Date:   Mon, 13 Sep 2021 21:57:05 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Baokun Li <libaokun1@...wei.com>,
        open list <linux-kernel@...r.kernel.org>,
        linux-stable <stable@...r.kernel.org>,
        Hulk Robot <hulkci@...wei.com>,
        Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        lkft-triage@...ts.linaro.org, llvm@...ts.linux.dev,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 5.14 018/334] nbd: add the check to prevent overflow in __nbd_ioctl()

On Mon, Sep 13, 2021 at 9:53 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> On Mon, Sep 13, 2021 at 11:39 AM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Mon, Sep 13, 2021 at 10:58 AM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.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?
>

"Merge branch 'gcc-min-version-5.1' (make gcc-5.1 the minimum version)"

- Sedat -

https://git.kernel.org/linus/316346243be6df12799c0b64b788e06bad97c30b

> 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
>
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