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

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Message-ID: <20201112111759.16377-1-rf@opensource.cirrus.com>
Date:   Thu, 12 Nov 2020 11:17:59 +0000
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     <pmladek@...e.com>, <rostedt@...dmis.org>,
        <sergey.senozhatsky@...il.com>
CC:     <linux-kernel@...r.kernel.org>, <patches@...nsource.cirrus.com>,
        Richard Fitzgerald <rf@...nsource.cirrus.com>
Subject: [PATCH] lib: vsprintf: Avoid 32-bit truncation in vsscanf number parsing

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)
-- 
2.20.1