linux-kernel - Re: [PATCH][next] tty: tty_buffer: Avoid hundreds of -Wflex-array-member-not-at-end warnings

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Message-ID: <2025020503-unnamable-canopener-ac71@gregkh>
Date: Wed, 5 Feb 2025 06:36:26 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Jiri Slaby <jirislaby@...nel.org>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] tty: tty_buffer: Avoid hundreds of
 -Wflex-array-member-not-at-end warnings

On Wed, Feb 05, 2025 at 03:51:35PM +1030, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> So, in order to avoid ending up with a flexible-array member in the
> middle of other structs, we use the `struct_group_tagged()` helper
> to create a new tagged `struct tty_buffer_hdr`. This structure
> groups together all the members of the flexible `struct tty_buffer`
> except the flexible array.
> 
> As a result, the array is effectively separated from the rest of the
> members without modifying the memory layout of the flexible structure.
> We then change the type of the middle struct member currently causing
> trouble from `struct tty_buffer` to `struct tty_buffer_hdr`.
> 
> We also want to ensure that when new members need to be added to the
> flexible structure, they are always included within the newly created
> tagged struct. For this, we use `static_assert()`. This ensures that the
> memory layout for both the flexible structure and the new tagged struct
> is the same after any changes.
> 
> This approach avoids having to implement `struct tty_buffer_hdr` as a
> completely separate structure, thus preventing having to maintain two
> independent but basically identical structures, closing the door to
> potential bugs in the future.

Why not just have a separate structure and embed that in the places it
is used?  No duplication should be needed or am I missing something?

I don't mind that, it would make this all much simpler and more obvious
over time, and the tty layer needs all the "simplification" it can get
:)

thanks,

greg k-h