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

Nick Desaulniers
2021-09-13T19:53:36Z

On Mon, Sep 13, 2021 at 11:39 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Mon, Sep 13, 2021 at 10:58 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Mon, Sep 13, 2021 at 09:52:33PM +0530, Naresh Kamboju wrote:
> > > [PATCH 00/10] raise minimum GCC version to 5.1
> > > https://lore.kernel.org/lkml/20210910234047.1019925-1-ndesaulniers@google.com/
> >
> > Has anyone submitted a fix for this upstream yet? I can't seem to find
> > one :(
>
> That lore link has a series to address this, though that's maybe
> something we don't want to backport to stable.
>
> I thought about this all weekend; I think I might be able to work
> around the one concern I had with my other approach, using
> __builtin_choose_expr().
>
> There's an issue with my alternative approach
> (https://gist.github.com/nickdesaulniers/2479818f4983bbf2d688cebbab435863)
> with declaring the local variable z in div_64() since either operand
> could be 64b, which result in an unwanted truncation if the dividend
> is 32b (or less, and divisor is 64b). I think (what I realized this
> weekend) is that we might be able to replace the `if` with
> `__builtin_choose_expr`, then have that whole expression be the final
> statement and thus the "return value" of the statement expression.

Christ...that...works? Though, did Linus just merge my patches for gcc 5.1?

Anyways, I'll send something like this for stable:
---

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 2928f03d6d46..e9ab8c25f8d3 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -11,6 +11,9 @@

#define div64_long(x, y) div64_s64((x), (y))
#define div64_ul(x, y) div64_u64((x), (y))
+#ifndef is_signed_type
+#define is_signed_type(type) (((type)(-1)) < (type)1)
+#endif

/**
* div_u64_rem - unsigned 64bit divide with 32bit divisor with remainder
@@ -112,6 +115,15 @@ extern s64 div64_s64(s64 dividend, s64 divisor);

#endif /* BITS_PER_LONG */

+#define div64_x64(dividend, divisor) ({ \
+ BUILD_BUG_ON_MSG(sizeof(dividend) < sizeof(u64),\
+ "prefer div_x64"); \
+ __builtin_choose_expr( \
+ is_signed_type(typeof(dividend)), \
+ div64_s64(dividend, divisor), \
+ div64_u64(dividend, divisor)); \
+})
+
/**
* div_u64 - unsigned 64bit divide with 32bit divisor
* @dividend: unsigned 64bit dividend
@@ -142,6 +154,28 @@ static inline s64 div_s64(s64 dividend, s32 divisor)
}
#endif

+#define div_x64(dividend, divisor) ({ \
+ BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u32),\
+ "prefer div64_x64"); \
+ __builtin_choose_expr( \
+ is_signed_type(typeof(dividend)), \
+ div_s64(dividend, divisor), \
+ div_u64(dividend, divisor)); \
+})
+
+// TODO: what if divisor is 128b?
+#define div_64(dividend, divisor) ({
\
+ __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);
\
+})
+
u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder);

#ifndef mul_u32_u32
---
--
Thanks,
~Nick Desaulniers