Merge branch 'bpf-fix-precision-backtracking-bug-with-linked-registers'

Eduard Zingerman says:

====================
bpf: Fix precision backtracking bug with linked registers

Emil Tsalapatis reported a verifier bug hit by the scx_lavd sched_ext
scheduler. The essential part of the verifier log looks as follows:

  436: ...
  // checkpoint hit for 438: (1d) if r7 == r8 goto ...
  frame 3: propagating r2,r7,r8
  frame 2: propagating r6
  mark_precise: frame3: last_idx ...
  mark_precise: frame3: regs=r2,r7,r8 stack= before 436: ...
  mark_precise: frame3: regs=r2,r7 stack= before 435: ...
  mark_precise: frame3: regs=r2,r7 stack= before 434: (85) call bpf_trace_vprintk#177
  verifier bug: backtracking call unexpected regs 84

The log complains that registers r2 and r7 are tracked as precise
while processing the bpf_trace_vprintk() call in precision backtracking.
This can't be right, as r2 is reset by the call and there is nothing
to backtrack it to. The precision propagation is triggered when
a checkpoint is hit at instruction 438, r2 is dead at that instruction.

This happens because of the following sequence of events:
- Instruction 438 is first reached with registers r2 and r7 having
  the same id via a path that does not call bpf_trace_vprintk():
  - Checkpoint is created at 438.
  - The jump at 438 is predicted, hence r7 and registers linked to it
    (r2) are propagated as precise, marking r2 and r7 precise in the
    checkpoint.
- Instruction 438 is reached a second time with r2 undefined and via
  a path that calls bpf_trace_vprintk():
  - Checkpoint is hit.
  - propagate_precision() picks registers r2 and r7 and propagates
    precision marks for those up to the helper call.

The root cause is the fact that states_equal() and
propagate_precision() assume that the precision flag can't be set for a
dead register (as computed by compute_live_registers()).
However, this is not the case when linked registers are at play.
Fix this by accounting for live register flags in
collect_linked_regs().
---
====================

Link: https://patch.msgid.link/20260306-linked-regs-and-propagate-precision-v1-0-18e859be570d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>