[PATCH v2 4/4] null_blk: Zero-initialize read buffers in non-memory-backed mode

[Bart Van Assche <bvanassche@acm.org>]

This patch suppresses an uninteresting KMSAN complaint without affecting
performance of the null_blk driver if CONFIG_KMSAN is disabled.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Cc: Alexander Potapenko <glider@google.com>
Reported-by: Alexander Potapenko <glider@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/block/null_blk_main.c | 50 +++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/drivers/block/null_blk_main.c b/drivers/block/null_blk_main.c
index 06f5761fccb6..0c1df6ecb30b 100644
--- a/drivers/block/null_blk_main.c
+++ b/drivers/block/null_blk_main.c
@@ -1250,8 +1250,58 @@ static inline blk_status_t null_handle_memory_backed(struct nullb_cmd *cmd,
 	return errno_to_blk_status(err);
 }
 
+static void zero_fill_bvec(const struct bio_vec *bvec)
+{
+	struct page *page = bvec->bv_page;
+	u32 offset = bvec->bv_offset;
+	u32 left = bvec->bv_len;
+
+	while (left) {
+		u32 len = min_t(u32, left, PAGE_SIZE - offset);
+		void *kaddr;
+
+		kaddr = kmap_atomic(page);
+		memset(kaddr + offset, 0, len);
+		flush_dcache_page(page);
+		kunmap_atomic(kaddr);
+		page++;
+		left -= len;
+		offset = 0;
+	}
+}
+
+static void nullb_zero_rq_data_buffer(const struct request *rq)
+{
+	struct req_iterator iter;
+	struct bio_vec bvec;
+
+	rq_for_each_bvec(bvec, rq, iter)
+		zero_fill_bvec(&bvec);
+}
+
+static void nullb_zero_read_cmd_buffer(struct nullb_cmd *cmd)
+{
+	struct nullb_device *dev = cmd->nq->dev;
+
+	if (dev->queue_mode == NULL_Q_BIO && bio_op(cmd->bio) == REQ_OP_READ)
+		zero_fill_bio(cmd->bio);
+	else if (req_op(cmd->rq) == REQ_OP_READ)
+		nullb_zero_rq_data_buffer(cmd->rq);
+}
+
+/* Complete a request. Only called if dev->memory_backed == 0. */
 static inline void nullb_complete_cmd(struct nullb_cmd *cmd)
 {
+	/*
+	 * Since root privileges are required to configure the null_blk
+	 * driver, it is fine that this driver does not initialize the
+	 * data buffers of read commands. Zero-initialize these buffers
+	 * anyway if KMSAN is enabled to prevent that KMSAN complains
+	 * about null_blk not initializing read data buffers.
+	 */
+	if (IS_ENABLED(CONFIG_KMSAN))
+		nullb_zero_read_cmd_buffer(cmd);
+
 	/* Complete IO by inline, softirq or timer */
 	switch (cmd->nq->dev->irqmode) {
 	case NULL_IRQ_SOFTIRQ: