linux-kernel - Re: [PATCH] lib: vsprintf: Avoid 32-bit truncation in vsscanf number parsing

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Message-ID: <20201112103546.5981815b@gandalf.local.home>
Date:   Thu, 12 Nov 2020 10:35:46 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc:     <pmladek@...e.com>, <sergey.senozhatsky@...il.com>,
        <linux-kernel@...r.kernel.org>, <patches@...nsource.cirrus.com>
Subject: Re: [PATCH] lib: vsprintf: Avoid 32-bit truncation in vsscanf
 number parsing

On Thu, 12 Nov 2020 11:17:59 +0000
Richard Fitzgerald <rf@...nsource.cirrus.com> wrote:

> Number conversion in vsscanf converts a whole string of digits and then
> extracts the field width part from the converted value. The maximum run
> of digits is limited by overflow. Conversion was using either
> simple_strto[u]l or simple_strto[u]ll based on the 'L' qualifier. This
> created a difference in truncation between builds where long is 32-bit
> and builds where it is 64-bit. This especially affects parsing a run of
> contiguous digits into separate fields - the maximum length of the run
> is 16 digits if long is 64-bit but only 8 digits if long is 32-bits.
> For example a conversion "%6x%6x" would convert both fields correctly if
> long is 64-bit but not if long is 32-bit.
> 
> It is undesirable for vsscanf to parse numbers differently depending on
> the size of long on the target build.
> 
> As simple_strto[u]l just calls simple_strto[u]ll anyway the conversion
> is always 64-bit, and the result is manipulated as a u64, so this is an
> avoidable behaviour difference between 32-bit and 64-bit builds. The
> conversion can call simple_strto[u]ll directly and preserve the full
> 64-bits that were parsed out of the string.
> 
> Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
> ---
>  lib/vsprintf.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 14c9a6af1b23..63b6cddfa7f7 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -3444,13 +3444,9 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
>  			break;
>  
>  		if (is_sign)
> -			val.s = qualifier != 'L' ?
> -				simple_strtol(str, &next, base) :
> -				simple_strtoll(str, &next, base);
> +			val.s = simple_strtoll(str, &next, base);
>  		else
> -			val.u = qualifier != 'L' ?
> -				simple_strtoul(str, &next, base) :
> -				simple_strtoull(str, &next, base);
> +			val.u = simple_strtoull(str, &next, base);
>  
>  		if (field_width > 0 && next - str > field_width) {
>  			if (base == 0)

It looks like this is fixing the symptom and not the disease. The real
issue I see here is that vsscanf is not honoring the '6' of '%6x' here. It
should only read the 6 characters then do the conversion, not the other
way around! This looks to me that the design is of issue.

-- Steve