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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Apr 23, 2024 at 7:26 PM Xu Kuohai <xukuohai@huaweicloud.com> 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

[...]

>
>      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) {

please keep if () condition aligned across multiple lines, it's super
confusing this way

> +               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);

do we need to update tnum parts as well (or reset and re-derive, probably)?

btw, can't we support src being a range here? the idea is that dst_reg
either all ones or all zeros. For and it means that it either stays
all zero, or will be *exactly equal* to src, right? So I think the
logic would be:

a) if [s32_min, s32_max] is on the same side of zero, then resulting
range would be [min(s32_min, 0), max(s32_max, 0)], just like you have
here

b) if [s32_min, s32_max] contains zero, then resulting range will be
exactly [s32_min, s32_max]

Or did I make a mistake above?

> +               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.
>           */

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