tracing: Move readpos from seq_buf to trace_seq

To make seq_buf more lightweight as a string buf, move the readpos member
from seq_buf to its container, trace_seq.  That puts the responsibility
of maintaining the readpos entirely in the tracing code.  If some future
users want to package up the readpos with a seq_buf, we can define a
new struct then.

Link: https://lore.kernel.org/linux-trace-kernel/20231020033545.2587554-2-willy@infradead.org

Cc: Kees Cook <keescook@chromium.org>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 10 insertions, 12 deletions

diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 45c450f423fa..b7477aefff53 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -324,23 +324,24 @@ int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc)
  * seq_buf_to_user - copy the sequence buffer to user space
  * @s: seq_buf descriptor
  * @ubuf: The userspace memory location to copy to
+ * @start: The first byte in the buffer to copy
  * @cnt: The amount to copy
  *
  * Copies the sequence buffer into the userspace memory pointed to
- * by @ubuf. It starts from the last read position (@s->readpos)
- * and writes up to @cnt characters or till it reaches the end of
- * the content in the buffer (@s->len), which ever comes first.
+ * by @ubuf. It starts from @start and writes up to @cnt characters
+ * or until it reaches the end of the content in the buffer (@s->len),
+ * whichever comes first.
  *
  * On success, it returns a positive number of the number of bytes
  * it copied.
  *
  * On failure it returns -EBUSY if all of the content in the
  * sequence has been already read, which includes nothing in the
- * sequence (@s->len == @s->readpos).
+ * sequence (@s->len == @start).
  *
  * Returns -EFAULT if the copy to userspace fails.
  */
-int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
+int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, size_t start, int cnt)
 {
 	int len;
 	int ret;
@@ -350,20 +351,17 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
 
 	len = seq_buf_used(s);
 
-	if (len <= s->readpos)
+	if (len <= start)
 		return -EBUSY;
 
-	len -= s->readpos;
+	len -= start;
 	if (cnt > len)
 		cnt = len;
-	ret = copy_to_user(ubuf, s->buffer + s->readpos, cnt);
+	ret = copy_to_user(ubuf, s->buffer + start, cnt);
 	if (ret == cnt)
 		return -EFAULT;
 
-	cnt -= ret;
-
-	s->readpos += cnt;
-	return cnt;
+	return cnt - ret;
 }
 
 /**