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Message-ID: <1445959501.6332.33.camel@linux.intel.com>
Date: Tue, 27 Oct 2015 17:25:01 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>,
Ulf Hansson <ulf.hansson@...aro.org>,
James Bottomley <JBottomley@...n.com>,
Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] lib/string_helpers.c: don't lose precision in
string_get_size()
On Tue, 2015-10-27 at 15:06 +0100, Vitaly Kuznetsov wrote:
> string_get_size() loses precision when there is a remainder for
> blk_size / divisor[units] and size is big enough. E.g
> string_get_size(8192, 4096, STRING_UNITS_10, ...) returns "32.7 MB"
> while it is supposed to return "33.5 MB". For some artificial inputs
> the result can be ridiculously wrong, e.g.
> string_get_size(3000, 1900, STRING_UNITS_10, ...) returns "3.00 MB"
> when "5.70 MB" is expected.
>
> The issues comes from the fact than we through away
> blk_size / divisor[units] remainder when size is > exp. This can be
> fixed
> by saving it and doing some non-trivial calculations later to fix the
> error
> but that would make this function even more cumbersome. Slightly re-
> factor
> the function to not lose the precision for all inputs.
>
> The overall complexity of this function comes from the fact that size
> can
> be huge and we don't want to do size * blk_size as it can overflow.
> Do the
> math in two steps:
> 1) Reduce size to something < blk_size * divisor[units]
> 2) Multiply the result (and the remainder) by blk_size and do final
> calculations.
>
> Reported-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> Changes since v1:
> - Check against blk_size == 0 [Rasmus Villemoes]
> - Do not rename 'i' to 'order' [Andy Shevchenko]
> ---
> lib/string_helpers.c | 37 ++++++++++++++++++++++++-------------
> 1 file changed, 24 insertions(+), 13 deletions(-)
>
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index f6c27dc..eba8e82 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -44,7 +44,8 @@ void string_get_size(u64 size, u32 blk_size, const
> enum string_size_units units,
> [STRING_UNITS_2] = 1024,
> };
> int i, j;
> - u32 remainder = 0, sf_cap, exp;
> + u64 remainder = 0;
> + u32 sf_cap;
> char tmp[8];
> const char *unit;
>
> @@ -53,28 +54,36 @@ void string_get_size(u64 size, u32 blk_size,
> const enum string_size_units units,
> if (!size)
> goto out;
>
> - while (blk_size >= divisor[units]) {
> - remainder = do_div(blk_size, divisor[units]);
> - i++;
> + if (!blk_size) {
> + WARN_ON(1);
Hmm... Isn't it too strong? WARN_ONCE() might reduce a noise. Or even
pr_warn_once/ratelimited().
> + size = 0;
> + goto out;
> }
What about doing it before if (!size) ?
Like
if (!blk_size) {
pr_warn_once(); /* or WARN_ONCE() ? */
/* Override size to follow error path */
size = 0;
}
if (!size)
Also, would it be separate patch?
>
> - exp = divisor[units] / blk_size;
> /*
> - * size must be strictly greater than exp here to ensure
> that remainder
> - * is greater than divisor[units] coming out of the if
> below.
> + * size can be huge and doing size * blk_size right away can
> overflow.
> + * As a first step reduce huge size to something less than
> + * blk_size * divisor[units].
> */
> - if (size > exp) {
> + while (size > (u64)blk_size * divisor[units]) {
> remainder = do_div(size, divisor[units]);
> - remainder *= blk_size;
> i++;
> - } else {
> - remainder *= size;
> }
>
> + /* Now we're OK with doing size * blk_size, it won't
> overflow. */
> size *= blk_size;
> + remainder *= blk_size;
> + /*
> + * We were doing partial multiplication by blk_size.
> + * remainder >= divisor[units] here means size should be
> increased.
> + */
> size += remainder / divisor[units];
> - remainder %= divisor[units];
> + remainder -= (remainder / divisor[units]) * divisor[units];
I'm sorry I didn't get what the purpose of change here.
(Yes, I was thinking about u64 on 32-bit architecture, but % and / are
working in the similar way aren't they?)
>
> + /*
> + * Normalize. size >= divisor[units] means we still have
> enough
> + * precision and dropping remainder is fine.
> + */
> while (size >= divisor[units]) {
> remainder = do_div(size, divisor[units]);
> i++;
> @@ -87,7 +96,8 @@ void string_get_size(u64 size, u32 blk_size, const
> enum string_size_units units,
> if (j) {
> remainder *= 1000;
> remainder /= divisor[units];
> - snprintf(tmp, sizeof(tmp), ".%03u", remainder);
> + /* remainder is < divisor[units] here, (u32) is
> legit */
> + snprintf(tmp, sizeof(tmp), ".%03u", (u32)remainder);
> tmp[j+1] = '\0';
> }
>
> @@ -97,6 +107,7 @@ void string_get_size(u64 size, u32 blk_size, const
> enum string_size_units units,
> else
> unit = units_str[units][i];
>
> + /* size is < divisor[units] here, (u32) is legit */
> snprintf(buf, len, "%u%s %s", (u32)size,
> tmp, unit);
> }
--
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy
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