Hi Shung-Hsi,

More importantly, 'bpf: make the verifier tracks the "not equal" for
regs' does not address root cause of the issue, it merely mask the issue
by making the two states different enough that the two is no longer
equal, which works for the Rust specific case you have, but won't work
if the value was slightly different (e.g. "r0 == 1" followed by "r0 !=
1").

Thanks for spelling this out. I now see that I did not fully
understand the point behind your suggested bpf-next-with-d028-reverted
check.

I was treating the not-equal refinement and the linked-scalar precision
issue as two ways to break the same failure chain, and chose the
d028-based path because it was smaller and easier for me to reason
about. With the `r0 == 1` variant, it became clear to me that this only
fixes the zero-valued branch shape from my original reproducer, while
the underlying linked-scalar pruning issue remains.

Could you give backporting the full "bpf: track find_equal_scalars history on
per-instruction level" series[3] a try? For 6.6 it should be doable, and
hopefully for 6.1, too, but not too sure about earlier ones. If you prefer I
work on it I can also give it a try later this week.

Sure, I will prepare v3 based on that series for 6.6.y, and then work
on the 6.1.y adaptation separately.

I tried applying the series starting from 6.1.y and still hit some
issues that need adaptation. 5.15.y and 5.10.y appear to need more
surrounding verifier changes, so they may be harder, but I will still
try to work through them. If I run into anything I am unsure about, I
will raise it earlier.

As for the selftest, it would need to be send separately and by itself
to bpf-next, and picked up there, before it can be backported to stable.
I suggest you look at [4] and have your test placed similarly, and
mention that your test specifically test a Rust/Aya pattern.

Thanks, I will send the selftest to bpf-next separately. I will also
change the test to use the `r0 == 1` / `r0 != 1` shape, so it covers
the broader linked-scalar pruning issue instead of only the original
zero-valued case.

Thanks again for the detailed explanation. I have only recently started
digging into the verifier implementation details, so this was very helpful!

BR,
Zhenzhong

On Mon, Jun 8, 2026 at 6:11 PM Shung-Hsi Yu [off-list ref] wrote:

Hi Zhenzhong,

On Mon, Jun 08, 2026 at 01:09:55AM +0800, Zhenzhong Wu wrote:

quoted

Hi,

This series backports two BPF verifier scalar range-tracking fixes to
6.6.y and adds a selftest. It fixes a verifier state-pruning issue where
an impossible linked-scalar path can be kept while the real success path is
pruned.

...

quoted

  15: (85) call bpf_get_func_ret#184    ; R0_w=scalar() fp-8_w=mmmmmmmm
  16: (79) r7 = *(u64 *)(r10 -8)        ; R7_w=scalar() R10=fp0
  17: (15) if r0 == 0x0 goto pc+1       ; R0_w=scalar()
  18: (bf) r7 = r0                      ; R0=scalar(id=1) R7=scalar(id=1)
  19: (55) if r0 != 0x0 goto pc+6       ; R0=0
  20: (67) r7 <<= 32                    ; R7_w=0
  21: (77) r7 >>= 32                    ; R7_w=0
  22: (b7) r1 = 1                       ; R1_w=1
  23: (55) if r7 != 0xf goto pc+1

...

quoted

I also checked bpf-next: bpf-next passes even when the d028f87517d6 JNE
refinement is reverted, because newer kernels also have the later
4bf79f9be434e ("bpf: Track equal scalars history on per-instruction level")
precision-tracking change. I did not use 4bf79f9be434e as the stable
backport base because it is a broader jmp_history/precision-tracking change
for linked scalars. For 6.6.y this series keeps the smaller stable backport
path that directly follows the bisected fix: preserve scalar bounds after
conditional refinement, then add the not-equal range refinement in the older
reg_set_min_max() layout.

...

To be honest I have not figure everything out yet, but I really much
prefer we backport commit 4bf79f9be434e ("bpf: Track equal scalars
history on per-instruction level") to address the issue instead. While
'bpf: make the verifier tracks the "not equal" for regs' itself is
self-contained and reasonable, "bpf: drop knowledge-losing
__reg_combine_{32,64}_into_{64,32} logic" comes from a much larger
series[1], and taking that out of context seems rather risky[2].

More importantly, 'bpf: make the verifier tracks the "not equal" for
regs' does not address root cause of the issue, it merely mask the issue
by making the two states different enough that the two is no longer
equal, which works for the Rust specific case you have, but won't work
if the value was slightly different (e.g. "r0 == 1" followed by "r0 !=
1").

The root cause to the problem have been stated by you already, it is:

quoted

The relevant pruning point is that regsafe()/states_equal() accepted the
real success-path state against an earlier cached state where r0 was an
imprecise scalar and r7 constraints were loose enough to cover the current
r7.

Looking at the verifier log you have, in the impossible path we have
r0.id == r7.id from instruction 18, where as the real success path (that
skips instruction 18) does not have that relationship, thus the two
should be considered different, and that seems just what "bpf: track
find_equal_scalars history on per-instruction level" solves by having
the correct precise mark.

Could you give backporting the full "bpf: track find_equal_scalars history on
per-instruction level" series[3] a try? For 6.6 it should be doable, and
hopefully for 6.1, too, but not too sure about earlier ones. If you prefer I
work on it I can also give it a try later this week.

As for the selftest, it would need to be send separately and by itself
to bpf-next, and picked up there, before it can be backported to stable.
I suggest you look at [4] and have your test placed similarly, and
mention that your test specifically test a Rust/Aya pattern.


Thanks,
Shung-Hsi

1: https://lore.kernel.org/r/20231102033759.2541186-1-andrii@kernel.org (local)
2: https://lore.kernel.org/bpf/20260601182508.29C811F00893@smtp.kernel.org/ (local)
3: https://lore.kernel.org/bpf/20240718202357.1746514-1-eddyz87@gmail.com/ (local)
4: https://lore.kernel.org/bpf/20240718202357.1746514-4-eddyz87@gmail.com/ (local)

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