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Subject[PATCH 72/77] perf augmented_syscalls: Avoid optimization to pass older BPF validators
Date
From: Arnaldo Carvalho de Melo <acme@redhat.com>

See https://www.spinics.net/lists/netdev/msg480099.html for the whole
discussio, but to make the augmented_syscalls.c BPF program to get built
and loaded successfully in a greater range of kernels, add an extra
check.

Related patch:

a60dd35d2e39 ("bpf: change bpf_perf_event_output arg5 type to ARG_CONST_SIZE_OR_ZERO")

That is in the kernel since v4.15, I couldn't figure why this is hitting
me with 4.17.17, but adding the workaround discussed there makes this
work with this fedora kernel and with 4.18.recent.

Before:

# uname -a
Linux seventh 4.17.17-100.fc27.x86_64 #1 SMP Mon Aug 20 15:53:11 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
# perf trace -e tools/perf/examples/bpf/augmented_syscalls.c cat /etc/passwd > /dev/null
libbpf: load bpf program failed: Permission denied
libbpf: -- BEGIN DUMP LOG ---
libbpf:
0: (bf) r6 = r1
1: (b7) r1 = 0
2: (7b) *(u64 *)(r10 -8) = r1
3: (7b) *(u64 *)(r10 -16) = r1
4: (7b) *(u64 *)(r10 -24) = r1
5: (7b) *(u64 *)(r10 -32) = r1
6: (7b) *(u64 *)(r10 -40) = r1
7: (7b) *(u64 *)(r10 -48) = r1
8: (7b) *(u64 *)(r10 -56) = r1
9: (7b) *(u64 *)(r10 -64) = r1
10: (7b) *(u64 *)(r10 -72) = r1
11: (7b) *(u64 *)(r10 -80) = r1
12: (7b) *(u64 *)(r10 -88) = r1
13: (7b) *(u64 *)(r10 -96) = r1
14: (7b) *(u64 *)(r10 -104) = r1
15: (7b) *(u64 *)(r10 -112) = r1
16: (7b) *(u64 *)(r10 -120) = r1
17: (7b) *(u64 *)(r10 -128) = r1
18: (7b) *(u64 *)(r10 -136) = r1
19: (7b) *(u64 *)(r10 -144) = r1
20: (7b) *(u64 *)(r10 -152) = r1
21: (7b) *(u64 *)(r10 -160) = r1
22: (7b) *(u64 *)(r10 -168) = r1
23: (7b) *(u64 *)(r10 -176) = r1
24: (7b) *(u64 *)(r10 -184) = r1
25: (7b) *(u64 *)(r10 -192) = r1
26: (7b) *(u64 *)(r10 -200) = r1
27: (7b) *(u64 *)(r10 -208) = r1
28: (7b) *(u64 *)(r10 -216) = r1
29: (7b) *(u64 *)(r10 -224) = r1
30: (7b) *(u64 *)(r10 -232) = r1
31: (7b) *(u64 *)(r10 -240) = r1
32: (7b) *(u64 *)(r10 -248) = r1
33: (7b) *(u64 *)(r10 -256) = r1
34: (7b) *(u64 *)(r10 -264) = r1
35: (7b) *(u64 *)(r10 -272) = r1
36: (7b) *(u64 *)(r10 -280) = r1
37: (7b) *(u64 *)(r10 -288) = r1
38: (7b) *(u64 *)(r10 -296) = r1
39: (7b) *(u64 *)(r10 -304) = r1
40: (7b) *(u64 *)(r10 -312) = r1
41: (bf) r7 = r10
42: (07) r7 += -312
43: (bf) r1 = r7
44: (b7) r2 = 48
45: (bf) r3 = r6
46: (85) call bpf_probe_read#4
47: (79) r3 = *(u64 *)(r6 +24)
48: (bf) r1 = r10
49: (07) r1 += -256
50: (b7) r8 = 256
51: (b7) r2 = 256
52: (85) call bpf_probe_read_str#45
53: (bf) r1 = r0
54: (67) r1 <<= 32
55: (77) r1 >>= 32
56: (bf) r5 = r0
57: (07) r5 += 56
58: (2d) if r8 > r1 goto pc+1
R0=inv(id=0) R1=inv(id=0,umin_value=256,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R5=inv(id=0) R6=ctx(id=0,off=0,imm=0) R7=fp-312,call_-1 R8=inv256 R10=fp0,call_-1 fp-264=0
59: (b7) r5 = 312
60: (63) *(u32 *)(r10 -264) = r0
61: (67) r5 <<= 32
62: (77) r5 >>= 32
63: (bf) r1 = r6
64: (18) r2 = 0xffff8b9120cc8500
66: (18) r3 = 0xffffffff
68: (bf) r4 = r7
69: (85) call bpf_perf_event_output#25
70: (b7) r0 = 0
71: (95) exit

