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Date:   Tue, 8 Nov 2022 16:34:15 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Deepak R Varma <drv@mailo.com>
Cc:     linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: staging/wlan-ng query: convert to flexible array member
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References: <Y2px+zOGjkpGh6qC@qemulion>
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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
:)

thanks,

greg k-h