linux-kernel - Re: staging/wlan-ng query: convert to flexible array member

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Date:   Tue, 8 Nov 2022 21:15:10 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: staging/wlan-ng query: convert to flexible array member

On Tue, Nov 08, 2022 at 04:34:15PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Nov 08, 2022 at 08:42:59PM +0530, Deepak R Varma wrote:
> > Hello,
> >
> > First, my apologies for the long email.
> > I am requesting guidance on how to approach resolving the zero element flexible
> > VLO struct implementation in this driver in file drivers/staging/waln-ng/hfa384x.f
> >
> > The struct hfa384x_pdrec contains nested structs with zero element arrays.  These
> > zero element structs are part of a union 'data' inside the struct container. This
> > union 'data' is the last element of this container. Please see the code snip below:
> >
> > <snip>
> >
> > 	1068 struct hfa384x_pdrec {
> > 	   1         __le16 len;             /* in words */
> > 	   2         __le16 code;
> > 	   3         union pdr {
> > 	   4                 struct hfa384x_pdr_pcb_partnum pcb_partnum;
> > 	  11                 struct hfa384x_pdr_nicid nicid;
> > 	  12                 struct hfa384x_pdr_refdac_measurements refdac_measurements;
> > 	  13                 struct hfa384x_pdr_vgdac_measurements vgdac_measurements;
> > 	  14                 struct hfa384x_pdr_level_comp_measurements level_compc_measurements;
> > 	  15                 struct hfa384x_pdr_mac_address mac_address;
> > 	  39         } data;
> > 	  40 } __packed;
> >
> > </snip>
> >
> > The three structures in question are declared as follows in the same file:
> >
> > <snip>
> > 	962  struct hfa384x_pdr_refdac_measurements {
> > 	   1         u16 value[0];
> > 	   2 } __packed;
> > 	   3
> > 	   4 struct hfa384x_pdr_vgdac_measurements {
> > 	   5         u16 value[0];
> > 	   6 } __packed;
> > 	   7
> > 	   8 struct hfa384x_pdr_level_comp_measurements {
> > 	   9         u16 value[0];
> > 	  10 } __packed;
> > </snip>
> >
> > As per the C99 specifications, the flexible array struct should have at least
> > one member other than the true flexible array member. So converting these from
> > [0] to [] is not feasible in the current form.
> >
> > I did not find these struct variables being used for memory allocation in the
> > code directly. My find may be short since the implementation appears to get very
> > complex as I tried to get deeper.
> >
> > Can you please suggest how should I approach correcting the zero element flex
> > array implementation here? Can these structs be removed if they are unused?
>
> Are you sure they are unused?
>
> They look like structures that are read from the memory of a device,
> right?  Try removing the structures from the union and see what happens
> :)

I did remove the structs from the union and it built fine. Is there anything else I
can check/test to verify the impact?

<snip>
	drv@...ulion:~/git/kernels/staging$ git diff
	diff --git a/drivers/staging/wlan-ng/hfa384x.h b/drivers/staging/wlan-ng/hfa384x.h
	index 0611e37df6ac..8fe10aa93dfb 100644
	--- a/drivers/staging/wlan-ng/hfa384x.h
	+++ b/drivers/staging/wlan-ng/hfa384x.h
	@@ -1077,9 +1077,6 @@ struct hfa384x_pdrec {
			struct hfa384x_pdr_mfisuprange mfisuprange;
			struct hfa384x_pdr_cfisuprange cfisuprange;
			struct hfa384x_pdr_nicid nicid;
	-               struct hfa384x_pdr_refdac_measurements refdac_measurements;
	-               struct hfa384x_pdr_vgdac_measurements vgdac_measurements;
	-               struct hfa384x_pdr_level_comp_measurements level_compc_measurements;
			struct hfa384x_pdr_mac_address mac_address;
			struct hfa384x_pdr_mkk_callname mkk_callname;
			struct hfa384x_pdr_regdomain regdomain;
	drv@...ulion:~/git/kernels/staging$ make M=drivers/staging/wlan-ng/
	  CC [M]  drivers/staging/wlan-ng/prism2usb.o
	  CC [M]  drivers/staging/wlan-ng/p80211netdev.o
	  LD [M]  drivers/staging/wlan-ng/prism2_usb.o
	  MODPOST drivers/staging/wlan-ng/Module.symvers
	  LD [M]  drivers/staging/wlan-ng/prism2_usb.ko
	  BTF [M] drivers/staging/wlan-ng/prism2_usb.ko
	drv@...ulion:~/git/kernels/staging$
</snip>

>
> thanks,
>
> greg k-h