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From:   Florent Revest <revest@chromium.org>
Date:   Thu, 15 Apr 2021 11:34:22 +0200
Message-ID: <CABRcYm+XPciihdZDWSMUBLmtBCuDSV=bHvtEUqZupfC=cng6FA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3 3/6] bpf: Add a bpf_snprintf helper
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Yonghong Song <yhs@fb.com>, KP Singh <kpsingh@kernel.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        open list <linux-kernel@vger.kernel.org>
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On Thu, Apr 15, 2021 at 12:57 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Apr 14, 2021 at 2:46 AM Florent Revest <revest@chromium.org> wrote:
> >
> > On Wed, Apr 14, 2021 at 1:16 AM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > > On Mon, Apr 12, 2021 at 8:38 AM Florent Revest <revest@chromium.org> wrote:
> > > > +
> > > > +       return err + 1;
> > >
> > > snprintf() already returns string length *including* terminating zero,
> > > so this is wrong
> >
> > lib/vsprintf.c says:
> >  * The return value is the number of characters which would be
> >  * generated for the given input, excluding the trailing null,
> >  * as per ISO C99.
> >
> > Also if I look at the "no arg" test case in the selftest patch.
> > "simple case" is asserted to return 12 which seems correct to me
> > (includes the terminating zero only once). Am I missing something ?
> >
>
> no, you are right, but that means that bpf_trace_printk is broken, it
> doesn't do + 1 (which threw me off here), shall we fix that?

Answered in the 1/6 thread

> > However that makes me wonder whether it would be more appropriate to
> > return the value excluding the trailing null. On one hand it makes
> > sense to be coherent with other BPF helpers that include the trailing
> > zero (as discussed in patch v1), on the other hand the helper is
> > clearly named after the standard "snprintf" function and it's likely
> > that users will assume it works the same as the std snprintf.
>
>
> Having zero included simplifies BPF code tremendously for cases like
> bpf_probe_read_str(). So no, let's stick with including zero
> terminator in return size.

Cool :)