Re: [PATCH] hwmon: (lm90) Fix error return value from detect function

[Kees Cook <keescook@chromium.org>]

On Tue, Aug 09, 2022 at 03:24:04PM -0700, Kees Cook wrote:
> On Mon, Aug 08, 2022 at 03:15:04AM -0700, Guenter Roeck wrote:
> > lm90_detect_nuvoton() is supposed to return NULL if it can not detect
> > a chip, or a pointer to the chip name if it does. Under some circumstances
> > it returns an error pointer instead. Some versions of gcc interpret an
> > ERR_PTR as region of size 0 and generate an error message.
> > 
> >   In function ‘__fortify_strlen’,
> >       inlined from ‘strlcpy’ at ./include/linux/fortify-string.h:159:10,
> >       inlined from ‘lm90_detect’ at drivers/hwmon/lm90.c:2550:2:
> >   ./include/linux/fortify-string.h:50:33: error:
> >       ‘__builtin_strlen’ reading 1 or more bytes from a region of size 0
> >      50 | #define __underlying_strlen     __builtin_strlen
> >         |                                 ^
> >   ./include/linux/fortify-string.h:141:24: note:
> >       in expansion of macro ‘__underlying_strlen’
> >     141 |                 return __underlying_strlen(p);
> >         |                        ^~~~~~~~~~~~~~~~~~~
> > 
> > Returning NULL instead of ERR_PTR() fixes the problem.
> > 
> > Fixes: c7cebce984a2 ("hwmon: (lm90) Rework detect function")
> > Reported-by: Ingo Molnar <mingo@kernel.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Kees Cook <keescook@chromium.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

> > Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> > ---
> > It is interesting that some versions of gcc interpret an ERR_PTR this way.
> > It did find a real bug, though the error message is quite confusing.
> > Would it be possible to enhance the fortify functions to detect a constant
> > ERR_PTR at compile time ? I think that might be quite useful.
> 
> Yeah, that should be possible. I suspect something like this might work:
> 
> 	BUILD_BUG_ON(__builtin_constant_p(src) && IS_ERR_VALUE(src));
> 	BUILD_BUG_ON(__builtin_constant_p(dst) && IS_ERR_VALUE(dst));
> 
> Though I'm not sure how it'd play with GCC value range checker.

Yeah, looks like this doesn't work. These are all only able to check for
a single value. The GCC diagnostics depend on its internal value range
checking. This tripped because of the (sometimes buggy) "void * cast of
a literal value is always a NULL pointer dereference, so its size must
always be zero" which we've had to repeatedly work around. In this case,
it was a real error, though. :P

I'm hoping we can teach future GCC "treat literal casts in range
$foo-$bar to be NULL derefs", and we can hand it the ERR_PTR range.

-Kees

-- 
Kees Cook