linux-kernel - Re: [PATCH 5.10 138/563] bpf: Remove config check to enable bpf support for branch records

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Message-ID: <20220125192735.GD5395@duo.ucw.cz>
Date:   Tue, 25 Jan 2022 20:27:35 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Kajol Jain <kjain@...ux.ibm.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.10 138/563] bpf: Remove config check to enable bpf
 support for branch records

Hi!

> Branch data available to BPF programs can be very useful to get stack traces
> out of userspace application.
> 
> Commit fff7b64355ea ("bpf: Add bpf_read_branch_records() helper") added BPF
> support to capture branch records in x86. Enable this feature also for other
> architectures as well by removing checks specific to x86.
> 
> If an architecture doesn't support branch records, bpf_read_branch_records()
> still has appropriate checks and it will return an -EINVAL in that scenario.
> Based on UAPI helper doc in include/uapi/linux/bpf.h, unsupported architectures
> should return -ENOENT in such case. Hence, update the appropriate check to
> return -ENOENT instead.
> 
> Selftest 'perf_branches' result on power9 machine which has the branch stacks
> support:
> 
>  - Before this patch:
> 
>   [command]# ./test_progs -t perf_branches
>    #88/1 perf_branches/perf_branches_hw:FAIL
>    #88/2 perf_branches/perf_branches_no_hw:OK
>    #88 perf_branches:FAIL
>   Summary: 0/1 PASSED, 0 SKIPPED, 1 FAILED
> 
>  - After this patch:
> 
>   [command]# ./test_progs -t perf_branches
>    #88/1 perf_branches/perf_branches_hw:OK
>    #88/2 perf_branches/perf_branches_no_hw:OK
>    #88 perf_branches:OK
>   Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
> 
> Selftest 'perf_branches' result on power9 machine which doesn't have branch
> stack report:
> 
>  - After this patch:
> 
>   [command]# ./test_progs -t perf_branches
>    #88/1 perf_branches/perf_branches_hw:SKIP
>    #88/2 perf_branches/perf_branches_no_hw:OK
>    #88 perf_branches:OK
>   Summary: 1/1 PASSED, 1 SKIPPED, 0 FAILED

This makes me nervous, it is not really a bugfix and probably noone
tested it on the stable branch. It would be safer to keep it disabled.

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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