from 58 to 60: R0=inv(id=0) R1=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R5=inv(id=0) R6=ctx(id=0,off=0,imm=0) R7=fp-312,call_-1 R8=inv256 R10=fp0,call_-1 fp-264=0
60: (63) *(u32 *)(r10 -264) = r0
61: (67) r5 <<= 32
62: (77) r5 >>= 32
63: (bf) r1 = r6
64: (18) r2 = 0xffff8b9120cc8500
66: (18) r3 = 0xffffffff
68: (bf) r4 = r7
69: (85) call bpf_perf_event_output#25
R5 unbounded memory access, use 'var &= const' or 'if (var < const)'

libbpf: -- END LOG --
libbpf: failed to load program 'syscalls:sys_enter_openat'
libbpf: failed to load object 'tools/perf/examples/bpf/augmented_syscalls.c'
bpf: load objects failed: err=-4007: (Kernel verifier blocks program loading)
event syntax error: 'tools/perf/examples/bpf/augmented_syscalls.c'
\___ Kernel verifier blocks program loading

After:

# perf trace -e tools/perf/examples/bpf/augmented_syscalls.c cat /etc/passwd > /dev/null
0.000 cat/29249 openat(dfd: CWD, filename: /etc/ld.so.cache, flags: CLOEXEC)
0.008 cat/29249 syscalls:sys_exit_openat:0x3
0.021 cat/29249 openat(dfd: CWD, filename: /lib64/libc.so.6, flags: CLOEXEC)
0.025 cat/29249 syscalls:sys_exit_openat:0x3
0.180 cat/29249 open(filename: /usr/lib/locale/locale-archive, flags: CLOEXEC)
0.185 cat/29249 syscalls:sys_exit_open:0x3
0.242 cat/29249 openat(dfd: CWD, filename: /etc/passwd)
0.245 cat/29249 syscalls:sys_exit_openat:0x3
#

It also works with a more recent kernel:

# uname -a
Linux jouet 4.18.0-00014-g4e67b2a5df5d #6 SMP Thu Aug 30 17:34:17 -03 2018 x86_64 x86_64 x86_64 GNU/Linux
# perf trace -e tools/perf/examples/bpf/augmented_syscalls.c cat /etc/passwd > /dev/null
0.000 cat/26451 openat(dfd: CWD, filename: /etc/ld.so.cache, flags: CLOEXEC)
0.020 cat/26451 syscalls:sys_exit_openat:0x3
0.039 cat/26451 openat(dfd: CWD, filename: /lib64/libc.so.6, flags: CLOEXEC)
0.044 cat/26451 syscalls:sys_exit_openat:0x3
0.231 cat/26451 open(filename: /usr/lib/locale/locale-archive, flags: CLOEXEC)
0.238 cat/26451 syscalls:sys_exit_open:0x3
0.278 cat/26451 openat(dfd: CWD, filename: /etc/passwd)
0.282 cat/26451 syscalls:sys_exit_openat:0x3
#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: Gianluca Borello <g.borello@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-wkpsivs1a9afwldbul46btbv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
tools/perf/examples/bpf/augmented_syscalls.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/perf/examples/bpf/augmented_syscalls.c b/tools/perf/examples/bpf/augmented_syscalls.c
index 0decbcfa8b90..2ae44813ef2d 100644
--- a/tools/perf/examples/bpf/augmented_syscalls.c
+++ b/tools/perf/examples/bpf/augmented_syscalls.c
@@ -51,8 +51,10 @@ int syscall_enter(syscall)(struct syscall_enter_##syscall##_args *args) \
augmented_args.filename.size = probe_read_str(&augmented_args.filename.value, \
sizeof(augmented_args.filename.value), \
args->filename_ptr); \
- if (augmented_args.filename.size < sizeof(augmented_args.filename.value)) \
+ if (augmented_args.filename.size < sizeof(augmented_args.filename.value)) { \
len -= sizeof(augmented_args.filename.value) - augmented_args.filename.size; \
+ len &= sizeof(augmented_args.filename.value) - 1; \
+ } \
perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
&augmented_args, len); \
return 0; \
--
2.14.4
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