linux-kernel - Re: [RFC PATCH 3/4] selftests/binderfs: Macro argument 'fd' may be better as '(fd)' to avoid precedence issues

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Date: Wed, 15 May 2024 11:32:48 -0700
From: Bill Wendling <morbo@...gle.com>
To: Abhinav Saxena <xandfury@...il.com>
Cc: linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, Shuah Khan <shuah@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [RFC PATCH 3/4] selftests/binderfs: Macro argument 'fd' may be
 better as '(fd)' to avoid precedence issues

On Tue, May 14, 2024 at 6:08 PM Abhinav Saxena <xandfury@...il.com> wrote:
>
> Change the macro argument 'fd' to '(fd)' to avoid potential precedence
> issues. Without parentheses, the macro expansion could lead to
> unexpected behavior when used with an expression having different
> precedence levels.
>
> Example Code:
>
>     #define CALC_SQR_BAD(x) (x*x)
>     #define CALC_SQR_GOOD(x) ((x)*(x))
>
> CALC_SQR_BAD(2+3) expands to 2+(3*3)+2, giving 11 as the incorrect
> answer. Enclosing with parathesis CALC_SQR_GOOD(2+3) sets the correct

s/parathesis/parentheses/

> order of precedence and expands to (2+3)*(2+3), giving 25 as the
> correct answer.
>
> Signed-off-by: Abhinav Saxena <xandfury@...il.com>
> ---
>  .../testing/selftests/filesystems/binderfs/binderfs_test.c  | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/filesystems/binderfs/binderfs_test.c b/tools/testing/selftests/filesystems/binderfs/binderfs_test.c
> index 4a149e3d4ba4..d5dba6c1e07f 100644
> --- a/tools/testing/selftests/filesystems/binderfs/binderfs_test.c
> +++ b/tools/testing/selftests/filesystems/binderfs/binderfs_test.c
> @@ -30,11 +30,11 @@
>
>  #define close_prot_errno_disarm(fd)      \
>         do {                             \
> -               if (fd >= 0) {           \
> +               if ((fd) >= 0) {         \
>                         int _e_ = errno; \
> -                       close(fd);       \
> +                       close((fd));     \
>                         errno = _e_;     \
> -                       fd = -EBADF;     \
> +                       (fd) = -EBADF;   \

While I agree that it's generally good to add parentheses in macros,
this line ensures that 'fd' must be an lvalue and not an arithmetic
expression.

-bw

>                 }                        \
>         } while (false)
>
> --
> 2.34.1
>