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From: Florent Revest <revest@chromium.org>
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>
Subject: Re: [PATCH bpf-next v3 3/6] bpf: Add a bpf_snprintf helper
Date: Thu, 15 Apr 2021 11:34:22 +0200	[thread overview]
Message-ID: <CABRcYm+XPciihdZDWSMUBLmtBCuDSV=bHvtEUqZupfC=cng6FA@mail.gmail.com> (raw)
In-Reply-To: <CAEf4Bza+rVCp=G5i97MuuBrTX+o1ZUBn3nzstssoS1KtE4F6vw@mail.gmail.com>

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 :)

  reply	other threads:[~2021-04-15  9:34 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-12 15:37 [PATCH bpf-next v3 0/6] Add a snprintf eBPF helper Florent Revest
2021-04-12 15:37 ` [PATCH bpf-next v3 1/6] bpf: Factorize bpf_trace_printk and bpf_seq_printf Florent Revest
2021-04-13 23:01   ` Andrii Nakryiko
2021-04-14  9:56     ` Florent Revest
2021-04-14 10:56       ` Florent Revest
2021-04-12 15:37 ` [PATCH bpf-next v3 2/6] bpf: Add a ARG_PTR_TO_CONST_STR argument type Florent Revest
2021-04-13 23:03   ` Andrii Nakryiko
2021-04-12 15:37 ` [PATCH bpf-next v3 3/6] bpf: Add a bpf_snprintf helper Florent Revest
2021-04-12 19:11   ` kernel test robot
2021-04-12 20:32   ` kernel test robot
2021-04-14 13:32     ` Florent Revest
2021-04-12 20:40   ` kernel test robot
2021-04-13 23:16   ` Andrii Nakryiko
2021-04-14  9:46     ` Florent Revest
2021-04-14 22:57       ` Andrii Nakryiko
2021-04-15  9:34         ` Florent Revest [this message]
2021-04-14 18:02     ` Geert Uytterhoeven
2021-04-14 18:30       ` Florent Revest
2021-04-14 22:58         ` Andrii Nakryiko
2021-04-15  7:18           ` Geert Uytterhoeven
2021-04-12 15:37 ` [PATCH bpf-next v3 4/6] libbpf: Initialize the bpf_seq_printf parameters array field by field Florent Revest
2021-04-13 23:18   ` Andrii Nakryiko
2021-04-12 15:37 ` [PATCH bpf-next v3 5/6] libbpf: Introduce a BPF_SNPRINTF helper macro Florent Revest
2021-04-13 23:18   ` Andrii Nakryiko
2021-04-12 15:37 ` [PATCH bpf-next v3 6/6] selftests/bpf: Add a series of tests for bpf_snprintf Florent Revest
2021-04-13 23:21   ` Andrii Nakryiko
2021-04-14  9:21     ` Florent Revest
2021-04-14 22:16       ` Andrii Nakryiko
2021-04-15  9:38         ` Florent Revest

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