Re: [PATCH] xfs: mark sb_fname as nonstring

[Eric Sandeen <sandeen@redhat.com>]

On 5/25/18 10:14 AM, Arnd Bergmann wrote:
> gcc-8 reports two warnings for the newly added getlabel/setlabel code:

Thanks for catching these.

The patch summary confuses me, what does "mark sb_fname as nonstring"
mean in the context of the actual patch?

I hate strings in C!  ;)

> fs/xfs/xfs_ioctl.c: In function 'xfs_ioc_getlabel':
> fs/xfs/xfs_ioctl.c:1822:38: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
>    strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));                                         ^
o_O hrpmh.

> In function 'strncpy',
>      inlined from 'xfs_ioc_setlabel' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1863:2,
>      inlined from 'xfs_file_ioctl' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1918:10:
> include/linux/string.h:254:9: error: '__builtin_strncpy' output may be truncated copying 12 bytes from a string of length 12 [-Werror=stringop-truncation]
>    return __builtin_strncpy(p, q, size);
> 
> In both cases, part of the problem is that one of the strncpy()
> arguments is a fixed-length character array with zero-padding rather
> than a zero-terminated string. In the first one case, we also get an
> odd warning about sizeof-pointer-memaccess, which doesn't seem right
> (the sizeof is for an array that happens to be the same as the second
> strncpy argument).
> 
> To work around the bogus warning, I use a plain 'XFSLABEL_MAX' for
> the strncpy() length when copying the label in getlabel. For setlabel(),
> using memcpy() with the correct length that is already known avoids
> the second warning and is slightly simpler.
> 
> In a related issue, it appears that we accidentally skip the trailing
> \0 when copying a 12-character label back to user space in getlabel().
> Using the correct sizeof() argument here copies the extra character.

Oops, you are correct.  Sigh, I wonder why testing didn't see that.

> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602
> Fixes: f7664b31975b ("xfs: implement online get/set fs label")
> Cc: Eric Sandeen <sandeen@redhat.com>
> Cc: Martin Sebor <msebor@gmail.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   fs/xfs/xfs_ioctl.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 84fbf164cbc3..eb79f2bc4dcc 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(
>   	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);
>   
>   	spin_lock(&mp->m_sb_lock);
> -	strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));
> +	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

ok

>   	spin_unlock(&mp->m_sb_lock);
>   
>   	/* xfs on-disk label is 12 chars, be sure we send a null to user */
>   	label[XFSLABEL_MAX] = '\0';
> -	if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))
> +	if (copy_to_user(user_label, label, sizeof(label)))


ok.  (odd how this is ok for copy_to_user but not for strncpy above) :)

>   		return -EFAULT;
>   	return 0;
>   }
> @@ -1860,7 +1860,7 @@ xfs_ioc_setlabel(
>   
>   	spin_lock(&mp->m_sb_lock);
>   	memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));
> -	strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));
> +	memcpy(sbp->sb_fname, label, len);

Hm but len = strnlen(label, XFSLABEL_MAX + 1);
which could be one longer than sbp->sb_fname, no?

>   	spin_unlock(&mp->m_sb_lock);
>   
>   	/*
>