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

[Yonghong Song <yonghong.song@linux.dev>]

On 4/23/24 7:25 PM, Xu Kuohai wrote:
> On 4/24/2024 5:55 AM, Yonghong Song wrote:
>>
>> 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@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@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.
>>
>
> Sure, already added, the commit message and the change now is like this:
>
> ---
>
>     bpf: Fix a false rejection caused by AND operation
>
>     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].
>
>     As explained by Eduard in [0], the clang transformation that 
> generates this
>     pattern 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.
>
>     As suggested by Eduard, this patch makes a special case for source
>     or destination 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.
>
>     And MIN and MAX values could be derived based on above two 
> observations.
>
>     [0] 
> https://lore.kernel.org/bpf/e62e2971301ca7f2e9eb74fc500c520285cad8f5.camel@gmail.com/
>     [1] 
> https://github.com/llvm/llvm-project/blob/4523a267829c807f3fc8fab8e5e9613985a51565/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>
>     Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
>     Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 640747b53745..30c551d39329 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -13374,6 +13374,24 @@ static void scalar32_min_max_and(struct 
> bpf_reg_state *dst_reg,
>         dst_reg->u32_min_value = var32_off.value;
>         dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
>
> +       /* Special case: src_reg is known and dst_reg is in range [-1, 
> 0] */
> +       if (src_known &&
> +               dst_reg->s32_min_value == -1 && dst_reg->s32_max_value 
> == 0 &&
> +               dst_reg->smin_value == -1 && dst_reg->smax_value == 0) {

do we need to check dst_reg->smin_value/smax_value here? They should not impact
final dst_reg->s32_{min,max}_value computation, right?
Similarly, for later 64bit min/max and, 32bit value does not really matter.

> + dst_reg->s32_min_value = min_t(s32, src_reg->s32_min_value, 0);
> +               dst_reg->s32_max_value = max_t(s32, 
> src_reg->s32_min_value, 0);
> +               return;
> +       }
> +
> +       /* Special case: dst_reg is known and src_reg is in range [-1, 
> 0] */
> +       if (dst_known &&
> +               src_reg->s32_min_value == -1 && src_reg->s32_max_value 
> == 0 &&
> +               src_reg->smin_value == -1 && src_reg->smax_value == 0) {
> +               dst_reg->s32_min_value = min_t(s32, 
> dst_reg->s32_min_value, 0);
> +               dst_reg->s32_max_value = max_t(s32, 
> dst_reg->s32_min_value, 0);
> +               return;
> +       }
> +
>         /* Safe to set s32 bounds by casting u32 result into s32 when u32
>          * doesn't cross sign boundary. Otherwise set s32 bounds to 
> unbounded.
>          */
> @@ -13404,6 +13422,24 @@ static void scalar_min_max_and(struct 
> bpf_reg_state *dst_reg,
>         dst_reg->umin_value = dst_reg->var_off.value;
>         dst_reg->umax_value = min(dst_reg->umax_value, umax_val);
>
> +       /* Special case: src_reg is known and dst_reg is in range [-1, 
> 0] */
> +       if (src_known &&
> +               dst_reg->smin_value == -1 && dst_reg->smax_value == 0 &&
> +               dst_reg->s32_min_value == -1 && dst_reg->s32_max_value 
> == 0) {
> +               dst_reg->smin_value = min_t(s64, src_reg->smin_value, 0);
> +               dst_reg->smax_value = max_t(s64, src_reg->smin_value, 0);
> +               return;
> +       }
> +
> +       /* Special case: dst_reg is known and src_reg is in range [-1, 
> 0] */
> +       if (dst_known &&
> +               src_reg->smin_value == -1 && src_reg->smax_value == 0 &&
> +               src_reg->s32_min_value == -1 && src_reg->s32_max_value 
> == 0) {
> +               dst_reg->smin_value = min_t(s64, dst_reg->smin_value, 0);
> +               dst_reg->smax_value = max_t(s64, dst_reg->smin_value, 0);
> +               return;
> +       }
> +
>         /* Safe to set s64 bounds by casting u64 result into s64 when u64
>          * doesn't cross sign boundary. Otherwise set s64 bounds to 
> unbounded.
>          */
>
>>>
>>>> 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
>>>
>>>
>>
>