Re: [PATCH 5.14 018/334] nbd: add the check to prevent overflow in __nbd_ioctl()

Nick Desaulniers
2021-09-13T21:15:49Z

On Mon, Sep 13, 2021 at 2:13 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Mon, Sep 13, 2021 at 1:50 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Mon, Sep 13, 2021 at 1:42 PM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > On Mon, Sep 13, 2021 at 1:16 PM Nick Desaulniers
> > > <ndesaulniers@google.com> wrote:
> > > >
> > > > Do we have access to _Generic in GCC 4.9?
> > >
> > > We've ended up using it unconditionally since last year, so yes.
> >
> > Sorry, grepping would have taken < 1s. I'm very lazy.
> > http://threevirtues.com/
> >
> > >
> > > In fact, the compiler version tests got removed when we raised the gcc
> > > version requirement to 4.9 in commit 6ec4476ac825 ("Raise gcc version
> > > requirement to 4.9"):
> > >
> > > "In particular, raising the minimum to 4.9 means that we can now just
> > > assume _Generic() exists, which is likely the much better replacement
> > > for a lot of very convoluted built-time magic with conditionals on
> > > sizeof and/or __builtin_choose_expr() with same_type() etc"
> > >
> > > but we haven't used it much since.
> > >
> > > The "seqprop" code for picking the right lock for seqlock is perhaps
> > > the main example, and staring at that code will make you go blind, so
> > > look away.
> >
> > Looking at my patch:
> > https://lore.kernel.org/stable/20210913203201.1844253-1-ndesaulniers@google.com/
> > I don't think _Generic helps us in the case of dispatching based on
> > the result of is_signed_type() (the operands could undergo type
> > promotion, so we'd need lots of cases that are more concisely covered
> > by is_signed_type()). It could replace the nested checks in div_64
> > with nested _Generics, I think. Not sure it's a huge win for
> > readability. Maybe cut the number of expansions of the parameters in
> > half though. Let me give it a try just to see what it looks like.
>
> Is this more readable? Same line count. I'm not sure if there's such
> a thing as "fallthrough" between the "cases" of _Generic to minimize
> duplication, or whether this could be factored further. Needs lots
> more () around macro param use and tab'ed out line endings...

Sorry wrong diff:
diff --git a/include/linux/math64.h b/include/linux/math64.h
index bc9c12c168d0..8fc4d56a45b9 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -162,18 +162,18 @@ static inline s64 div_s64(s64 dividend, s32 divisor)
div_u64(dividend, divisor)); \
})

+#define __div_64(dividend, divisor) _Generic((divisor), \
+ s64: div64_x64(dividend, divisor), \
+ u64: div64_x64(dividend, divisor), \
+ default: div_x64(dividend, divisor))
+
#define div_64(dividend, divisor) ({
\
BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u64),
\
"128b div unsupported");
\
- __builtin_choose_expr(
\
- __builtin_types_compatible_p(typeof(dividend), s64) ||
\
- __builtin_types_compatible_p(typeof(dividend), u64),
\
- __builtin_choose_expr(
\
- __builtin_types_compatible_p(typeof(divisor),
s64) || \
- __builtin_types_compatible_p(typeof(divisor),
u64), \
- div64_x64(dividend, divisor),
\
- div_x64(dividend, divisor)),
\
- dividend / divisor);
\
+ _Generic((dividend), \
+ s64: __div_64(dividend, divisor), \
+ u64: __div_64(dividend, divisor), \
+ default: dividend / divisor); \
})
--
Thanks,
~Nick Desaulniers