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

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Date: Tue, 5 Mar 2024 15:52:12 -0800
From: Kees Cook <keescook@...omium.org>
To: "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 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.

-- 
Kees Cook