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Message-Id: <20230519153622.f95f74819ecf467c5291de7d@linux-foundation.org>
Date: Fri, 19 May 2023 15:36:22 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix mult_frac() multiple argument evaluation bug
On Fri, 19 May 2023 23:24:54 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:
> mult_frac() evaluates _all_ arguments multiple times in the body.
>
> --- a/include/linux/math.h
> +++ b/include/linux/math.h
> @@ -118,17 +118,17 @@ __STRUCT_FRACT(s32)
> __STRUCT_FRACT(u32)
> #undef __STRUCT_FRACT
>
> -/*
> - * Multiplies an integer by a fraction, while avoiding unnecessary
> - * overflow or loss of precision.
> - */
> -#define mult_frac(x, numer, denom)( \
> -{ \
> - typeof(x) quot = (x) / (denom); \
> - typeof(x) rem = (x) % (denom); \
> - (quot * (numer)) + ((rem * (numer)) / (denom)); \
> -} \
> -)
> +/* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
> +#define mult_frac(x, n, d) \
> +({ \
> + typeof(x) x_ = (x); \
> + typeof(n) n_ = (n); \
> + typeof(d) d_ = (d); \
> + \
> + typeof(x) q = x_ / d_; \
> + typeof(x) r = x_ % d_; \
> + q * n_ + r * n_ / d_; \
> +})
>
I like, but the compiler doesn't. x86_64 allmodconfig, gcc-12.2.0.
In file included from ./include/linux/math64.h:6,
from ./include/linux/time.h:6,
from ./include/linux/ktime.h:24,
from ./include/linux/timer.h:6,
from drivers/target/target_core_iblock.c:16:
drivers/target/target_core_iblock.c: In function 'iblock_configure_device':
drivers/target/target_core_iblock.c:127:73: error: passing argument 1 of 'queue_max_hw_sectors' makes pointer from integer without a cast [-Werror=int-conversion]
127 | dev->dev_attrib.hw_max_sectors = mult_frac(queue_max_hw_sectors(q),
| ^
| |
| unsigned int
./include/linux/math.h:129:16: note: in definition of macro 'mult_frac'
129 | typeof(x) r = x_ % d_; \
| ^
In file included from drivers/target/target_core_iblock.c:18:
./include/linux/blkdev.h:1112:77: note: expected 'const struct request_queue *' but argument is of type 'unsigned int'
1112 | static inline unsigned int queue_max_hw_sectors(const struct request_queue *q)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
cc1: all warnings being treated as errors
make[3]: *** [scripts/Makefile.build:252: drivers/target/target_core_iblock.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [scripts/Makefile.build:494: drivers/target] Error 2
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [scripts/Makefile.build:494: drivers] Error 2
make: *** [Makefile:2026: .] Error 2
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