Re: [PATCH bpf-next] selftests/bpf: Workaround iters/iter_arr_with_actual_elem_count failure when -mcpu=cpuv4

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

On 7/8/24 9:27 AM, Alexei Starovoitov wrote:
> On Mon, Jul 8, 2024 at 8:46 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>> With latest llvm19, the selftest iters/iter_arr_with_actual_elem_count
>> failed with -mcpu=v4.
>>
>> The following are the details:
>>    0: R1=ctx() R10=fp0
>>    ; int iter_arr_with_actual_elem_count(const void *ctx) @ iters.c:1420
>>    0: (b4) w7 = 0                        ; R7_w=0
>>    ; int i, n = loop_data.n, sum = 0; @ iters.c:1422
>>    1: (18) r1 = 0xffffc90000191478       ; R1_w=map_value(map=iters.bss,ks=4,vs=1280,off=1144)
>>    3: (61) r6 = *(u32 *)(r1 +128)        ; R1_w=map_value(map=iters.bss,ks=4,vs=1280,off=1144) R6_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
>>    ; if (n > ARRAY_SIZE(loop_data.data)) @ iters.c:1424
>>    4: (26) if w6 > 0x20 goto pc+27       ; R6_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f))
>>    5: (bf) r8 = r10                      ; R8_w=fp0 R10=fp0
>>    6: (07) r8 += -8                      ; R8_w=fp-8
>>    ; bpf_for(i, 0, n) { @ iters.c:1427
>>    7: (bf) r1 = r8                       ; R1_w=fp-8 R8_w=fp-8
>>    8: (b4) w2 = 0                        ; R2_w=0
>>    9: (bc) w3 = w6                       ; R3_w=scalar(id=1,smin=smin32=0,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f)) R6_w=scalar(id=1,smin=smin32=0,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f))
>>    10: (85) call bpf_iter_num_new#45179          ; R0=scalar() fp-8=iter_num(ref_id=2,state=active,depth=0) refs=2
>>    11: (bf) r1 = r8                      ; R1=fp-8 R8=fp-8 refs=2
>>    12: (85) call bpf_iter_num_next#45181 13: R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R6=scalar(id=1,smin=smin32=0,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f)) R7=0 R8=fp-8 R10=fp0 fp-8=iter_num(ref_id=2,state=active,depth=1) refs=2
>>    ; bpf_for(i, 0, n) { @ iters.c:1427
>>    13: (15) if r0 == 0x0 goto pc+2       ; R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) refs=2
>>    14: (81) r1 = *(s32 *)(r0 +0)         ; R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R1_w=scalar(smin=0xffffffff80000000,smax=0x7fffffff) refs=2
>>    15: (ae) if w1 < w6 goto pc+4 20: R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R1=scalar(smin=0xffffffff80000000,smax=smax32=umax32=31,umax=0xffffffff0000001f,smin32=0,var_off=(0x0; 0xffffffff0000001f)) R6=scalar(id=1,smin=umin=smin32=umin32=1,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f)) R7=0 R8=fp-8 R10=fp0 fp-8=iter_num(ref_id=2,state=active,depth=1) refs=2
>>    ; sum += loop_data.data[i]; @ iters.c:1429
>>    20: (67) r1 <<= 2                     ; R1_w=scalar(smax=0x7ffffffc0000007c,umax=0xfffffffc0000007c,smin32=0,smax32=umax32=124,var_off=(0x0; 0xfffffffc0000007c)) refs=2
>>    21: (18) r2 = 0xffffc90000191478      ; R2_w=map_value(map=iters.bss,ks=4,vs=1280,off=1144) refs=2
>>    23: (0f) r2 += r1
>>    math between map_value pointer and register with unbounded min value is not allowed
>>
>> The source code:
>>    int iter_arr_with_actual_elem_count(const void *ctx)
>>    {
>>          int i, n = loop_data.n, sum = 0;
>>
>>          if (n > ARRAY_SIZE(loop_data.data))
>>                  return 0;
>>
>>          bpf_for(i, 0, n) {
>>                  /* no rechecking of i against ARRAY_SIZE(loop_data.n) */
>>                  sum += loop_data.data[i];
>>          }
>>
>>          return sum;
>>    }
>>
>> The insn #14 is a sign-extenstion load which is related to 'int i'.
>> The insn #15 did a subreg comparision. Note that smin=0xffffffff80000000 and this caused later
>> insn #23 failed verification due to unbounded min value.
>>
>> Actually insn #15 smin range can be better. Since after comparison, we know smin32=0 and smax32=32.
>> With insn #14 being a sign-extension load. We will know top 32bits should be 0 as well.
>> Current verifier is not able to handle this, and this patch is a workaround to fix
>> test failure by changing variable 'i' type from 'int' to 'unsigned' which will give
>> proper range during comparison.
>>
>>    ; bpf_for(i, 0, n) { @ iters.c:1428
>>    13: (15) if r0 == 0x0 goto pc+2       ; R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) refs=2
>>    14: (61) r1 = *(u32 *)(r0 +0)         ; R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R1_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff)) refs=2
>>    ...
>>    from 15 to 20: R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R1=scalar(smin=smin32=0,smax=umax=smax32=umax32=31,var_off=(0x0; 0x1f)) R6=scalar(id=1,smin=umin=smin32=umin32=1,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f)) R7=0 R8=fp-8 R10=fp0 fp-8=iter_num(ref_id=2,state=active,depth=1) refs=2
>>    20: R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R1=scalar(smin=smin32=0,smax=umax=smax32=umax32=31,var_off=(0x0; 0x1f)) R6=scalar(id=1,smin=umin=smin32=umin32=1,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f)) R7=0 R8=fp-8 R10=fp0 fp-8=iter_num(ref_id=2,state=active,depth=1) refs=2
>>    ; sum += loop_data.data[i]; @ iters.c:1430
>>    20: (67) r1 <<= 2                     ; R1_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=124,var_off=(0x0; 0x7c)) refs=2
>>    21: (18) r2 = 0xffffc90000185478      ; R2_w=map_value(map=iters.bss,ks=4,vs=1280,off=1144) refs=2
>>    23: (0f) r2 += r1
>>    mark_precise: frame0: last_idx 23 first_idx 20 subseq_idx -1
>>    ...
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   tools/testing/selftests/bpf/progs/iters.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/bpf/progs/iters.c b/tools/testing/selftests/bpf/progs/iters.c
>> index 16bdc3e25591..d1801d151a12 100644
>> --- a/tools/testing/selftests/bpf/progs/iters.c
>> +++ b/tools/testing/selftests/bpf/progs/iters.c
>> @@ -1419,7 +1419,8 @@ SEC("raw_tp")
>>   __success
>>   int iter_arr_with_actual_elem_count(const void *ctx)
>>   {
>> -       int i, n = loop_data.n, sum = 0;
>> +       unsigned i;
>> +       int n = loop_data.n, sum = 0;
>>
>>          if (n > ARRAY_SIZE(loop_data.data))
>>                  return 0;
> I think we only have one realistic test that
> checks 'range vs range' verifier logic.
> Since "int i; bpf_for(i"
> is a very common pattern in all other bpf_for tests it feels
> wrong to workaround like this.

