On Mon, Jul 8, 2024 at 11:34 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
> On 7/8/24 9:27 AM, Alexei Starovoitov wrote:
> > On Mon, Jul 8, 2024 at 8:46 AM Yonghong Song <yonghong.song@xxxxxxxxx> 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@xxxxxxxxx>
> >> ---
> >> 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.
Just rephrasing for myself...
Because smin=0xffffffff80000000 if upper 32-bit == 0xffffFFFF
then lower 32-bit has to be negative.
and because we're doing unsigned compare w1 < w6
and w6 is less than 80000000
we can conclude that upper bits are zero.
right?
> <=== 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.
yep. looks like it.
We can hard code this special logic for this specific smin/smax pair,
but the gut feel is that we can generalize it further.
