eeprom: at25: Rework buggy read splitting

The recent change to split reads into chunks has several problems:
  1. If an SPI controller has no transfer size limit, max_chunk is
     SIZE_MAX, and num_msgs becomes zero, causing no data to be read
     into the buffer, and exposing the original contents of the buffer
     to userspace,
  2. If the requested read size is not a multiple of the maximum
     transfer size, the last transfer reads too much data, overflowing
     the buffer,
  3. The loop logic differs from the write case.

Fix the above by:
  1. Keeping track of the number of bytes that are still to be
     transferred, instead of precalculating the number of messages and
     keeping track of the number of bytes tranfered,
  2. Calculating the transfer size of each individual message, taking
     into account the number of bytes left,
  3. Switching from a "while"-loop to a "do-while"-loop, and renaming
     "msg_count" to "segment".

While at it, drop the superfluous cast from "unsigned int" to "unsigned
int", also from at25_ee_write(), where it was probably copied from.

Fixes: 0a35780 ("eeprom: at25: Split reads into chunks and cap write size")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/7ae260778d2c08986348ea48ce02ef148100e088.1655817534.git.geert+renesas@glider.be
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: geertu

drivers/misc/eeprom/at25.c

@@ -80,10 +80,9 @@ static int at25_ee_read(void *priv, unsigned int offset,
 	struct at25_data *at25 = priv;
 	char *buf = val;
 	size_t max_chunk = spi_max_transfer_size(at25->spi);
-	size_t num_msgs = DIV_ROUND_UP(count, max_chunk);
-	size_t nr_bytes = 0;
-	unsigned int msg_offset;
-	size_t msg_count;
+	unsigned int msg_offset = offset;
+	size_t bytes_left = count;
+	size_t segment;
 	u8			*cp;
 	ssize_t			status;
 	struct spi_transfer	t[2];
@@ -97,9 +96,8 @@ static int at25_ee_read(void *priv, unsigned int offset,
 	if (unlikely(!count))
 		return -EINVAL;
 
-	msg_offset = (unsigned int)offset;
-	msg_count = min(count, max_chunk);
-	while (num_msgs) {
+	do {
+		segment = min(bytes_left, max_chunk);
 		cp = at25->command;
 
 		instr = AT25_READ;
@@ -131,8 +129,8 @@ static int at25_ee_read(void *priv, unsigned int offset,
 		t[0].len = at25->addrlen + 1;
 		spi_message_add_tail(&t[0], &m);
 
-		t[1].rx_buf = buf + nr_bytes;
-		t[1].len = msg_count;
+		t[1].rx_buf = buf;
+		t[1].len = segment;
 		spi_message_add_tail(&t[1], &m);
 
 		status = spi_sync(at25->spi, &m);
@@ -142,10 +140,10 @@ static int at25_ee_read(void *priv, unsigned int offset,
 		if (status)
 			return status;
 
-		--num_msgs;
-		msg_offset += msg_count;
-		nr_bytes += msg_count;
-	}
+		msg_offset += segment;
+		buf += segment;
+		bytes_left -= segment;
+	} while (bytes_left > 0);
 
 	dev_dbg(&at25->spi->dev, "read %zu bytes at %d\n",
 		count, offset);
@@ -229,7 +227,7 @@ static int at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
 	do {
 		unsigned long	timeout, retries;
 		unsigned	segment;
-		unsigned	offset = (unsigned) off;
+		unsigned	offset = off;
 		u8		*cp = bounce;
 		int		sr;
 		u8		instr;