[PATCH] kunit: string-stream: Replace strlcat() with strscpy() and seq_buf

[PATCH] kunit: string-stream: Replace strlcat() with strscpy() and seq_buf

[Ian Bridges]

In preparation for removing the strlcat() API[1], replace its uses in
string-stream.

string_stream_vadd() appends at most a single newline into space that
was explicitly reserved when the fragment was sized, so a bounded copy
at the end of the string is enough. The return value of strscpy()
keeps the length accounting unchanged. string_stream_get_string()
concatenates a variable number of fragments into a buffer sized to
hold them all, which is what seq_buf is for.

Link: https://github.com/KSPP/linux/issues/370 [1]
Signed-off-by: Ian Bridges <icb@fastmail.org>
---
Truncation semantics differ between the two APIs. On overflow,
strlcat() keeps whatever prefix of the fragment fits, while
seq_buf_puts() drops any fragment that does not fit whole. Neither
path is reachable here. string_stream_get_string() sizes the buffer
to hold all fragments exactly. The newline lands in a byte that
string_stream_vadd() reserved when it sized the fragment.

The patch was tested as follows:
 - The string-stream KUnit suite passes all 12 tests before and after
   the change. The full lib/kunit KUnit config (95 tests) also shows
   identical results before and after.
 - An x86_64 build of lib/kunit/string-stream.o at W=1 produces no
   warnings.
 - A userspace comparison of the old and new construction ran
   with different fragment lengths for the newline append and with
   randomized fragment lists for the concatenation loop. All outputs
   were identical.

 lib/kunit/string-stream.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/kunit/string-stream.c b/lib/kunit/string-stream.c
index 0d8f1b30559b..51ba40ebf19f 100644
--- a/lib/kunit/string-stream.c
+++ b/lib/kunit/string-stream.c
@@ -9,6 +9,7 @@
 #include <kunit/static_stub.h>
 #include <kunit/test.h>
 #include <linux/list.h>
+#include <linux/seq_buf.h>
 #include <linux/slab.h>
 
 #include "string-stream.h"
@@ -74,7 +75,8 @@ int string_stream_vadd(struct string_stream *stream,
 
 		/* Append newline if necessary. */
 		if (frag_container->fragment[result_len - 1] != '\n')
-			result_len = strlcat(frag_container->fragment, "\n", buf_len);
+			result_len += strscpy(frag_container->fragment + result_len,
+					      "\n", buf_len - result_len);
 	} else {
 		result_len = vsnprintf(frag_container->fragment, buf_len, fmt, args);
 	}
@@ -118,15 +120,18 @@ char *string_stream_get_string(struct string_stream *stream)
 {
 	struct string_stream_fragment *frag_container;
 	size_t buf_len = stream->length + 1; /* +1 for null byte. */
+	struct seq_buf sb;
 	char *buf;
 
 	buf = kzalloc(buf_len, stream->gfp);
 	if (!buf)
 		return NULL;
 
+	seq_buf_init(&sb, buf, buf_len);
+
 	spin_lock(&stream->lock);
 	list_for_each_entry(frag_container, &stream->fragments, node)
-		strlcat(buf, frag_container->fragment, buf_len);
+		seq_buf_puts(&sb, frag_container->fragment);
 	spin_unlock(&stream->lock);
 
 	return buf;
-- 
2.47.3