[PATCH v2] xfs: xattr: replace strncpy and check for truncation

[Justin Stitt <justinstitt@google.com>]

strncpy is deprecated and as such we should prefer less ambiguous and
more robust string interfaces [1].

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 while enabling us to check for
possible truncation, resulting in a soft warning.

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 "%.*s" format specifier
- use != instead of < to check for truncation (Christoph H.)
- Link to v1: https://lore.kernel.org/r/20240405-strncpy-xattr-split2-v1-1-90ab18232407@google.com
---
Tested with https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git
but using scripts + image from: https://github.com/tytso/xfstests-bld

test results: https://pastebin.com/44bjhpCp (no failures)

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_xattr.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
index 364104e1b38a..54e7e7d24ce9 100644
--- a/fs/xfs/xfs_xattr.c
+++ b/fs/xfs/xfs_xattr.c
@@ -206,6 +206,7 @@ __xfs_xattr_put_listent(
 {
 	char *offset;
 	int arraytop;
+	size_t combined_len, actual_len;
 
 	if (context->count < 0 || context->seen_enough)
 		return;
@@ -220,11 +221,16 @@ __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';
+
+	combined_len = prefix_len + namelen;
+
+	/* plus one byte for \0 */
+	actual_len = scnprintf(offset, combined_len + 1, "%.*s%.*s",
+			       prefix_len, prefix, namelen, name);
+
+	if (actual_len != combined_len)
+		xfs_warn(context->dp->i_mount,
+	"cannot completely copy context buffer resulting in truncation");
 
 compute_size:
 	context->count += prefix_len + namelen + 1;

---
base-commit: c85af715cac0a951eea97393378e84bb49384734
change-id: 20240405-strncpy-xattr-split2-0a3aff0c6a20

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