seq_buf: Add seq_buf_init_append()

Several strlcat() call sites being converted to seq_buf need behavior
seq_buf doesn't currently provide. The normal seq_buf_init() always
empties the buffer (sets its "len" to 0) via seq_buf_clear(). Code
migrating from strlcat(buf, ...), which appends to whatever buf already
contains, can't use seq_buf_init() without discarding that existing
content. Add seq_buf_init_append(), which preserves the existing
contents and positions the seq_buf to append after it. A buffer with
no NUL within its size starts out overflowed, as strlcat() treats it
as already truncated. Add KUnit tests for behavior coverage.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.2.0.

Assisted-by: LLM
Signed-off-by: Bill Wendling <morbo@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20261005155708.1471260-9-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 9830f905f0ee..be979089228f 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -71,6 +71,31 @@ seq_buf_set_overflow(struct seq_buf *s)
 	s->len = s->size + 1;
 }
 
+/**
+ * seq_buf_init_append - initialize a seq_buf over a buffer that may
+ *			 already hold NUL-terminated content
+ * @s: the seq_buf handle
+ * @buf: pointer to the (possibly non-empty) buffer
+ * @size: total size of @buf
+ *
+ * Unlike seq_buf_init(), which always starts @buf at len=0, this
+ * preserves whatever NUL-terminated content @buf already holds and
+ * positions @s to append after it. Useful for converting code that used
+ * to append to an existing buffer with strlcat()/scnprintf() and friends.
+ *
+ * If @buf holds no NUL within @size, @s starts out overflowed, as
+ * strlcat() treats such a buffer as already truncated.
+ */
+static inline void
+seq_buf_init_append(struct seq_buf *s, char *buf, unsigned int size)
+{
+	s->buffer = buf;
+	s->size = size;
+	s->len = strnlen(buf, size);
+	if (s->len == size)
+		seq_buf_set_overflow(s);
+}
+
 /*
  * How much buffer is left on the seq_buf?
  */