[PATCH v2] xfs: replace deprecated strncpy with memtostr_pad

[PATCH v2] xfs: replace deprecated strncpy with memtostr_pad

[Justin Stitt]

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

sbp->sb_fname may not be NUL-terminated while label is expected to be.
memtostr best describes this behavior, specifically, use the pad variant
since we're copying out to userspace.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org

Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v2:
- use memtostr_pad (thanks Kees)
- Link to v1: https://lore.kernel.org/r/20240405-strncpy-xfs-split1-v1-1-3e3df465adb9@google.com
---
Note: This patch relies on the memtostr{_pad} implementation from Kees' patch:
https://lore.kernel.org/all/20240410023155.2100422-1-keescook@chromium.org/

Split from https://lore.kernel.org/all/20240401-strncpy-fs-xfs-xfs_ioctl-c-v1-1-02b9feb1989b@google.com/
with feedback from Christoph H.
---
 fs/xfs/xfs_ioctl.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index d0e2cec6210d..7ed7a5d57094 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1750,15 +1750,14 @@ xfs_ioc_getlabel(
 	char			__user *user_label)
 {
 	struct xfs_sb		*sbp = &mp->m_sb;
+	/* 1 larger than sb_fname, for a NULL byte */
 	char			label[XFSLABEL_MAX + 1];
 
 	/* Paranoia */
 	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);
 
-	/* 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);
+	memtostr_pad(label, sbp->sb_fname);
 	spin_unlock(&mp->m_sb_lock);
 
 	if (copy_to_user(user_label, label, sizeof(label)))

---
base-commit: c85af715cac0a951eea97393378e84bb49384734
change-id: 20240405-strncpy-xfs-split1-a2c408b934c6

Best regards,
--
Justin Stitt <justinstitt@google.com>