Subject: Re: [PATCH] powerpc/rtas: Replace one-element arrays with flexible arrays

From: Kees Cook <[email-masked]>
Date: Fri, 27 Jan 2023 11:23:57 -0800

On Fri, Jan 27, 2023 at 07:10:28AM -0600, Nathan Lynch wrote:
> Andrew Donnellan <[email-masked]> writes:
> > Using a one-element array as a fake flexible array is deprecated.
> >
> > Replace the one-element flexible arrays in rtas-types.h with C99 standard
> > flexible array members instead.
> >
> > This helps us move towards enabling -fstrict-flex-arrays=3 in future.
> >
> > Found using scripts/coccinelle/misc/flexible_array.cocci.
> >
> > Cc: Nathan Lynch <[email-masked]>
> > Cc: Leonardo Bras <[email-masked]>
> > Cc: [email-masked]
> > Link: https://github.com/KSPP/linux/issues/21
> > Link: https://github.com/KSPP/linux/issues/79
> > Signed-off-by: Andrew Donnellan <[email-masked]>
> > ---
> >  arch/powerpc/include/asm/rtas-types.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/rtas-types.h b/arch/powerpc/include/asm/rtas-types.h
> > index 8df6235d64d1..40ec03a05c0b 100644
> > --- a/arch/powerpc/include/asm/rtas-types.h
> > +++ b/arch/powerpc/include/asm/rtas-types.h
> > @@ -44,7 +44,7 @@ struct rtas_error_log {
> >  	 */
> >  	u8		byte3;			/* General event or error*/
> >  	__be32		extended_log_length;	/* length in bytes */
> > -	unsigned char	buffer[1];		/* Start of extended log */
> > +	unsigned char	buffer[];		/* Start of extended log */
> >  						/* Variable length.      */
> >  };
> >  
> > @@ -85,7 +85,7 @@ struct rtas_ext_event_log_v6 {
> >  					/* that defines the format for	*/
> >  					/* the vendor specific log type	*/
> >  	/* Byte 16-end of log */
> > -	u8 vendor_log[1];		/* Start of vendor specific log	*/
> > +	u8 vendor_log[];		/* Start of vendor specific log	*/
> >  					/* Variable length.		*/
> >  };
> 
> I see at least one place that consults the size of one of these structs,
> in get_pseries_errorlog():
> 
> 	/* Check that we understand the format */
> 	if (ext_log_length < sizeof(struct rtas_ext_event_log_v6) || ...
> 
> Don't all such sites need to be audited/adjusted for changes like this?

Yeah, I'd expect a binary comparison[1] before/after to catch things like
this. E.g. the following C files mention those structs:

arch/powerpc/platforms/pseries/io_event_irq.c
arch/powerpc/platforms/pseries/ras.c
arch/powerpc/kernel/rtasd.c
arch/powerpc/kernel/rtas.c

-Kees

[1] https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/

-- 
Kees Cook