linux-kernel - [PATCH] seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf

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Message-Id: <20231026170722.work.638-kees@kernel.org>
Date:   Thu, 26 Oct 2023 10:07:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Christoph Hellwig <hch@....de>,
        Justin Stitt <justinstitt@...gle.com>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Yun Zhou <yun.zhou@...driver.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-trace-kernel@...r.kernel.org,
        Yosry Ahmed <yosryahmed@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH] seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf_cstr()

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 direction after terminating it
in refactored seq_buf_terminate(), now known as seq_buf_cstr():

	do_soemthing(seq_buf_cstr(s));

Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc: Christoph Hellwig <hch@....de>
Cc: Justin Stitt <justinstitt@...gle.com>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Petr Mladek <pmladek@...e.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Jonathan Corbet <corbet@....net>
Cc: Yun Zhou <yun.zhou@...driver.com>
Cc: Jacob Keller <jacob.e.keller@...el.com>
Cc: Zhen Lei <thunder.leizhen@...wei.com>
Cc: linux-trace-kernel@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/linux/seq_buf.h | 19 +++++++++++++++----
 kernel/trace/trace.c    | 11 +----------
 lib/seq_buf.c           |  4 +---
 3 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 8483e4b2d0d2..8896b830eb3d 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -21,9 +21,16 @@ struct seq_buf {
 	size_t			len;
 };
 
+#define DECLARE_SEQ_BUF(NAME, SIZE)					\
+	char __ ## NAME ## _buffer[SIZE] = "";				\
+	struct seq_buf NAME = { .buffer = &__ ## NAME ## _buffer,	\
+				.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 +76,8 @@ 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_cstr - 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 +88,20 @@ 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 char *seq_buf_cstr(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;
 }
 
 /**
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d629065c2383..d83f36dc4bf8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -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_cstr(&iter->seq.seq))) {
 			int ret;
 
 			/* Try to safely read the string */
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index b7477aefff53..165caed5a74e 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -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_cstr(s);
 	while ((lf = strchr(start, '\n'))) {
 		int len = lf - start + 1;
 
-- 
2.34.1