linux-kernel - Re: [PATCH] HID: Remove default case statement in fetch

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Message-ID: <nycvar.YFH.7.76.2410161017120.20286@cbobk.fhfr.pm>
Date: Wed, 16 Oct 2024 10:17:21 +0200 (CEST)
From: Jiri Kosina <jikos@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
cc: Benjamin Tissoires <bentiss@...nel.org>, 
    Dmitry Torokhov <dmitry.torokhov@...il.com>, linux-input@...r.kernel.org, 
    linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
    patches@...ts.linux.dev, "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] HID: Remove default case statement in fetch_item()

On Tue, 15 Oct 2024, Nathan Chancellor wrote:

> A default case statement with a bare unreachable() was recently added to
> fetch_item(), which by itself introduces undefined behavior. objtool
> points this out with a few different warnings, depending on
> configuration and compiler:
> 
>   vmlinux.o: warning: objtool: fetch_item() falls through to next function ...
> 
>   vmlinux.o: warning: objtool: hid_open_report() falls through to next function hid_parser_main()
>   vmlinux.o: warning: objtool: hid_scan_report() falls through to next function hid_allocate_device()
> 
>   vmlinux.o: warning: objtool: hid_open_report+0x21b: can't find jump dest instruction at .text.hid_open_report+0x40f
> 
> Replacing unreachable() with BUG() is a typical fix to eliminate the
> undefined behavior and make the default case well defined. However, in
> this case, all possible values are enumerated in the switch statement,
> so the default case can never actually happen, as proven with the
> comment next to the item->size assignment. Just remove the default case
> altogether, as the return statement would still be valid if the switch
> statement were ever to be skipped.
> 
> Fixes: 61595012f280 ("HID: simplify code in fetch_item()")
> Suggested-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> Closes: https://lore.kernel.org/20241010222451.GA3571761@thelio-3990X/
> Reported-by: Paul E. McKenney <paulmck@...nel.org>
> Closes: https://lore.kernel.org/fe8c909e-bf02-4466-b3eb-0a4747df32e3@paulmck-laptop/
> Tested-by: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Applied, thanks.

-- 
Jiri Kosina
SUSE Labs