linux-kernel - Re: [PATCH 2/4] power_supply: add eeprom dump file to olpc

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Date:	Wed, 30 Apr 2008 14:53:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andres Salomon <dilinger@...ued.net>
Cc:	cbou@...l.ru, linux-kernel@...r.kernel.org, dwmw2@...radead.org
Subject: Re: [PATCH 2/4] power_supply: add eeprom dump file to
 olpc_battery's sysfs

On Wed, 30 Apr 2008 16:30:08 -0400
Andres Salomon <dilinger@...ued.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@...ian.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 :(

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