seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf_str()

Solve two ergonomic issues with struct seq_buf;

1) Too much boilerplate is required to initialize:

	struct seq_buf s;
	char buf[32];

	seq_buf_init(s, buf, sizeof(buf));

Instead, we can build this directly on the stack. Provide DECLARE_SEQ_BUF() macro to do this:

	DECLARE_SEQ_BUF(s, 32);

2) %NUL termination is fragile and requires 2 steps to get a valid

    C String (and is a layering violation exposing the "internals" of
    seq_buf):

	seq_buf_terminate(s);
	do_something(s->buffer);

Instead, we can just return s->buffer directly after terminating it in the refactored seq_buf_terminate(), now known as seq_buf_str():

	do_something(seq_buf_str(s));

Cc Steven Rostedt <[email protected]>

Cc "Matthew Wilcox (Oracle)" <[email protected]>

Cc Christoph Hellwig <[email protected]>

Cc Justin Stitt <[email protected]>

Cc Kent Overstreet <[email protected]>

Cc Petr Mladek <[email protected]>

Cc Andy Shevchenko <[email protected]>

Cc Rasmus Villemoes <[email protected]>

Cc Sergey Senozhatsky <[email protected]>

Cc Masami Hiramatsu <[email protected]>

Cc Greg Kroah-Hartman <[email protected]>

Cc Arnd Bergmann <[email protected]>

Cc Jonathan Corbet <[email protected]>

Cc Yun Zhou <[email protected]>

Cc Jacob Keller <[email protected]>

Cc Zhen Lei <[email protected]>

Cc [email protected]

Link https://lore.kernel.org/r/[email protected] open here

Signed-off-by Kees Cook <[email protected]>

---
v3
- fix commit log typos
- improve code style for DECLARE_SEQ_BUF (shevchenko)
- const-ify seq_bug_str() return (rostedt)
v2 - https://lore.kernel.org/lkml/[email protected]
v1 - https://lore.kernel.org/lkml/[email protected]

---
  include/linux/seq_buf.h | 21 +++++++++++++++++----
  kernel/trace/trace.c    | 11 +----------
  lib/seq_buf.c           |  4 +---
  3 files changed, 19 insertions(+), 17 deletions(-)

include/linux/seq_buf.h +0 -0

index 8483e4b2d0d2..5fb1f12c33f9 100644

include/linux/seq_buf.h +13 -4

+		.size = SIZE,				\
+	}
+
  static inline void seq_buf_clear(struct seq_buf *s)
  {
  	s->len = 0;
+	if (s->size)
+		s->buffer[0] = '\0';
  }
 
  static inline void

@@ -69,8 +78,8 @@ static inline unsigned int seq_buf_used(struct seq_buf *s)
  }
 
  /**
- * seq_buf_terminate - Make sure buffer is nul terminated
- * @s: the seq_buf descriptor to terminate.
+ * seq_buf_str - get %NUL-terminated C string from seq_buf
+ * @s: the seq_buf handle
   *
   * This makes sure that the buffer in @s is nul terminated and
   * safe to read as a string.

@@ -81,16 +90,20 @@ static inline unsigned int seq_buf_used(struct seq_buf *s)
   *
   * After this function is called, s->buffer is safe to use
   * in string operations.
+ *
+ * Returns @s->buf after making sure it is terminated.
   */
-static inline void seq_buf_terminate(struct seq_buf *s)
+static inline const char *seq_buf_str(struct seq_buf *s)
  {
  	if (WARN_ON(s->size == 0))
-		return;
+		return "";
 
  	if (seq_buf_buffer_left(s))
  		s->buffer[s->len] = 0;
  	else
  		s->buffer[s->size - 1] = 0;
+
+	return s->buffer;
  }
 
  /**

kernel/trace/trace.c +1 -10

index d629065c2383..2539cfc20a97 100644
@@ -3828,15 +3828,6 @@ static bool trace_safe_str(struct trace_iterator *iter, const char *str,
  	return false;
  }
 
-static const char *show_buffer(struct trace_seq *s)
-{
-	struct seq_buf *seq = &s->seq;
-
-	seq_buf_terminate(seq);
-
-	return seq->buffer;
-}
-
  static DEFINE_STATIC_KEY_FALSE(trace_no_verify);
 
  static int test_can_verify_check(const char *fmt, ...)

@@ -3976,7 +3967,7 @@ void trace_check_vprintf(struct trace_iterator *iter, const char *fmt,
  		 */
  		if (WARN_ONCE(!trace_safe_str(iter, str, star, len),
  			      "fmt: '%s' current_buffer: '%s'",
-			      fmt, show_buffer(&iter->seq))) {
+			      fmt, seq_buf_str(&iter->seq.seq))) {
  			int ret;
 
  			/* Try to safely read the string */

lib/seq_buf.c +1 -3

index b7477aefff53..23518f77ea9c 100644
@@ -109,9 +109,7 @@ void seq_buf_do_printk(struct seq_buf *s, const char *lvl)
  	if (s->size == 0 || s->len == 0)
  		return;
 
-	seq_buf_terminate(s);
-
-	start = s->buffer;
+	start = seq_buf_str(s);
  	while ((lf = strchr(start, '\n'))) {
  		int len = lf - start + 1;