[PATCH 1/3] relay: Fix 4 off-by-one errors occuring when writing to a CPU buffer.

[Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>]

Semantically, start + offset is always lower than the total size in bytes,
so the code should check against equality to size, not size + 1. When
userspace did read() (but not necessarily restricted to this), this
resulted in all-zeros data at the end of the buffer.

Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
---
 include/linux/relay.h |    4 ++--
 kernel/relay.c        |    4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/relay.h b/include/linux/relay.h
index 6cd8c44..8593ca1 100644
--- a/include/linux/relay.h
+++ b/include/linux/relay.h
@@ -202,7 +202,7 @@ static inline void relay_write(struct rchan *chan,
 
 	local_irq_save(flags);
 	buf = chan->buf[smp_processor_id()];
-	if (unlikely(buf->offset + length > chan->subbuf_size))
+	if (unlikely(buf->offset + length >= chan->subbuf_size))
 		length = relay_switch_subbuf(buf, length);
 	memcpy(buf->data + buf->offset, data, length);
 	buf->offset += length;
@@ -228,7 +228,7 @@ static inline void __relay_write(struct rchan *chan,
 	struct rchan_buf *buf;
 
 	buf = chan->buf[get_cpu()];
-	if (unlikely(buf->offset + length > buf->chan->subbuf_size))
+	if (unlikely(buf->offset + length >= buf->chan->subbuf_size))
 		length = relay_switch_subbuf(buf, length);
 	memcpy(buf->data + buf->offset, data, length);
 	buf->offset += length;
diff --git a/kernel/relay.c b/kernel/relay.c
index 7de644c..07f25e7 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -622,7 +622,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 	if (unlikely(length > buf->chan->subbuf_size))
 		goto toobig;
 
-	if (buf->offset != buf->chan->subbuf_size + 1) {
+	if (buf->offset != buf->chan->subbuf_size) {
 		buf->prev_padding = buf->chan->subbuf_size - buf->offset;
 		old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
 		buf->padding[old_subbuf] = buf->prev_padding;
@@ -645,7 +645,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 	new = buf->start + new_subbuf * buf->chan->subbuf_size;
 	buf->offset = 0;
 	if (!buf->chan->cb->subbuf_start(buf, new, old, buf->prev_padding)) {
-		buf->offset = buf->chan->subbuf_size + 1;
+		buf->offset = buf->chan->subbuf_size;
 		return 0;
 	}
 	buf->data = new;
-- 
1.5.5.4