linux-kernel - Re: [PATCH bpf-next] bpf: Allow bpf_d_path in perf_event

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Message-ID: <CABRcYmLWAp6kYJBA2g+DvNQcg-5NaAz7u51ucBMPfW0dGykZAg@mail.gmail.com>
Date:   Fri, 29 Oct 2021 19:02:13 +0200
From:   Florent Revest <revest@...omium.org>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kpsingh@...nel.org, jackmanb@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Allow bpf_d_path in perf_event_mmap

On Fri, Oct 29, 2021 at 12:47 AM Martin KaFai Lau <kafai@...com> wrote:
>
> On Thu, Oct 28, 2021 at 06:43:57PM +0200, Florent Revest wrote:
> > Allow the helper to be called from the perf_event_mmap hook. This is
> > convenient to lookup vma->vm_file and implement a similar logic as
> > perf_event_mmap_event in BPF.
> From struct vm_area_struct:
>         struct file * vm_file;          /* File we map to (can be NULL). */
>
> Under perf_event_mmap, vm_file won't be NULL or bpf_d_path can handle it?

Thanks Martin, this is a very good point. :) Yes, vm_file can be NULL
in perf_event_mmap.
I wonder what would happen (and what we could do about it? :|).
bpf_d_path is called on &vma->vm_file->f_path So without NULL checks
(of vm_file) in BPF, the helper wouldn't be called with a NULL pointer
but rather with an address that is offsetof(struct file, f_path).