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Date: Sat, 19 Nov 2022 20:08:10 +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, gustavoars@...nel.org Subject: Re: [PATCH] staging: wlan-ng: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper On Thu, Nov 17, 2022 at 07:03:21PM +0100, Greg Kroah-Hartman wrote: > On Thu, Nov 17, 2022 at 06:50:55PM +0530, Deepak R Varma wrote: > > On Thu, Nov 17, 2022 at 01:54:49PM +0100, Greg Kroah-Hartman wrote: > > > On Thu, Nov 17, 2022 at 03:48:45PM +0530, Deepak R Varma wrote: > > > > > > > > struct hfa384x_pdr_refdac_measurements { > > > > - u16 value[0]; > > > > + DECLARE_FLEX_ARRAY(u16, value); > > > > } __packed; > > > > > > Why? This structure is never used anywhere, right? So why is this > > > needed to be changed and not just removed entirely? Same for the other > > > structures in this patch. > > > > Hello Greg, > > I am unable to confirm that these structures are truly not needed in the absence > > if a real device based testing. I could only validate that using the compile > > build and driver loading. > > Think this through, if no one is actually using this structure, and it > is of 0 size, then do you think it is being used? Hello Greg, I did not find any memory allocation for these zero length array structures. Also, the union or its enclosing structure do not appear to access the members. Hence I am leaning towards concluding that these zero length array structures do not appear to be necessary. There are a few other structures that are part of the same union, however, they too do not appear to be used for accessing the memory assigned to the union [or its enclosing structure]. I think most of the members of these unions can be replaced by one max size structure of this union [e.g. struct hfa384x_pdr_mkk_measurements]. Could you please comment if I am reading the code right? For your quick reference, the zero length structure declaration are online 963 whereas the union is on line number 1080 of the file drivers/staging/wlan-ng/hfa384x.h Thank you, ./drv > > > This change that I am proposing in the interim would enable the compiler to > > protect the structure from addition of a new member below the zero length array. > > Why would you want to add a new member below this? That's not what is > happening here at all. I came across this one old commit where such an accident happened. This is from a recent LWN article: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e48f129c2f20 I understand the C99 now protects from such an attempt at the compile time itself. Thank you, ./drv > > Please think this through a bit more. > > good luck! > > greg k-h >
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