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selftests/bpf: Fix flaky test ptr_untrusted

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Somehow recently I frequently hit the following test failure
with either ./test_progs or ./test_progs-cpuv4:
  serial_test_ptr_untrusted:PASS:skel_open 0 nsec
  serial_test_ptr_untrusted:PASS:lsm_attach 0 nsec
  serial_test_ptr_untrusted:PASS:raw_tp_attach 0 nsec
  serial_test_ptr_untrusted:FAIL:cmp_tp_name unexpected cmp_tp_name: actual -115 != expected 0
  #182     ptr_untrusted:FAIL

Further investigation found the failure is due to
  bpf_probe_read_user_str()
where reading user-level string attr->raw_tracepoint.name
is not successfully, most likely due to the
string itself still in disk and not populated into memory yet.

One solution is do a printf() call of the string before doing bpf
syscall which will force the raw_tracepoint.name into memory.
But I think a more robust solution is to use bpf_copy_from_user()
which is used in sleepable program and can tolerate page fault,
and the fix here used the latter approach.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240204194452.2785936-1-yonghong.song@linux.dev
This commit is contained in:
Yonghong Song 2024-02-04 11:44:52 -08:00 committed by Andrii Nakryiko
parent df9705eaa0
commit 7e428638bd
1 changed files with 3 additions and 3 deletions
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6
tools/testing/selftests/bpf/progs/test_ptr_untrusted.c
View file

@ -6,13 +6,13 @@
char tp_name[128];
SEC("lsm/bpf")
SEC("lsm.s/bpf")
int BPF_PROG(lsm_run, int cmd, union bpf_attr *attr, unsigned int size)
{
switch (cmd) {
case BPF_RAW_TRACEPOINT_OPEN:
bpf_probe_read_user_str(tp_name, sizeof(tp_name) - 1,
(void *)attr->raw_tracepoint.name);
bpf_copy_from_user(tp_name, sizeof(tp_name) - 1,
(void *)attr->raw_tracepoint.name);
break;
default:
break;