verify instruction "r0 &= -13", it
>>> fails to deduce the correct value range of r0.
>>>
>>> 7: (c7) r0 s>>= 63                    ; 
>>> R0_w=scalar(smin=smin32=-1,smax=smax32=0)
>>> 8: (57) r0 &= -13                     ; 
>>> R0_w=scalar(smax=0x7ffffffffffffff3,umax=0xfffffffffffffff3,smax32=0x7ffffff3,umax32=0xfffffff3,var_off=(0x0; 
>>> 0xfffffffffffffff3))
>>>
>>> So why the verifier fails to deduce the result of 'r0 &= -13'?
>>>
>>> The verifier uses tnum to track values, and the two ranges "[-1, 0]" 
>>> and
>>> "[0, -1ULL]" are encoded to the same tnum. When verifing instruction
>>> "r0 &= -13", the verifier erroneously deduces the result from
>>> "[0, -1ULL] AND -13", which is out of the expected return range
>>> [-4095, 0].
>>>
>>> To fix it, this patch simply adds a special SCALAR32 case for the
>>> verifier. That is, when the source operand of the AND instruction is
>>> a constant and the destination operand changes from negative to
>>> non-negative and falls in range [-256, 256], deduce the result range
>>> by enumerating all possible AND results.
>>>
>>> Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
>>> ---
>>
>> Hello,
>>
>> Sorry for the delay, I had to think about this issue a bit.
>> I found the clang transformation that generates the pattern this patch
>> tries to handle.
>> It is located in DAGCombiner::SimplifySelectCC() method (see [1]).
>> The transformation happens as a part of DAG to DAG rewrites
>> (LLVM uses several internal representations:
>>   - generic optimizer uses LLVM IR, most of the work is done
>>     using this representation;
>>   - before instruction selection IR is converted to Selection DAG,
>>     some optimizations are applied at this stage,
>>     all such optimizations are a set of pattern replacements;
>>   - Selection DAG is converted to machine code, some optimizations
>>     are applied at the machine code level).
>>
>> Full pattern is described as follows:
>>
>>    // fold (select_cc seteq (and x, y), 0, 0, A) -> (and (sra (shl 
>> x)) A)
>>    // where y is has a single bit set.
>>    // A plaintext description would be, we can turn the SELECT_CC 
>> into an AND
>>    // when the condition can be materialized as an all-ones 
>> register.  Any
>>    // single bit-test can be materialized as an all-ones register with
>>    // shift-left and shift-right-arith.
>>
>> For this particular test case the DAG is converted as follows:
>>
>>                      .---------------- lhs         The meaning of 
>> this select_cc is:
>>                      |        .------- rhs         `lhs == rhs ? true 
>> value : false value`
>>                      |        | .----- true value
>>                      |        | |  .-- false value
>>                      v        v v  v
>>    (select_cc seteq (and X 2) 0 0 -13)
>>                            ^
>> ->                        '---------------.
>>    (and (sra (sll X 62) 63)                |
>>         -13)                               |
>>                                            |
>> Before pattern is applied, it checks that second 'and' operand has
>> only one bit set, (which is true for '2').
>>
>> The pattern itself generates logical shift left / arithmetic shift
>> right pair, that ensures that result is either all ones (-1) or all
>> zeros (0). Hence, applying 'and' to shifts result and false value
>> generates a correct result.
>>
>
> Thanks for your detailed and invaluable explanation!

Thanks Eduard for detailed explanation. It looks like we could
resolve this issue without adding too much complexity to verifier.
Also, this code pattern above seems generic enough to be worthwhile
with verifier change.

Kuohai, please added detailed explanation (as described by Eduard)
in the commit message.

>
>> In my opinion the approach taken by this patch is sub-optimal:
>> - 512 iterations is too much;
>> - this does not cover all code that could be generated by the above
>>    mentioned LLVM transformation
>>    (e.g. second 'and' operand could be 1 << 16).
>>
>> Instead, I suggest to make a special case for source or dst register
>> of '&=' operation being in range [-1,0].
>> Meaning that one of the '&=' operands is either:
>> - all ones, in which case the counterpart is the result of the 
>> operation;
>> - all zeros, in which case zero is the result of the operation;
>> - derive MIN and MAX values based on above two observations.
>>
>
> Totally agree, I'll cook a new patch as you suggested.
>
>> [1] 
>> https://github.com/llvm/llvm-project/blob/4523a267829c807f3fc8fab8e5e9613985a51565/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L5391
>>
>> Best regards,
>> Eduard
>
>