bpf: collect only live registers in linked regs#11307
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eddyz87 wants to merge 3 commits intokernel-patches:bpf-next_basefrom
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bpf: collect only live registers in linked regs#11307eddyz87 wants to merge 3 commits intokernel-patches:bpf-next_basefrom
eddyz87 wants to merge 3 commits intokernel-patches:bpf-next_basefrom
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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().
--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
"series": {
"revision": 1,
"change-id": "20260306-linked-regs-and-propagate-precision-3d28e2ef5cf5",
"prefixes": [
"bpf"
]
}
}
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Fix an inconsistency between func_states_equal() and
collect_linked_regs():
- regsafe() uses check_ids() to verify that cached and current states
have identical register id mapping.
- func_states_equal() calls regsafe() only for registers computed as
live by compute_live_registers().
- clean_live_states() is supposed to remove dead registers from cached
states, but it can skip states belonging to an iterator-based loop.
- collect_linked_regs() collects all registers sharing the same id,
ignoring the marks computed by compute_live_registers().
Linked registers are stored in the state's jump history.
- backtrack_insn() marks all linked registers for an instruction
as precise whenever one of the linked registers is precise.
The above might lead to a scenario:
- There is an instruction I with register rY known to be dead at I.
- Instruction I is reached via two paths: first A, then B.
- On path A:
- There is an id link between registers rX and rY.
- Checkpoint C is created at I.
- Linked register set {rX, rY} is saved to the jump history.
- rX is marked as precise at I, causing both rX and rY
to be marked precise at C.
- On path B:
- There is no id link between registers rX and rY,
otherwise register states are sub-states of those in C.
- Because rY is dead at I, check_ids() returns true.
- Current state is considered equal to checkpoint C,
propagate_precision() propagates spurious precision
mark for register rY along the path B.
- Depending on a program, this might hit verifier_bug()
in the backtrack_insn(), e.g. if rY ∈ [r1..r5]
and backtrack_insn() spots a function call.
The reproducer program is in the next patch.
This was hit by sched_ext scx_lavd scheduler code.
Changes in tests:
- verifier_scalar_ids.c selftests need modification to preserve
some registers as live for __msg() checks.
- exceptions_assert.c adjusted to match changes in the verifier log,
R0 is dead after conditional instruction and thus does not get
range.
- precise.c adjusted to match changes in the verifier log, register r9
is dead after comparison and it's range is not important for test.
Reported-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
…ugh calls Add a test for the scenario described in the previous commit: an iterator loop with two paths where one ties r2/r7 via shared scalar id and skips a call, while the other goes through the call. Precision marks from the linked registers get spuriously propagated to the call path via propagate_precision(), hitting "backtracking call unexpected regs" in backtrack_insn(). Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
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Same can't be done for stack slots at the moment.