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Subject[PATCH AUTOSEL 5.4 267/350] bpf, testing: Workaround a verifier failure for test_progs
Date
From: Yonghong Song <yhs@fb.com>

[ Upstream commit b7a0d65d80a0c5034b366392624397a0915b7556 ]

With latest llvm compiler, running test_progs will have the following
verifier failure for test_sysctl_loop1.o:

libbpf: load bpf program failed: Permission denied
libbpf: -- BEGIN DUMP LOG ---
libbpf:
invalid indirect read from stack var_off (0x0; 0xff)+196 size 7
...
libbpf: -- END LOG --
libbpf: failed to load program 'cgroup/sysctl'
libbpf: failed to load object 'test_sysctl_loop1.o'

The related bytecode looks as below:

0000000000000308 LBB0_8:
97: r4 = r10
98: r4 += -288
99: r4 += r7
100: w8 &= 255
101: r1 = r10
102: r1 += -488
103: r1 += r8
104: r2 = 7
105: r3 = 0
106: call 106
107: w1 = w0
108: w1 += -1
109: if w1 > 6 goto -24 <LBB0_5>
110: w0 += w8
111: r7 += 8
112: w8 = w0
113: if r7 != 224 goto -17 <LBB0_8>

And source code:

for (i = 0; i < ARRAY_SIZE(tcp_mem); ++i) {
ret = bpf_strtoul(value + off, MAX_ULONG_STR_LEN, 0,
tcp_mem + i);
if (ret <= 0 || ret > MAX_ULONG_STR_LEN)
return 0;
off += ret & MAX_ULONG_STR_LEN;
}

Current verifier is not able to conclude that register w0 before '+'
at insn 110 has a range of 1 to 7 and thinks it is from 0 - 255. This
leads to more conservative range for w8 at insn 112, and later verifier
complaint.

Let us workaround this issue until we found a compiler and/or verifier
solution. The workaround in this patch is to make variable 'ret' volatile,
which will force a reload and then '&' operation to ensure better value
range. With this patch, I got the below byte code for the loop:

0000000000000328 LBB0_9:
101: r4 = r10
102: r4 += -288
103: r4 += r7
104: w8 &= 255
105: r1 = r10
106: r1 += -488
107: r1 += r8
108: r2 = 7
109: r3 = 0
110: call 106
111: *(u32 *)(r10 - 64) = r0
112: r1 = *(u32 *)(r10 - 64)
113: if w1 s< 1 goto -28 <LBB0_5>
114: r1 = *(u32 *)(r10 - 64)
115: if w1 s> 7 goto -30 <LBB0_5>
116: r1 = *(u32 *)(r10 - 64)
117: w1 &= 7
118: w1 += w8
119: r7 += 8
120: w8 = w1
121: if r7 != 224 goto -21 <LBB0_9>

Insn 117 did the '&' operation and we got more precise value range
for 'w8' at insn 120. The test is happy then:

#3/17 test_sysctl_loop1.o:OK

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191107170045.2503480-1-yhs@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/bpf/progs/test_sysctl_loop1.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c b/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c
index 608a06871572d..d22e438198cf7 100644
--- a/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c
+++ b/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c
@@ -44,7 +44,10 @@ int sysctl_tcp_mem(struct bpf_sysctl *ctx)
unsigned long tcp_mem[TCP_MEM_LOOPS] = {};
char value[MAX_VALUE_STR_LEN];
unsigned char i, off = 0;
- int ret;
+ /* a workaround to prevent compiler from generating
+ * codes verifier cannot handle yet.
+ */
+ volatile int ret;

if (ctx->write)
return 0;
--
2.20.1
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