Re: [PATCH 1/2] kernel.h: use parentheses around argument in u64_to_user_ptr()

Mukesh Ojha
2019-03-29T05:59:44Z

On 3/29/2019 2:53 AM, Jann Horn wrote:
> Use parentheses around uses of the argument in u64_to_user_ptr() to ensure
> that the cast doesn't apply to part of the argument.
>
> There are existing uses of the macro of the form `u64_to_user_ptr(A + B)`,
> which expands to `(void __user *)(uintptr_t)A + B` (the cast applies to the
> first operand of the addition, the addition is a pointer addition). This
> happens to still work as intended, the semantic difference doesn't cause a
> difference in behavior.
> But I want to use u64_to_user_ptr() with a ternary operator in the
> argument, like so: `u64_to_user_ptr(A ? B : C)`. This currently doesn't
> work as intended.
>
> Fixes: f09174c501f8 ("x86: add user_atomic_cmpxchg_inatomic at uaccess.h")
> Signed-off-by: Jann Horn <jannh@google.com>

Looks good to me.
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>

-Mukesh

> ---
> Can we take this patch through the x86 tree with the following one, or
> do we need to get this one through akpm's tree first?
>
> include/linux/kernel.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 34a5036debd3..2d14e21c16c0 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -47,8 +47,8 @@
>
> #define u64_to_user_ptr(x) ( \
> { \
> - typecheck(u64, x); \
> - (void __user *)(uintptr_t)x; \
> + typecheck(u64, (x)); \
> + (void __user *)(uintptr_t)(x); \
> } \
> )
>