linux-kernel - Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

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Date:   Tue, 24 Sep 2019 11:38:27 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linux@...ck-us.net, clang-built-linux@...glegroups.com,
        jdelvare@...e.com,
        Tomasz Paweł Gajc <tpgxyz@...il.com>,
        Henrik Rydberg <rydberg@...math.org>,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

On Tue, Sep 24, 2019 at 10:47:28AM -0700, Nick Desaulniers wrote:
> Fixes the following 2 issues in the driver:
> 1. Left shifting a signed integer is undefined behavior. Unsigned
>    integral types should be used for bitwise operations.
> 2. The delay scales from 0x0010 to 0x20000 by powers of 2, but udelay
>    will result in a linkage failure when given a constant that's greater
>    than 20000 (0x4E20). Agressive loop unrolling can fully unroll the
>    loop, resulting in later iterations overflowing the call to udelay.
> 
> 2 is fixed via splitting the loop in two, iterating the first up to the
> point where udelay would overflow, then switching to mdelay, as
> suggested in Documentation/timers/timers-howto.rst.
> 
> Reported-by: Tomasz Paweł Gajc <tpgxyz@...il.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/678
> Debugged-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Changes V1 -> V2:
> * The first loop in send_byte() needs to break out on the same condition
>   now. Technically, the loop condition could even be removed. The diff
>   looks funny because of the duplicated logic between existing and newly
>   added for loops.
> 
>  drivers/hwmon/applesmc.c | 35 +++++++++++++++++++++++++++++++----
>  1 file changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
> index 183ff3d25129..c76adb504dff 100644
> --- a/drivers/hwmon/applesmc.c
> +++ b/drivers/hwmon/applesmc.c
> @@ -46,6 +46,7 @@
>  #define APPLESMC_MIN_WAIT	0x0010
>  #define APPLESMC_RETRY_WAIT	0x0100
>  #define APPLESMC_MAX_WAIT	0x20000
> +#define APPLESMC_UDELAY_MAX	20000
>  
>  #define APPLESMC_READ_CMD	0x10
>  #define APPLESMC_WRITE_CMD	0x11
> @@ -157,14 +158,23 @@ static struct workqueue_struct *applesmc_led_wq;
>  static int wait_read(void)
>  {
>  	u8 status;
> -	int us;
> -	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> +	unsigned int us;
> +
> +	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_UDELAY_MAX; us <<= 1) {
>  		udelay(us);
>  		status = inb(APPLESMC_CMD_PORT);
>  		/* read: wait for smc to settle */
>  		if (status & 0x01)
>  			return 0;
>  	}
> +	/* switch to mdelay for longer sleeps */
> +	for (; us < APPLESMC_MAX_WAIT; us <<= 1) {
> +		mdelay(us);
> +		status = inb(APPLESMC_CMD_PORT);
> +		/* read: wait for smc to settle */
> +		if (status & 0x01)
> +			return 0;
> +	}
>  
>  	pr_warn("wait_read() fail: 0x%02x\n", status);
>  	return -EIO;
> @@ -177,10 +187,10 @@ static int wait_read(void)
>  static int send_byte(u8 cmd, u16 port)
>  {
>  	u8 status;
> -	int us;
> +	unsigned int us;
>  
>  	outb(cmd, port);
> -	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> +	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_UDELAY_MAX; us <<= 1) {
>  		udelay(us);
>  		status = inb(APPLESMC_CMD_PORT);
>  		/* write: wait for smc to settle */
> @@ -190,6 +200,23 @@ static int send_byte(u8 cmd, u16 port)
>  		if (status & 0x04)
>  			return 0;
>  		/* timeout: give up */
> +		if (us << 1 == APPLESMC_UDELAY_MAX)
> +			break;
> +		/* busy: long wait and resend */
> +		udelay(APPLESMC_RETRY_WAIT);
> +		outb(cmd, port);
> +	}
> +	/* switch to mdelay for longer sleeps */
> +	for (; us < APPLESMC_MAX_WAIT; us <<= 1) {
> +		mdelay(us);
> +		status = inb(APPLESMC_CMD_PORT);
> +		/* write: wait for smc to settle */
> +		if (status & 0x02)
> +			continue;
> +		/* ready: cmd accepted, return */
> +		if (status & 0x04)
> +			return 0;
> +		/* timeout: give up */
>  		if (us << 1 == APPLESMC_MAX_WAIT)
>  			break;
>  		/* busy: long wait and resend */
> -- 
> 2.23.0.351.gc4317032e6-goog
> 

This resolves the __bad_udelay appearance at -O3 for me. I am not
familiar enough with this code to give a reviewed by though!

Also, for some odd reason, I couldn't apply your patch with 'git apply':

% curl -LSs https://lore.kernel.org/lkml/20190924174728.201464-1-ndesaulniers@google.com/raw | git apply
error: corrupt patch at line 117

It looks like some of the '=' got changed into =3D and some spaces got
changed into =20. Weird encoding glitch?

Cheers,
Nathan