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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Bill Wendling' <morbo@google.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Bill Wendling <isanbard@gmail.com>,
	Tony Luck <tony.luck@intel.com>, Borislav Petkov <bp@alien8.de>,
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	Dave Hansen <dave.hansen@linux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
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Subject: RE: [PATCH 00/12] Clang -Wformat warning fixes
Date: Fri, 10 Jun 2022 08:17:25 +0000	[thread overview]
Message-ID: <01da36bfd13e421aadb2eff661e7a959@AcuMS.aculab.com> (raw)
In-Reply-To: <CAGG=3QXDt9AeCQOAp1311POFRSByJru4=Q=oFiQn3u2iZYk2_w@mail.gmail.com>

From: Bill Wendling
> Sent: 09 June 2022 23:49
> 
> On Thu, Jun 9, 2022 at 3:25 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Thu,  9 Jun 2022 22:16:19 +0000 Bill Wendling <morbo@google.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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