From: Eugene Loh <eugene.loh@oracle.com>
To: bpf@vger.kernel.org
Subject: using skip>0 with bpf_get_stack()
Date: Fri, 28 May 2021 18:16:15 -0400 [thread overview]
Message-ID: <30a7b5d5-6726-1cc2-eaee-8da2828a9a9c@oracle.com> (raw)
I have a question about bpf_get_stack(). I'm interested in the case
skip > 0
user_build_id == 0
num_elem < sysctl_perf_event_max_stack
The function sets
init_nr = sysctl_perf_event_max_stack - num_elem;
which means that get_perf_callchain() will return "num_elem" stack
frames. Then, since we skip "skip" frames, we'll fill the user buffer
with only "num_elem - skip" frames, the remaining frames being filled zero.
For example, let's say the call stack is
leaf <- caller <- foo1 <- foo2 <- foo3 <- foo4 <- foo5 <- foo6
Let's say I pass bpf_get_stack() a buffer with num_elem==4 and ask
skip==2. I would expect to skip 2 frames then get 4 frames, getting back:
foo1 foo2 foo3 foo4
Instead, I get
foo1 foo2 0 0
skipping 2 frames but also leaving frames zeroed out.
I think the init_nr computation should be:
- if (sysctl_perf_event_max_stack < num_elem)
+ if (sysctl_perf_event_max_stack <= num_elem + skip)
init_nr = 0;
else
- init_nr = sysctl_perf_event_max_stack - num_elem;
+ init_nr = sysctl_perf_event_max_stack - num_elem - skip;
Incidentally, the return value of the function is presumably the size of
the returned data. Would it make sense to say so in
include/uapi/linux/bpf.h?
next reply other threads:[~2021-05-28 22:16 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-28 22:16 Eugene Loh [this message]
2021-06-01 21:48 ` using skip>0 with bpf_get_stack() Yonghong Song
2021-06-26 1:22 ` Eugene Loh
2021-06-29 3:33 ` Yonghong Song
2022-03-04 20:37 ` Namhyung Kim
2022-03-04 20:50 ` Eugene Loh
2022-03-04 21:16 ` Namhyung Kim
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