CERT C:Rec.INT13-C

Use bitwise operators only on unsigned operands

描述

规则定义

Use bitwise operators only on unsigned operands.¹

Polyspace 实现

规则检查项检查是否存在对负值进行按位运算。

扩展检查项

当输入值未知且只有部分输入会导致问题时，默认的 Bug Finder 分析可能不会触发此规则的违规。要检查由特定系统输入值引起的违规，请运行更严格的 Bug Finder 分析。请参阅Extend Bug Finder Checkers to Find Defects from Specific System Input Values。

示例

全部展开

对负值执行按位运算

问题

负值上的位运算检测对负值有符号整数变量使用的位运算符（>>、^、|、~、but、not、&）。

风险

如果有符号整数的值为负数，则按位运算可能会出现意外结果，因为：

对负值的位运算因编译器而异。
意外的计算可能会导致其他漏洞，例如缓冲区溢出。

修复

执行按位运算时，请使用 unsigned 整数以避免出现意外结果。

示例 - 负整数的右移

#include <stdio.h>
#include <stdarg.h>

static void demo_sprintf(const char *format, ...)
{
    int rc;
    va_list ap;
    char buf[sizeof("256")];

    va_start(ap, format);
    rc = vsprintf(buf, format, ap);
    if (rc == -1 || rc >= sizeof(buf)) {
        /* Handle error */
    }
    va_end(ap);
}

void bug_bitwiseneg()
{
    int stringify = 0x80000000;
    demo_sprintf("%u", stringify >> 24); //Noncompliant
}

在此示例中，语句 demo_sprintf("%u", stringify >> 24) 意外地停止了程序。您期望 stringify >> 24 的结果是 0x80。然而，实际结果是 0xffffff80，因为 stringify 是有符号的负数。符号位也已移位。

更正 - 添加 unsigned 关键字

通过添加 unsigned 关键字，stringify 不是负数，右移运算给出预期结果 0x80。

#include <stdio.h>
#include <stdarg.h>

static void demo_sprintf(const char *format, ...)
{
    int rc;
    va_list ap;
    char buf[sizeof("256")];

    va_start(ap, format);
    rc = vsprintf(buf, format, ap);
    if (rc == -1 || rc >= sizeof(buf)) {
        /* Handle error */
    }
    va_end(ap);
}

void corrected_bitwiseneg()
{
    unsigned int stringify = 0x80000000;
    demo_sprintf("%u", stringify >> 24);
}

检查信息

组：Rec.04.整数 (INT)

版本历史记录

在 R2019a 中推出

另请参阅

检查 SEI CERT-C (-cert-c)

主题

检查并审查编码标准违规

外部网站

INT13-C

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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This software and associated documentation has not been reviewed nor is it endorsed by Carnegie Mellon University or its Software Engineering Institute.