Re: [PATCH bpf v2 0/2] bpf, x86: Fix i386 JIT signed ALU32 DIV/MOD and MOVSX

[Kuan-Wei Chiu <visitorckw@gmail.com>]

On Fri, Nov 13, 2026 at 12:23:41PM -0500, Nicholas Dudar wrote:
> The i386 JIT accepts signed ALU32 DIV/MOD and register MOVSX, but
> ignores the offset that selects them and emits an unsigned div and a
> plain move. The results differ from RFC9669 and the interpreter, and a
> verifier-accepted program can use values outside the range proved by
> the verifier. In i386 VM tests, three verifier-accepted programs
> supplied helper lengths beyond the verifier's bounds before these
> fixes, and the helper wrote that many bytes. With the fixes the lengths
> stayed within the bounds. The test programs are not included here and
> are available to maintainers on request.
> 
> The series is meant as a minimal fix that closes this gap between the
> verifier and the JIT. It makes the two instructions the JIT already
> accepts produce the values the verifier assumes and adds no new
> instruction support.
> 
> Tested on i386 under KVM. vmtest.sh has no i386 target and test_progs
> does not build in full as a 32-bit binary, so it was built with
> jit_insns, verifier_sdiv and verifier_movsx only, with a strtol() to
> strtoll() change in test_loader.c. The commands and output are in the
> patches. The six jit_insns subtests that still fail (arena_zero_off,
> cast_to_user, staggered_jumps, acq_rel, probe_mem, shift_counts) fail
> without the series as well.
> 
> On the unpatched kernel the unprivileged subtest "MOV32SX, S8,
> range_check" hangs in the kernel: the wrong value takes a branch the
> verifier proved dead, which for unprivileged programs is a jump to
> itself. The unprivileged verifier_movsx subtests were therefore run
> with both patches only. There it passes, and two others differ from the
> expected xlated text by an additional zero extension on i386.
> 
> Claude Mythos 5.2 assisted with this patch series.
> 
> v1 -> v2:
>   - Dropped MEMSX, 32-bit-offset unconditional jumps, unconditional
>     byte swaps and the load-emitter refactor
>   - Added the signed DIV/MOD change, which was posted on its own [1]
>     and is superseded by this series; the code is unchanged
>   - Sent as fixes for bpf with Fixes tags instead of new support for
>     bpf-next, since the JIT already accepts both instructions and
>     miscompiles them
>   - Tested with jit_insns and the verifier tests of test_progs
> 
> [1] https://lore.kernel.org/bpf/20260731002950.179573-1-main.kalliope@gmail.com/
> 
> v1: https://lore.kernel.org/bpf/20260915021338.1520964-1-main.kalliope@gmail.com/
> ---
> Nicholas Dudar (2):
>       bpf, x86: Fix signed division and modulo in the i386 JIT
>       bpf, x86: Fix sign-extension moves in the i386 JIT
> 
>  arch/x86/net/bpf_jit_comp32.c | 76 ++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 68 insertions(+), 8 deletions(-)

IIRC Alexei mentioned previously that he didn't think anyone still
cared about i386 JIT, so the i386 JIT patches were not merged?
Now we are seeing more i386 JIT patches. If we really think no one
cares about this platform anymore, maybe we should remove it to avoid
receiving more patches that we don't plan to merge?

Regards,
Kuan-Wei

> ---
> base-commit: b7cc45994c3f9c5f24397fcc0251f6c23bff9bd8
> change-id: 20261112-i386_bpf_jit_wrong_code_20261028-7246b0231fce
> 
> Best regards,
> --  
> Nicholas Dudar <main.kalliope@gmail.com>
>