linux-kernel - Re: [PATCH] bcachefs: ioctl: avoid stack overflow warning

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Message-ID: <okwdyhfmx6qocem3ckgvt3jhdglyjav7tdrjzgquugj3kkolvn@ontqnranltb6>
Date: Tue, 10 Jun 2025 09:37:43 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Thomas Bertschinger <tahbertschinger@...il.com>, 
	linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] bcachefs: ioctl: avoid stack overflow warning

On Tue, Jun 10, 2025 at 11:24:04AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Multiple ioctl handlers individually use a lot of stack space, and clang chooses
> to inline them into the bch2_fs_ioctl() function, blowing through the warning
> limit:
> 
> fs/bcachefs/chardev.c:655:6: error: stack frame size (1032) exceeds limit (1024) in 'bch2_fs_ioctl' [-Werror,-Wframe-larger-than]
>   655 | long bch2_fs_ioctl(struct bch_fs *c, unsigned cmd, void __user *arg)
> 
> By marking the largest two of them as noinline_for_stack, no indidual code path
> ends up using this much, which avoids the warning and reduces the possible
> total stack usage in the ioctl handler.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Applied, thanks

> ---
>  fs/bcachefs/chardev.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/bcachefs/chardev.c b/fs/bcachefs/chardev.c
> index 2d38466eddfd..fde3c2380e28 100644
> --- a/fs/bcachefs/chardev.c
> +++ b/fs/bcachefs/chardev.c
> @@ -399,7 +399,7 @@ static long bch2_ioctl_data(struct bch_fs *c,
>  	return ret;
>  }
>  
> -static long bch2_ioctl_fs_usage(struct bch_fs *c,
> +static noinline_for_stack long bch2_ioctl_fs_usage(struct bch_fs *c,
>  				struct bch_ioctl_fs_usage __user *user_arg)
>  {
>  	struct bch_ioctl_fs_usage arg = {};
> @@ -469,7 +469,7 @@ static long bch2_ioctl_query_accounting(struct bch_fs *c,
>  }
>  
>  /* obsolete, didn't allow for new data types: */
> -static long bch2_ioctl_dev_usage(struct bch_fs *c,
> +static noinline_for_stack long bch2_ioctl_dev_usage(struct bch_fs *c,
>  				 struct bch_ioctl_dev_usage __user *user_arg)
>  {
>  	struct bch_ioctl_dev_usage arg;
> -- 
> 2.39.5
>