seq_buf: Add seq_buf_do_printk() helper

Sometimes we use seq_buf to format a string buffer, which
we then pass to printk(). However, in certain situations
the seq_buf string buffer can get too big, exceeding the
PRINTKRB_RECORD_MAX bytes limit, and causing printk() to
truncate the string.

Add a new seq_buf helper. This helper prints the seq_buf
string buffer line by line, using \n as a delimiter,
rather than passing the whole string buffer to printk()
at once.

Link: https://lkml.kernel.org/r/20230415100110.1419872-1-senozhatsky@chromium.org

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: sergey-senozhatsky

include/linux/seq_buf.h

@@ -159,4 +159,6 @@ extern int
 seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
 #endif
 
+void seq_buf_do_printk(struct seq_buf *s, const char *lvl);
+
 #endif /* _LINUX_SEQ_BUF_H */

lib/seq_buf.c

@@ -93,6 +93,38 @@ int seq_buf_printf(struct seq_buf *s, const char *fmt, ...)
 }
 EXPORT_SYMBOL_GPL(seq_buf_printf);
 
+/**
+ * seq_buf_do_printk - printk seq_buf line by line
+ * @s: seq_buf descriptor
+ * @lvl: printk level
+ *
+ * printk()-s a multi-line sequential buffer line by line. The function
+ * makes sure that the buffer in @s is nul terminated and safe to read
+ * as a string.
+ */
+void seq_buf_do_printk(struct seq_buf *s, const char *lvl)
+{
+	const char *start, *lf;
+
+	if (s->size == 0 || s->len == 0)
+		return;
+
+	seq_buf_terminate(s);
+
+	start = s->buffer;
+	while ((lf = strchr(start, '\n'))) {
+		int len = lf - start + 1;
+
+		printk("%s%.*s", lvl, len, start);
+		start = ++lf;
+	}
+
+	/* No trailing LF */
+	if (start < s->buffer + s->len)
+		printk("%s%s\n", lvl, start);
+}
+EXPORT_SYMBOL_GPL(seq_buf_do_printk);
+
 #ifdef CONFIG_BINARY_PRINTF
 /**
  * seq_buf_bprintf - Write the printf string from binary arguments