This is a test case minimized from a syzbot reproducer from [1].
The test case triggers verifier.c:maybe_exit_scc() w/o
preceding call to verifier.c:maybe_enter_scc() on a speculative
symbolic execution path.
Here is verifier log for the test case:
Live regs before insn:
0: .......... (b7) r0 = 100
1 1: 0......... (7b) *(u64 *)(r10 -512) = r0
1 2: 0......... (b5) if r0 <= 0x0 goto pc-2
3: 0......... (95) exit
0: R1=ctx() R10=fp0
0: (b7) r0 = 100 ; R0_w=100
1: (7b) *(u64 *)(r10 -512) = r0 ; R0_w=100 R10=fp0 fp-512_w=100
2: (b5) if r0 <= 0x0 goto pc-2
mark_precise: ...
2: R0_w=100
3: (95) exit
from 2 to 1 (speculative execution): R0_w=scalar() R1=ctx() R10=fp0 fp-512_w=100
1: R0_w=scalar() R1=ctx() R10=fp0 fp-512_w=100
1: (7b) *(u64 *)(r10 -512) = r0
processed 5 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
- Non-speculative execution path 0-3 does not allocate any checkpoints
(and hence does not call maybe_enter_scc()), and schedules a
speculative jump from 2 to 1.
- Speculative execution path stops immediately because of an infinite
loop detection and triggers verifier.c:update_branch_counts() ->
maybe_exit_scc() calls.
[1] https://lore.kernel.org/bpf/68c85acd.050a0220.2ff435.03a4.GAE@google.com/
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250916212251.3490455-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Verify that infinite loop detection logic separates states with
identical register states but different imprecise scalars spilled to
stack.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240108205209.838365-4-maxtram95@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
When BPF program is verified in privileged mode, BPF verifier allows
bounded loops. This means that from CFG point of view there are
definitely some back-edges. Original commit adjusted check_cfg() logic
to not detect back-edges in control flow graph if they are resulting
from conditional jumps, which the idea that subsequent full BPF
verification process will determine whether such loops are bounded or
not, and either accept or reject the BPF program. At least that's my
reading of the intent.
Unfortunately, the implementation of this idea doesn't work correctly in
all possible situations. Conditional jump might not result in immediate
back-edge, but just a few unconditional instructions later we can arrive
at back-edge. In such situations check_cfg() would reject BPF program
even in privileged mode, despite it might be bounded loop. Next patch
adds one simple program demonstrating such scenario.
To keep things simple, instead of trying to detect back edges in
privileged mode, just assume every back edge is valid and let subsequent
BPF verification prove or reject bounded loops.
Note a few test changes. For unknown reason, we have a few tests that
are specified to detect a back-edge in a privileged mode, but looking at
their code it seems like the right outcome is passing check_cfg() and
letting subsequent verification to make a decision about bounded or not
bounded looping.
Bounded recursion case is also interesting. The example should pass, as
recursion is limited to just a few levels and so we never reach maximum
number of nested frames and never exhaust maximum stack depth. But the
way that max stack depth logic works today it falsely detects this as
exceeding max nested frame count. This patch series doesn't attempt to
fix this orthogonal problem, so we just adjust expected verifier failure.
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Fixes: 2589726d12 ("bpf: introduce bounded loops")
Reported-by: Hao Sun <sunhao.th@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231110061412.2995786-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Test verifier/loops1 automatically converted to use inline assembly.
There are a few modifications for the converted tests.
"tracepoint" programs do not support test execution, change program
type to "xdp" (which supports test execution) for the following tests
that have __retval tags:
- bounded loop, count to 4
- bonded loop containing forward jump
Also, remove the __retval tag for test:
- bounded loop, count from positive unknown to 4
As it's return value is a random number.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230421174234.2391278-10-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Eduard Zingerman