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Message-ID: <CAEf4BzYrEfkdvP+k+np0S9-Rtf=xnpgVhL25wFgPQ81bm-_h_Q@mail.gmail.com>
Date: Wed, 27 Apr 2022 11:53:40 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Alexander Lobakin <alobakin@...me>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
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Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>,
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Subject: Re: [PATCH bpf-next 08/11] samples: bpf: fix shifting unsigned long
by 32 positions
On Wed, Apr 27, 2022 at 8:55 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 4/20/22 10:18 AM, Andrii Nakryiko wrote:
> > On Thu, Apr 14, 2022 at 3:46 PM Alexander Lobakin <alobakin@...me> wrote:
> >>
> >> On 32 bit systems, shifting an unsigned long by 32 positions
> >> yields the following warning:
> >>
> >> samples/bpf/tracex2_kern.c:60:23: warning: shift count >= width of type [-Wshift-count-overflow]
> >> unsigned int hi = v >> 32;
> >> ^ ~~
> >>
> >
> > long is always 64-bit in BPF, but I suspect this is due to
> > samples/bpf/Makefile still using this clang + llc combo, where clang
> > is called with native target and llc for -target bpf. Not sure if we
> > are ready to ditch that complicated combination. Yonghong, do we still
> > need that or can we just use -target bpf in samples/bpf?
>
> Current most bpf programs in samples/bpf do not use vmlinux.h and CO-RE.
> They direct use kernel header files. That is why clang C -> IR
> compilation still needs to be native.
>
> We could just use -target bpf for the whole compilation but that needs
> to change the code to use vmlinux.h and CO-RE. There are already a
> couple of sample bpf programs did this.
Right, I guess I'm proposing to switch samples/bpf to vmlinux.h. Only
purely networking BPF apps can get away with not using vmlinux.h
because they might avoid dependency on kernel types. But even then a
lot of modern networking apps seem to be gaining elements of more
generic tracing and would rely on CO-RE for staying "portable" between
kernels. So it might be totally fine to just use CO-RE universally in
samples/bpf?
>
> >
> >
> >> The usual way to avoid this is to shift by 16 two times (see
> >> upper_32_bits() macro in the kernel). Use it across the BPF sample
> >> code as well.
> >>
> >> Fixes: d822a1926849 ("samples/bpf: Add counting example for kfree_skb() function calls and the write() syscall")
> >> Fixes: 0fb1170ee68a ("bpf: BPF based latency tracing")
> >> Fixes: f74599f7c530 ("bpf: Add tests and samples for LWT-BPF")
> >> Signed-off-by: Alexander Lobakin <alobakin@...me>
> >> ---
> >> samples/bpf/lathist_kern.c | 2 +-
> >> samples/bpf/lwt_len_hist_kern.c | 2 +-
> >> samples/bpf/tracex2_kern.c | 2 +-
> >> 3 files changed, 3 insertions(+), 3 deletions(-)
> >>
> >
> > [...]
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