linux-kernel - RE: [PATCH 00/12] Clang -Wformat warning fixes

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Date:   Fri, 10 Jun 2022 08:17:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bill Wendling' <morbo@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Bill Wendling <isanbard@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Jan Kara <jack@...e.com>,
        "Pablo Neira Ayuso" <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        "Florian Westphal" <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        "Eric Dumazet" <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Ross Philipson <ross.philipson@...cle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "coreteam@...filter.org" <coreteam@...filter.org>,
        Networking <netdev@...r.kernel.org>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: RE: [PATCH 00/12] Clang -Wformat warning fixes

From: Bill Wendling
> Sent: 09 June 2022 23:49
> 
> On Thu, Jun 9, 2022 at 3:25 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Thu,  9 Jun 2022 22:16:19 +0000 Bill Wendling <morbo@...gle.com> wrote:
> >
> > > This patch set fixes some clang warnings when -Wformat is enabled.
> > >
> >
> > tldr:
> >
> > -       printk(msg);
> > +       printk("%s", msg);
> >
> > the only reason to make this change is where `msg' could contain a `%'.
> > Generally, it came from userspace.
> 
> It helps kernel developers not accidentally to insert an unescaped '%'
> in their messages, potentially exposing their code to an attack
> vector.
> 
> > Otherwise these changes are a
> > useless consumer of runtime resources.
> 
> Calling a "printf" style function is already insanely expensive. :-) I
> understand that it's not okay blithely to increase runtime resources
> simply because it's already slow, but in this case it's worthwhile.

Yep, IMHO definitely should be fixed.
It is even possible that using "%s" is faster because the printf
code doesn't have to scan the string for format effectors.

> > I think it would be better to quieten clang in some fashion.
> 
> The "printk" and similar functions all have the "__printf" attribute.
> I don't know of a modification to that attribute which can turn off
> this type of check.

And you wouldn't want to for these cases.

The only problems arise when the format is calculated
(or passed in from a caller).
But that is likely to be dangerous - reading formats from
files (eg for language translation) isn't a good idea at all.

	David

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