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Date: Tue, 24 Sep 2019 12:36:44 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Nathan Chancellor <natechancellor@...il.com>
Cc: Guenter Roeck <linux@...ck-us.net>,
clang-built-linux <clang-built-linux@...glegroups.com>,
jdelvare@...e.com,
Tomasz Paweł Gajc <tpgxyz@...il.com>,
Henrik Rydberg <rydberg@...math.org>,
linux-hwmon@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow
On Tue, Sep 24, 2019 at 11:38 AM Nathan Chancellor
<natechancellor@...il.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@...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!
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
View attachment "v2-0001-hwmon-applesmc-fix-UB-and-udelay-overflow.patch" of type "text/x-patch" (3718 bytes)
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