Re: [PATCH] bcache: fix bch_hprint crash and improve output

[Kent Overstreet <kent.overstreet@gmail.com>]

On Mon, Sep 04, 2017 at 02:55:24PM -0700, Michael Lyle wrote:
> Most importantly, solve a crash where %llu was used to format signed
> numbers.  This would cause a buffer overflow when reading sysfs
> writeback_rate_debug, as only 20 bytes were allocated for this and
> %llu writes 20 characters plus a null.
> 
> Always use the units mechanism rather than having different output
> paths for simplicity.
> 
> Also, correct problems with display output where 1.10 was a larger
> number than 1.09, by multiplying by 10 and then dividing by 1024 instead
> of dividing by 100.  (Remainders of >= 1000 would print as .10).
> 
> Minor changes: Always display the decimal point instead of trying to
> omit it based on number of digits shown.  Decide what units to use
> based on 1000 as a threshold, not 1024 (in other words, always print
> at most 3 digits before the decimal point).
> 
> Signed-off-by: Michael Lyle <mlyle@lyle.org>
> Reported-by: Dmitry Yu Okunev <dyokunev@ut.mephi.ru>

Acked-by: Kent Overstreet <kent.overstreet@gmail.com>

> ---
>  drivers/md/bcache/util.c | 50 +++++++++++++++++++++++++++++++++---------------
>  1 file changed, 35 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
> index 8c3a938f4bf0..176d3c2ef5f5 100644
> --- a/drivers/md/bcache/util.c
> +++ b/drivers/md/bcache/util.c
> @@ -74,24 +74,44 @@ STRTO_H(strtouint, unsigned int)
>  STRTO_H(strtoll, long long)
>  STRTO_H(strtoull, unsigned long long)
>  
> +/**
> + * bch_hprint() - formats @v to human readable string for sysfs.
> + *
> + * @v - signed 64 bit integer
> + * @buf - the (at least 8 byte) buffer to format the result into.
> + *
> + * Returns the number of bytes used by format.
> + */
>  ssize_t bch_hprint(char *buf, int64_t v)
>  {
>  	static const char units[] = "?kMGTPEZY";
> -	char dec[4] = "";
> -	int u, t = 0;
> -
> -	for (u = 0; v >= 1024 || v <= -1024; u++) {
> -		t = v & ~(~0 << 10);
> -		v >>= 10;
> -	}
> -
> -	if (!u)
> -		return sprintf(buf, "%llu", v);
> -
> -	if (v < 100 && v > -100)
> -		snprintf(dec, sizeof(dec), ".%i", t / 100);
> -
> -	return sprintf(buf, "%lli%s%c", v, dec, units[u]);
> +	int u = 0, t;
> +
> +	uint64_t q;
> +
> +	if (v < 0)
> +		q = -v;
> +	else
> +		q = v;
> +
> +	/* For as long as the number is more than 3 digits, but at least
> +	 * once, shift right / divide by 1024.  Keep the remainder for
> +	 * a digit after the decimal point.
> +	 */
> +	do {
> +		u++;
> +
> +		t = q & ~(~0 << 10);
> +		q >>= 10;
> +	} while (q >= 1000);
> +
> +	if (v < 0)
> +		/* '-', up to 3 digits, '.', 1 digit, 1 character, null;
> +		 * yields 8 bytes.
> +		 */
> +		return sprintf(buf, "-%llu.%i%c", q, t * 10 / 1024, units[u]);
> +	else
> +		return sprintf(buf, "%llu.%i%c", q, t * 10 / 1024, units[u]);
>  }
>  
>  ssize_t bch_snprint_string_list(char *buf, size_t size, const char * const list[],
> -- 
> 2.11.0
> 
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