From: Tao Lyu <tao.lyu@epfl.ch>
To: <andrii@kernel.org>, <ast@kernel.org>, <daniel@iogearbox.net>,
<song@kernel.org>, <yonghong.song@linux.dev>, <haoluo@google.com>,
<martin.lau@linux.dev>
Cc: <bpf@vger.kernel.org>, <sanidhya.kashyap@epfl.ch>,
<mathias.payer@nebelwelt.net>, <meng.xu.cs@uwaterloo.ca>,
Tao Lyu <tao.lyu@epfl.ch>
Subject: max<min after jset
Date: Tue, 21 Nov 2023 18:32:06 +0100 [thread overview]
Message-ID: <20231121173206.3594040-1-tao.lyu@epfl.ch> (raw)
Hi,
The eBPF program shown below leads to an reversed min and max
after insn 6 "if w0 & 0x894b6a55 goto +2",
whic means max < min.
Here is the introduction how it happens.
Before insn 6,
the range of r0 expressed by the min and max field is
min1 = 884670597, max1 = 900354100
And the range expressed by the var_off=(0x34000000; 0x1ff5fbf))
is min2=872415232, max2=905928639.
---min2-----------------------min1-----max1-----max2---
Here we can see that the range expressed by var_off is wider than that of min and max.
When verifying insn6,
it first uses the var_off and immediate "0x894b6a55" to
calculate the new var_off=(0x34b00000; 0x415aa).
The range expressed by the new var_off is:
min3=883949568, max3=884217258
---min2-----min3-----max3-----min1-----max1-----max2---
And then it will calculate the new min and max by:
(1) new-min = MAX(min3, min1) = min1
(2) new-max = MIN(max3, max1) = max3
---min2-----min3-----max3-----min1-----max1-----max2---
"new-max" "new-min"
Now, the new-max becomes less than the new min.
Notably, [min1, max1] can never make "w0 & 0x894b6a55 == 0"
and thus cannot goes the fall-through branch.
In other words, actually the fall-trough branch is a dead path.
BTW, I cannot successfully compile this instruciton "if w0 != 0 goto +2;\"
in the c inline assembly code.
So I can only attach the bytecodes.
Signed-off-by: Tao Lyu <tao.lyu@epfl.ch>
---
.../selftests/bpf/verifier/jset_reversed_range.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
create mode 100644 tools/testing/selftests/bpf/verifier/jset_reversed_range.c
diff --git a/tools/testing/selftests/bpf/verifier/jset_reversed_range.c b/tools/testing/selftests/bpf/verifier/jset_reversed_range.c
new file mode 100644
index 000000000000..734f492a2a96
--- /dev/null
+++ b/tools/testing/selftests/bpf/verifier/jset_reversed_range.c
@@ -0,0 +1,15 @@
+{
+ "BPF_JSET: incorrect scalar range",
+ .insns = {
+ BPF_MOV64_IMM(BPF_REG_5, 100),
+ BPF_ALU64_IMM(BPF_DIV, BPF_REG_5, 3),
+ BPF_ALU32_IMM(BPF_RSH, BPF_REG_5, 7),
+ BPF_ALU64_IMM(BPF_AND, BPF_REG_5, -386969681),
+ BPF_ALU64_IMM(BPF_SUB, BPF_REG_5, -884670597),
+ BPF_MOV32_REG(BPF_REG_0, BPF_REG_5),
+ BPF_JMP32_IMM(BPF_JSET, BPF_REG_0, 0x894b6a55, 1),
+ BPF_MOV64_IMM(BPF_REG_0, 1),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+},
--
2.25.1
next reply other threads:[~2023-11-21 17:32 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-21 17:32 Tao Lyu [this message]
2023-11-22 0:25 ` max<min after jset Yonghong Song
2023-11-22 14:40 ` [PATCH] C inlined assembly for reproducing max<min Tao Lyu
2023-11-22 18:08 ` Yonghong Song
2023-11-22 18:15 ` Alexei Starovoitov
2023-11-22 18:37 ` Jose E. Marchesi
2023-11-22 18:51 ` Yonghong Song
2023-11-22 18:39 ` Eduard Zingerman
2023-11-28 4:16 ` max<min after jset Yonghong Song
2023-12-02 10:44 ` [PATCH] C inlined assembly for reproducing max<min Tao Lyu
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