linux-kernel - Re: [PATCH][next] drm/i915/guc: Replace zero-length arrays with flexible-array members

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Message-ID: <Y72/wTm7eIIylTqf@work>
Date:   Tue, 10 Jan 2023 13:42:57 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gfx@...ts.freedesktop.org, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH][next] drm/i915/guc: Replace zero-length arrays with
 flexible-array members

On Tue, Jan 10, 2023 at 02:28:11PM -0500, Rodrigo Vivi wrote:
> 
> On Tue, Jan 10, 2023 at 10:44:53AM -0600, Gustavo A. R. Silva wrote:
> > Zero-length arrays are deprecated[1] and we are moving towards
> > adopting C99 flexible-array members, instead. So, replace zero-length
> > arrays in a couple of structures (three, actually) with flex-array
> > members.
> > 
> > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > routines on memcpy() and help us make progress towards globally
> > enabling -fstrict-flex-arrays=3 [2].
> > 
> > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays [1]
> > Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [2]
> > Link: https://github.com/KSPP/linux/issues/78
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> 
> > ---
> >  drivers/gpu/drm/i915/gt/uc/guc_capture_fwif.h | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/uc/guc_capture_fwif.h b/drivers/gpu/drm/i915/gt/uc/guc_capture_fwif.h
> > index 3624abfd22d1..9d589c28f40f 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/guc_capture_fwif.h
> > +++ b/drivers/gpu/drm/i915/gt/uc/guc_capture_fwif.h
> > @@ -73,7 +73,7 @@ struct guc_debug_capture_list_header {
> >  
> >  struct guc_debug_capture_list {
> >  	struct guc_debug_capture_list_header header;
> > -	struct guc_mmio_reg regs[0];
> > +	struct guc_mmio_reg regs[];
> >  } __packed;
> >  
> >  /**
> > @@ -125,7 +125,7 @@ struct guc_state_capture_header_t {
> >  
> >  struct guc_state_capture_t {
> >  	struct guc_state_capture_header_t header;
> > -	struct guc_mmio_reg mmio_entries[0];
> > +	struct guc_mmio_reg mmio_entries[];
> >  } __packed;
> >  
> >  enum guc_capture_group_types {
> > @@ -145,7 +145,7 @@ struct guc_state_capture_group_header_t {
> >  /* this is the top level structure where an error-capture dump starts */
> >  struct guc_state_capture_group_t {
> >  	struct guc_state_capture_group_header_t grp_header;
> > -	struct guc_state_capture_t capture_entries[0];
> > +	struct guc_state_capture_t capture_entries[];
> 
> Please notice we are currently using sizeof(struct ...).

Yep; I noticed that. :)

> Along with your proposed changes, shouldn't we also start using
> the struct_size() which already take the flexible array into account?

Not necessarily. In recent times, we don't include the struct_size
changes in the same patch as the flex-array transformation. That's
usually a follow-up patch.

--
Gustavo