lkml.org 
[lkml]   [2020]   [Nov]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH] lib: vsprintf: Avoid 32-bit truncation in vsscanf number parsing
On Thu, 12 Nov 2020 11:17:59 +0000
Richard Fitzgerald <rf@opensource.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@opensource.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

\
 
 \ /
  Last update: 2020-11-12 16:36    [W:0.067 / U:0.668 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site