Re: [PATCH 2/4] power_supply: add eeprom dump file to olpc_battery's sysfs

Andrew Morton
2008-04-30T21:53:31Z

On Wed, 30 Apr 2008 16:30:08 -0400
Andres Salomon <dilinger@queued.net> wrote:

>
> This allows you to dump 0x60 bytes from the battery's EEPROM (starting
> at address 0x20). Note that it does an EC command for each byte, so
> it's pretty slow. OTOH, if you want to grab just a single byte from
> somewhere in the EEPROM, you can do something like:
>
> dd bs=1 count=1 skip=16 if=/sys/class/power_supply/olpc-battery/eeprom | od -x
>
> Userspace battery collection/logging information needs this.
>
> Signed-off-by: Andres Salomon <dilinger@debian.org>
> ---
> drivers/power/olpc_battery.c | 49 ++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 49 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/power/olpc_battery.c b/drivers/power/olpc_battery.c
> index 9d9dd09..e5ecf27 100644
> --- a/drivers/power/olpc_battery.c
> +++ b/drivers/power/olpc_battery.c
> @@ -283,6 +283,48 @@ static enum power_supply_property olpc_bat_props[] = {
> POWER_SUPPLY_PROP_SERIAL_NUMBER,
> };
>
> +/* EEPROM reading goes completely around the power_supply API, sadly */
> +
> +#define EEPROM_START 0x20
> +#define EEPROM_END 0x80
> +#define EEPROM_SIZE (EEPROM_END - EEPROM_START)
> +
> +static ssize_t olpc_bat_eeprom_read(struct kobject *kobj,
> + struct bin_attribute *attr, char *buf, loff_t off, size_t count)
> +{
> + uint8_t ec_byte;
> + int ret, end;
> +
> + if (off >= EEPROM_SIZE)
> + return 0;

loff_t is signed.

Happily, the VFS prevents negative loff_t's from being passed into read()
handlers.

> + if (off + count > EEPROM_SIZE)
> + count = EEPROM_SIZE - off;

But the vfs doesn't check for (offset+len) going negative (I think?)

However you got lucky (or smart ;)) - size_t is unsigned so the comparison
will dtrt.

Plus the first `if' will prevent us getting here with huge-nearly-negative
`off'.

This stuff is harder than it should be :(