Re: [PATCH v9 03/27] kallsyms: add static relationship between `KSYM_NAME_LEN{,_BUFFER}`

[Kees Cook]

On Wed, Aug 17, 2022 at 12:50:50PM -0700, Boqun Feng wrote:
> 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@xxxxxxxxx>
> > > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> > > ---
> > >  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().

Ah yeah. Thanks!

> 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.

Yeah, please add a source comment for that. :)

-- 
Kees Cook