linux-kernel - Re: [PATCH v2] HID: ishtp-hid-client: replace fake-flex arrays with flex-array members

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Message-ID: <nycvar.YFH.7.76.2410080852440.20286@cbobk.fhfr.pm>
Date: Tue, 8 Oct 2024 08:52:55 +0200 (CEST)
From: Jiri Kosina <jikos@...nel.org>
To: Kees Cook <kees@...nel.org>
cc: srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>, 
    Erick Archer <erick.archer@...look.com>, 
    Benjamin Tissoires <bentiss@...nel.org>, linux-kernel@...r.kernel.org, 
    linux-input@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] HID: ishtp-hid-client: replace fake-flex arrays with
 flex-array members

On Sun, 22 Sep 2024, Kees Cook wrote:

> From: Erick Archer <erick.archer@...look.com>
> 
> One-element arrays as fake flex arrays are deprecated[1] as the kernel
> has switched to C99 flexible-array members instead. This case, however,
> has more complexity because it is a flexible array of flexible arrays
> and this patch needs to be ready to enable the new compiler flag
> -Wflex-array-member-not-at-end (coming in GCC-14) globally.
> 
> So, define a new struct type for the single reports:
> 
> struct report {
> 	uint16_t size;
> 	struct hostif_msg_hdr msg;
> } __packed;
> 
> but without the payload (flex array) in it. And add this payload to the
> "hostif_msg" structure. This way, in the "report_list" structure we can
> declare a flex array of single reports which now do not contain another
> flex array.
> 
> struct report_list {
> 	[...]
>         struct report reports[];
> } __packed;
> 
> Therefore, the "struct hostif_msg" is now made up of a header and a
> payload. And the "struct report" uses only the "hostif_msg" header.
> The perfect solution would be for the "report" structure to use the
> whole "hostif_msg" structure but this is not possible due to nested
> flexible arrays. Anyway, the end result is equivalent since this patch
> does attempt to change the behaviour of the code.
> 
> Now as well, we have more clarity after the cast from the raw bytes to
> the new structures. Refactor the code accordingly to use the new
> structures.
> 
> Also, use "container_of()" whenever we need to retrieve a pointer to
> the flexible structure, through which we can access the flexible array
> if needed.
> 
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays [1]
> Closes: https://github.com/KSPP/linux/issues/333
> Signed-off-by: Erick Archer <erick.archer@...look.com>
> Link: https://lore.kernel.org/r/AS8PR02MB723760CB93942370E92F00638BF72@AS8PR02MB7237.eurprd02.prod.outlook.com
> [kees: tweaked commit log and dropped struct_size() uses]
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
>  v2: - update based on feedback
>  rfc: https://lore.kernel.org/lkml/AS8PR02MB723760CB93942370E92F00638BF72@AS8PR02MB7237.eurprd02.prod.outlook.com/

Applied, thanks.

-- 
Jiri Kosina
SUSE Labs