linux-kernel - Re: [PATCH] [v2] statmount: reduce runtime stack usage

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Date:   Wed, 13 Dec 2023 11:15:10 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Jan Kara <jack@...e.cz>,
        Ian Kent <raven@...maw.net>,
        Miklos Szeredi <mszeredi@...hat.com>,
        "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [v2] statmount: reduce runtime stack usage

On Wed, Dec 13, 2023 at 10:00:03AM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> prepare_kstatmount() constructs a copy of 'struct kstatmount' on the stack
> and copies it into the local variable on the stack of its caller. Because
> of the size of this structure, this ends up overflowing the limit for
> a single function's stack frame when prepare_kstatmount() gets inlined
> and both copies are on the same frame without the compiler being able
> to collapse them into one:
> 
> fs/namespace.c:4995:1: error: stack frame size (1536) exceeds limit (1024) in '__se_sys_statmount' [-Werror,-Wframe-larger-than]
>  4995 | SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
> 
> Replace the assignment with an in-place memset() plus assignment that
> should always be more efficient for both stack usage and runtime cost.
> 
> Fixes: 49889374ab92 ("statmount: simplify string option retrieval")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---

I folded this patch and placed a link here. Thanks!