linux-hardening - Re: [PATCH][next] fsnotify: Avoid -Wflex-array-member-not-at-end warning

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Date: Wed, 6 Mar 2024 09:42:17 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] fsnotify: Avoid -Wflex-array-member-not-at-end
 warning

On 3/5/24 17:52, Kees Cook wrote:
> On Tue, Mar 05, 2024 at 04:18:46PM -0600, Gustavo A. R. Silva wrote:
>> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
>> ready to enable it globally.
>>
>> There is currently a local structure `f` that is using a flexible
>> `struct file_handle` as header for an on-stack place-holder for the
>> flexible-array member `unsigned char f_handle[];`.
>>
>> struct {
>> 	struct file_handle handle;
>> 	u8 pad[MAX_HANDLE_SZ];
>> } f;
> 
> This code pattern is "put a flex array struct on the stack", but we have
> a macro for this now:
> 
> DEFINE_FLEX(struct file_handle, handle, f_handle, MAX_HANDLE_SZ);
> 
> And you can even include the initializer:
> 
> _DEFINE_FLEX(struct file_handle, handle, f_handle, MAX_HANDLE_SZ,
> 	     = { .handle_bytes = MAX_HANDLE_SZ });
> 
> I think this would be a simpler conversion.
> 
> Also, this could use a __counted_by tag...
> 
> I need to improve the DEFINE_FLEX macro a bit, though, to take advantage
> of __counted_by.
> 

Yep, I like it.

I'll go and hunt down all those on-stack -Wflex-array-member-not-at-end
issues with this helper. :)

Thanks
--
Gustavo