linux-kernel - Re: [PATCH v2 0/2] perf trace: Fix support for the new BPF feature in clang 12

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Message-ID: <ae55ea37-0e0d-4fe1-9b1c-6a31dd20ea30@linaro.org>
Date: Fri, 11 Oct 2024 09:18:21 +0100
From: James Clark <james.clark@...aro.org>
To: Howard Chu <howardchu95@...il.com>
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
 mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
 irogers@...gle.com, adrian.hunter@...el.com, kan.liang@...ux.intel.com,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 alan.maguire@...cle.com, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 0/2] perf trace: Fix support for the new BPF feature in
 clang 12



On 11/10/2024 3:14 am, Howard Chu wrote:
> Changes in v2:
> - Resolved a clang-16 build error pointed out by Namhyung Kim
>    <namhyung@...nel.org>
> 
> The new augmentation feature in perf trace, along with the protocol
> change (from payload to payload->value), breaks the clang 12 build.
> 
> perf trace actually builds for any clang version newer than clang 16.
> However, as pointed out by Namhyung Kim <namhyung@...nel.org> and Ian
> Rogers <irogers@...gle.com>, clang 16, which was released in 2023, is
> still too new for most users. Additionally, as James Clark
> <james.clark@...aro.org> noted, some commonly used distributions do not
> yet support clang 16. Therefore, breaking BPF features between clang 12
> and clang 15 is not a good approach.
> 
> This patch series rewrites the BPF program in a way that allows it to
> pass the BPF verifier, even when the BPF bytecode is generated by older
> versions of clang.
> 
> However, I have only tested it till clang 14, as older versions are not
> supported by my distribution.
> 
> Howard Chu (2):
>    perf build: Change the clang check back to 12.0.1
>    perf trace: Rewrite BPF code to pass the verifier
> 
>   tools/perf/Makefile.config                    |   4 +-
>   .../bpf_skel/augmented_raw_syscalls.bpf.c     | 122 ++++++++++--------
>   2 files changed, 69 insertions(+), 57 deletions(-)
> 


Tested with clang 15:

$ sudo perf trace -e write --max-events=100 -- echo hello

    0.000 ( 0.014 ms): echo/976249 write(fd: 1, buf: hello\10, count: 6)
                                             =

Tested-by: James Clark <james.clark@...aro.org>