linux-kernel - Re: [PATCH] fs/xfs: replace strncpy with strscpy

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Message-ID: <qlogdnggv2y4nbzzt62oq4yguitq4ytkqavdwele3xrqi6gwfo@aj45rl7f3eik>
Date: Mon, 30 Jun 2025 10:38:59 +0200
From: Carlos Maiolino <cem@...nel.org>
To: Pranav Tyagi <pranav.tyagi03@...il.com>
Cc: skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev, 
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/xfs: replace strncpy with strscpy

On Tue, Jun 17, 2025 at 06:15:46PM +0530, Pranav Tyagi wrote:
> Replace the deprecated strncpy() with strscpy() as the destination
> buffer should be NUL-terminated and does not require any trailing
> NUL-padding. Also, since NUL-termination is guaranteed,

NUL-termination is only guaranteed if you copy into the buffer one less
byte than the label requires, i.e XFSLABEL_MAX.

> use sizeof(label) in place of XFSLABEL_MAX as the size
> parameter.

This is wrong, see below why.

> 
> Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.com>
> ---
>  fs/xfs/xfs_ioctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index d250f7f74e3b..9f4d68c5b5ab 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -992,7 +992,7 @@ xfs_ioc_getlabel(
>  	/* 1 larger than sb_fname, so this ensures a trailing NUL char */
>  	memset(label, 0, sizeof(label));
>  	spin_lock(&mp->m_sb_lock);
> -	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);
> +	strscpy(label, sbp->sb_fname, sizeof(label));

This is broken and you created a buffer overrun here.

XFSLABEL_MAX is set to 12 bytes. The current label size is 13 bytes:

char                    label[XFSLABEL_MAX + 1];

This ensures the label will always have a null termination character as
long as you copy XFSLABEL_MAX bytes into the label.

- strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

Copies 12 bytes from sb_fname into label. This ensures we always have a
trailing \0 at the last byte.

Your version:

strscpy(label, sbp->sb_fname, sizeof(label));

Copies 13 bytes from sb_fname into the label buffer.

This not only could have copied a non-null byte to the last byte in the
label buffer, but also But sbp->sb_fname size is XFSLABEL_MAX, so you
are reading beyond the source buffer size, causing a buffer overrun as you
can see on the kernel test robot report.

Carlos

>  	spin_unlock(&mp->m_sb_lock);
> 
>  	if (copy_to_user(user_label, label, sizeof(label)))
> --
> 2.49.0
>