[PATCH] xfs: cleanup deprecated uses of strncpy

[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.

In xfs_ioctl.c:
The current code has taken care NUL-termination by memset()'ing @label.
This is followed by a strncpy() to perform the string copy.

Use strscpy_pad() to get both 1) NUL-termination and 2) NUL-padding
which may be needed as this is copied out to userspace.

Note that this patch uses the new 2-argument version of strscpy_pad
introduced in Commit e6584c3964f2f ("string: Allow 2-argument
strscpy()").

In xfs_xattr.c:
There's a lot of manual memory management to get a prefix and name into
a string. Let's use an easier to understand and more robust interface in
scnprintf() to accomplish the same task.

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@xxxxxxxxxxxxxxx
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 fs/xfs/xfs_ioctl.c | 4 +---
 fs/xfs/xfs_xattr.c | 6 +-----
 2 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index d0e2cec6210d..abef9707a433 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1755,10 +1755,8 @@ xfs_ioc_getlabel(
 	/* 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);
+	strscpy_pad(label, sbp->sb_fname);
 	spin_unlock(&mp->m_sb_lock);
 
 	if (copy_to_user(user_label, label, sizeof(label)))
diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
index 364104e1b38a..b9256988830f 100644
--- a/fs/xfs/xfs_xattr.c
+++ b/fs/xfs/xfs_xattr.c
@@ -220,11 +220,7 @@ __xfs_xattr_put_listent(
 		return;
 	}
 	offset = context->buffer + context->count;
-	memcpy(offset, prefix, prefix_len);
-	offset += prefix_len;
-	strncpy(offset, (char *)name, namelen);			/* real name */
-	offset += namelen;
-	*offset = '\0';
+	scnprintf(offset, prefix_len + namelen + 1, "%s%s", prefix, name);
 
 compute_size:
 	context->count += prefix_len + namelen + 1;

---
base-commit: 928a87efa42302a23bb9554be081a28058495f22
change-id: 20240401-strncpy-fs-xfs-xfs_ioctl-c-8af7a895bff0

Best regards,
--
Justin Stitt <justinstitt@xxxxxxxxxx>