linux-kernel - Re: [PATCH] posix-clock: Explicitly handle compat ioctls

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Message-Id: <1ba5d3a4-7931-455b-a3ce-85a968a7cb10@app.fastmail.com>
Date: Thu, 23 Jan 2025 22:10:52 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: Thomas Weißschuh <linux@...ssschuh.net>,
 "Richard Cochran" <richardcochran@...il.com>
Cc: "Andrew Lunn" <andrew+netdev@...n.ch>,
 "David S . Miller" <davem@...emloft.net>,
 "Eric Dumazet" <edumazet@...gle.com>, "Jakub Kicinski" <kuba@...nel.org>,
 "Paolo Abeni" <pabeni@...hat.com>,
 "Anna-Maria Gleixner" <anna-maria@...utronix.de>,
 "Frederic Weisbecker" <frederic@...nel.org>,
 "Thomas Gleixner" <tglx@...utronix.de>, "John Stultz" <johnstul@...ibm.com>,
 Netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
 "Cyrill Gorcunov" <gorcunov@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH] posix-clock: Explicitly handle compat ioctls

On Thu, Jan 23, 2025, at 17:22, Thomas Weißschuh wrote:
> On 2025-01-22 18:15:13+0100, Arnd Bergmann wrote:
>> A simpler variant of the patch would move the switch/case logic
>> into posix_clock_compat_ioctl() and avoid the extra function
>> pointer, simply calling posix_clock_ioctl() with the modified
>> argument.
>
> That would work, but be a layering violation.
> Or a "compat_mode" argument to ptp_ioctl()
>
> I'm fine with either approach.

You don't even need to add an argument, just use
in_compat_syscall() to check for compat mode inside
of the ioctl handler. If CONFIG_COMPAT is disabled, that
gets optimized out through dead-code-elimination.

      Arnd