linux-kernel - Re: [PATCH bpf-next] selftests/bpf: Return -1 from get_preempt

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Message-Id: <3141d10de14ab669ef0927bdb175b0fd1e1e0d1b0c6b6092385e3bcb1898ee4d@mail.kernel.org>
Date: Thu, 29 Jan 2026 05:38:40 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: skb99@...ux.ibm.com,linux-kernel@...r.kernel.org,linux-kselftest@...r.kernel.org,bpf@...r.kernel.org
Cc: hbathini@...ux.ibm.com,sachinpb@...ux.ibm.com,venkat88@...ux.ibm.com,andrii@...nel.org,eddyz87@...il.com,ast@...nel.org,daniel@...earbox.net,martin.lau@...ux.dev,song@...nel.org,yonghong.song@...ux.dev,john.fastabend@...il.com,kpsingh@...nel.org,sdf@...ichev.me,haoluo@...gle.com,jolsa@...nel.org,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next] selftests/bpf: Return -1 from get_preempt_count() for unsupported architecture

> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
> index a39576c8ba..87706a213a 100644
> --- a/tools/testing/selftests/bpf/bpf_experimental.h
> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> @@ -628,7 +628,7 @@ static inline int get_preempt_count(void)
>  #elif defined(bpf_target_arm64)
>  	return bpf_get_current_task_btf()->thread_info.preempt.count;
>  #endif
> -	return 0;
> +	return -1;
>  }

Returning -1 here causes incorrect behavior in all the bpf_in_* helper
functions that call get_preempt_count(). Since -1 is 0xFFFFFFFF in
twos complement, the bitmask operations in the callers produce non-zero
results:

    bpf_in_nmi():     -1 & NMI_MASK     = 0x00F00000 (non-zero)
    bpf_in_hardirq(): -1 & HARDIRQ_MASK = 0x000F0000 (non-zero)
    bpf_in_interrupt(): returns non-zero (falsely indicates interrupt ctx)
    bpf_in_serving_softirq(): returns non-zero
    bpf_in_task(): returns 0 (falsely indicates NOT in task context)

Previously with return 0, these functions would return 0 on unsupported
architectures. Now they return false positives indicating the system is
in NMI/hardirq/softirq context when it may not be.

Should the callers be updated to check for -1 before performing the
bitmask operations? For example:

    static inline int bpf_in_nmi(void)
    {
        int pcnt = get_preempt_count();
        if (pcnt < 0)
            return -1;  /* or 0? */
        return pcnt & NMI_MASK;
    }

---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21466700415

AI-authorship-score: low
AI-authorship-explanation: Simple one-line change with clear human-authored commit message following standard kernel conventions.
issues-found: 1
issue-severity-score: low
issue-severity-explanation: Incomplete API change causes bpf_in_* functions to return false positives on unsupported architectures, but this only affects selftest code on non-x86/arm64 systems.