Merge branch 'bpf-fix-x86-jit-stopping-before-jumps-converge'

Mykyta Yatsenko says:

====================
bpf: Fix x86 JIT stopping before jumps converge

The x86 JIT stops its sizing passes once two passes give the same image
length. A backward jump can grow while other jumps shrink, so the length
may repeat with jump sizes still changing. The final pass then emits code
of a different size than was sized, hits "bpf_jit: fatal error" and the
program fails to load with -ENOTSUPP.

Patch 1 sizes backward jumps by the insns they jump over, so no insn
grows between passes and a repeated length means nothing moved.
Patch 2 adds a selftest which fails to load without the fix.

This was hit in CI by cgroup_tcp_skb with the unpublished branch
inversion series, whose layout change happened to trigger it.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
Changes in v2:
- Size backward jumps by the insns they jump over instead of checking
  for moved insns, drop the ja -1 special case and the imm8 limit of 123
  (Alexei)
- Start every insn at BPF_MAX_INSN_SIZE, so none grows in the first pass
- Check that the selftest program is JITed
- Link to v1: https://patch.msgid.link/20261021-jit_size_fix-v1-0-b89fd32cafd6@meta.com
====================

Link: https://patch.msgid.link/20261022-jit_size_fix-v2-0-4bee6921b218@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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