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

[Nick Desaulniers <ndesaulniers@google.com>]

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On Tue, Sep 24, 2019 at 11:38 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> 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@gmail.com>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/678
> > Debugged-by: Nathan Chancellor <natechancellor@gmail.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.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!

Does that constitute a Tested-by tag?

>
> 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?

The text in my email client shows no encoding error; the link above
shows the issue.  Attaching a copy here, in case git-send-email
related.
-- 
Thanks,
~Nick Desaulniers

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From e112935cb4361aa820b89588e5764bfc3dfd7f9f Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Tue, 24 Sep 2019 10:26:34 -0700
Subject: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/678
Debugged-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.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