Agree. We should fix this in verifier to be user friendly.

>
> What exactly needs to be improved in 'range vs range' logic to
> handle this case?

We can add a bit in struct bpf_reg_state like below
	struct bpf_reg_state {
		...
		enum bpf_reg_liveness live;
		bool precise;
	}
to
	struct bpf_reg_state {
		...
		enum bpf_reg_liveness live;
		unsigned precise:1;
		unsigned 32bit_sign_ext:1;  /* for *(s32 *)(...) */
	}
When the insn 15 is processed with 'true' branch
   14: (81) r1 = *(s32 *)(r0 +0)         ; R0=rdonly_mem(id=3,ref_obj_id=2,sz=4) R1_w=scalar(smin=0xffffffff80000000,smax=0x7fffffff) refs=2
   15: (ae) if w1 < w6 goto pc+4

the 32bit_sign_ext will indicate the register r1 is from 32bit sign extension, so once w1 range is refined, the upper 32bit can be recalculated.

Can we avoid 32bit_sign_exit in the above? Let us say we have
   r1 = ...;  R1_w=scalar(smin=0xffffffff80000000,smax=0x7fffffff), R6_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=32,var_off=(0x0; 0x3f))
   if w1 < w6 goto pc+4
where r1 achieves is trange through other means than 32bit sign extension e.g.
   call bpf_get_prandom_u32;
   r1 = r0;
   r1 <<= 32;
   call bpf_get_prandom_u32;
   r1 |= r0;  /* r1 is 64bit random number */
   r2 = 0xffffffff80000000 ll;
   if r1 s< r2 goto end;
   if r1 s> 0x7fffFFFF goto end; /* after this r1 range (smin=0xffffffff80000000,smax=0x7fffffff) */
   if w1 < w6 goto end;
   ...  <=== w1 range [0,31]
        <=== but if we have upper bit as 0xffffffff........, then the range will be
        <=== [0xffffffff0000001f, 0xffffffff00000000] and this range is not possible compared to original r1 range.
        <=== so the only possible way for upper 32bit range is 0.
end:

Therefore, looks like we do not need 32bit_sign_exit. Just from
R1_w=scalar(smin=0xffffffff80000000,smax=0x7fffffff)
with refined range in true path of 'if w1 < w6 goto ...',
we can further refine w1 range properly.