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

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Sep 24, 2019 at 12:36:44PM -0700, Nick Desaulniers wrote:
> 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?

Given that this could have real implications for the hardware, I would
think tested by would imply running this on said hardware. FWIW:

Build-tested-by: Nathan Chancellor <natechancellor@gmail.com>

> >
> > 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.

Yes, appears git send-email related because your attached patch applies
just fine.

Cheers,
Nathan