Messages in this thread | | | Subject | Re: [RFC] [PATCH bpf-next 1/1] bpf: Clear the noisy tail buffer for bpf_d_path() helper | From | xufeng zhang <> | Date | Thu, 25 Nov 2021 09:47:56 +0800 |
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Hi Tao,
在 2021/11/24 下午4:49, Hou Tao 写道: > Hi, > > On 11/24/2021 12:15 PM, xufeng zhang wrote: >> Jiri and KP, >> >> Any suggestion? >> >> >> Thanks in advance! >> >> Xufeng >> >> 在 2021/11/20 下午1:18, Xufeng Zhang 写道: >>> From: "Xufeng Zhang" <yunbo.xufeng@linux.alibaba.com> >>> >>> The motivation behind this change is to use the returned full path >>> for lookup keys in BPF_MAP_TYPE_HASH map. >>> bpf_d_path() prepend the path string from the end of the input >>> buffer, and call memmove() to copy the full path from the tail >>> buffer to the head of buffer before return. So although the >>> returned buffer string is NULL terminated, there is still >>> noise data at the tail of buffer. >>> If using the returned full path buffer as the key of hash map, >>> the noise data is also calculated and makes map lookup failed. >>> To resolve this problem, we could memset the noisy tail buffer >>> before return. >>> >>> Signed-off-by: Xufeng Zhang <yunbo.xufeng@linux.alibaba.com> >>> --- >>> kernel/trace/bpf_trace.c | 2 ++ >>> 1 file changed, 2 insertions(+) >>> >>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c >>> index 25ea521fb8f1..ec4a6823c024 100644 >>> --- a/kernel/trace/bpf_trace.c >>> +++ b/kernel/trace/bpf_trace.c >>> @@ -903,6 +903,8 @@ BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, >>> u32, sz) >>> } else { >>> len = buf + sz - p; >>> memmove(buf, p, len); >>> + /* Clear the noisy tail buffer before return */ >>> + memset(buf + len, 0, sz - len); > Is implementing bpf_memset() helper a better idea ? So those who need to > clear the buffer after the terminated null character can use the helper to > do that.
This is a good point.
I think the reason why mainline has not such a helper yet is because a LLVM __builtin_memset() is
already available, but clearly this __builtin_memset() has too much limitation which can't meet all the needs,
there might be other concerns to implement such a memset helper which I don't know, but I think your suggestion
is a good idea.
Xufeng
> > Regards, > Tao > >>> } >>> return len; >> .
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