linux-kernel - Re: [PATCH v9 03/27] kallsyms: add static relationship between `KSYM_NAME_LEN{,

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Date:   Wed, 17 Aug 2022 12:50:50 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, patches@...ts.linux.dev,
        Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH v9 03/27] kallsyms: add static relationship between
 `KSYM_NAME_LEN{,_BUFFER}`

On Wed, Aug 17, 2022 at 12:39:48PM -0700, Kees Cook wrote:
> On Fri, Aug 05, 2022 at 05:41:48PM +0200, Miguel Ojeda wrote:
> > This adds a static assert to ensure `KSYM_NAME_LEN_BUFFER`
> > gets updated when `KSYM_NAME_LEN` changes.
> > 
> > The relationship used is one that keeps the new size (512+1)
> > close to the original buffer size (500).
> > 
> > Co-developed-by: Boqun Feng <boqun.feng@...il.com>
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> > ---
> >  scripts/kallsyms.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> > index f3c5a2623f71..f543b1c4f99f 100644
> > --- a/scripts/kallsyms.c
> > +++ b/scripts/kallsyms.c
> > @@ -33,7 +33,11 @@
> >  #define KSYM_NAME_LEN		128
> >  
> >  /* A substantially bigger size than the current maximum. */
> > -#define KSYM_NAME_LEN_BUFFER	499
> > +#define KSYM_NAME_LEN_BUFFER	512
> > +_Static_assert(
> > +	KSYM_NAME_LEN_BUFFER == KSYM_NAME_LEN * 4,
> > +	"Please keep KSYM_NAME_LEN_BUFFER in sync with KSYM_NAME_LEN"
> > +);
> 
> Why not just make this define:
> 
> #define KSYM_NAME_LEN_BUFFER (KSYM_NAME_LEN * 4)
> 
> ? If there's a good reason not it, please put it in the commit log.
> 

Because KSYM_NAME_LEN_BUFFER is used as a string by stringify() in
fscanf(), defining it as (KSYM_NAME_LEN * 4) will produce a string

	"128 * 4"

after stringify() and that doesn't work with fscanf().

Miguel, maybe we can add something below in the commit log?

`KSYM_NAME_LEN_BUFFER` cannot be defined as an expression, because it
gets stringified in the fscanf() format. Therefore a _Static_assert() is
needed.

Thoughts?

Regards,
Boqun

> -Kees
> 
> -- 
> Kees Cook