[RFC PATCH 08/17] tracing: Have seq_buf use full buffer

[Wang Long]

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

commit 8cd709ae7658a7fd7f6630699e3229188c2591e4 upstream.

Currently seq_buf is full when all but one byte of the buffer is
filled. Change it so that the seq_buf is full when all of the
buffer is filled.

Some of the functions would fill the buffer completely and report
everything was fine. This was inconsistent with the max of size - 1.
Changing this to be max of size makes all functions consistent.

Link: http://lkml.kernel.org/r/20141104160222.502133196@xxxxxxxxxxx
Link: http://lkml.kernel.org/r/20141114011412.811957882@xxxxxxxxxxx

Tested-by: Jiri Kosina <jkosina@xxxxxxx>
Acked-by: Jiri Kosina <jkosina@xxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxx>
[wanglong: backport to 3.10 stable]
Signed-off-by: Wang Long <long.wanglong@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 include/linux/seq_buf.h | 6 +++---
 kernel/trace/seq_buf.c  | 9 ++++++---
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 93718e5..0800a24 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -43,13 +43,13 @@ seq_buf_init(struct seq_buf *s, unsigned char *buf, unsigned int size)
 static inline bool
 seq_buf_has_overflowed(struct seq_buf *s)
 {
-	return s->len == s->size;
+	return s->len > s->size;
 }
 
 static inline void
 seq_buf_set_overflow(struct seq_buf *s)
 {
-	s->len = s->size;
+	s->len = s->size + 1;
 }
 
 /*
@@ -61,7 +61,7 @@ seq_buf_buffer_left(struct seq_buf *s)
 	if (seq_buf_has_overflowed(s))
 		return 0;
 
-	return (s->size - 1) - s->len;
+	return s->size - s->len;
 }
 
 /* How much buffer was written? */
diff --git a/kernel/trace/seq_buf.c b/kernel/trace/seq_buf.c
index 6fc9d02..c53f1d5 100644
--- a/kernel/trace/seq_buf.c
+++ b/kernel/trace/seq_buf.c
@@ -26,7 +26,7 @@
  */
 static bool seq_buf_can_fit(struct seq_buf *s, size_t len)
 {
-	return s->len + len < s->size;
+	return s->len + len <= s->size;
 }
 
 /**
@@ -110,8 +110,11 @@ int seq_buf_bitmask(struct seq_buf *s, const unsigned long *maskp,
 	WARN_ON(s->size == 0);
 
 	/*
-	 * The last byte of the buffer is used to determine if we
-	 * overflowed or not.
+	 * Note, because bitmap_scnprintf() only returns the number of bytes
+	 * written and not the number that would be written, we use the last
+	 * byte of the buffer to let us know if we overflowed. There's a small
+	 * chance that the bitmap could have fit exactly inside the buffer, but
+	 * it's not that critical if that does happen.
 	 */
 	if (len > 1) {
 		ret = bitmap_scnprintf(s->buffer + s->len, len, maskp, nmaskbits);
-- 
1.8.3.4

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