Re: [PATCH] eeprom: at25: Rework buggy read splitting

[Joel Stanley <joel@xxxxxxxxx>]

On Tue, 21 Jun 2022 at 13:22, Geert Uytterhoeven
<geert+renesas@xxxxxxxxx> wrote:
>
> 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,

sp: transferred

>   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: 0a35780c755ccec0 ("eeprom: at25: Split reads into chunks and cap write size")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>

Thanks Geert for the patch. This is particularly important as I
noticed the "fix" has been backported to stable trees.

I was surprised that patch went in as-is; I thought it needed some
discussion before merging. I wasn't sure if it was the correct fix at
all; I wondered if it should have been fixed at the spi layer. Do you
have an opinion there?

Eddie, can you jump in for some testing and a review of this one?

Cheers,

Joel

> ---
> Tested on Ebisu-4D with 25LC040 EEPROM.
> ---
>  drivers/misc/eeprom/at25.c | 26 ++++++++++++--------------
>  1 file changed, 12 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/misc/eeprom/at25.c b/drivers/misc/eeprom/at25.c
> index c9c56fd194c1301f..bdffc6543f6f8b7f 100644
> --- a/drivers/misc/eeprom/at25.c
> +++ b/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;
> --
> 2.25.1
>