sprintf/snprintf 哪个更安全? [英] Which of sprintf/snprintf is more secure?

问题描述

我想知道这两个选项中哪个更安全:

I wish to know which of these two options is the more secure one to use:

#define MAXLEN 255
char buff[MAXLEN + 1]

1. sprintf(buff, "%.*s", MAXLEN, name)

snprintf(buff, MAXLEN, "%s", name)

我的理解是两者是一样的.请提出建议.

My understanding is that both are same. Please suggest.

推荐答案

你给出的两个表达式不是等价的:sprintf 没有指定最大字节数的参数来写;它只需要一个目标缓冲区、一个格式字符串和一堆参数.因此，它可能会写入比缓冲区空间更多的字节，并在这样做时写入任意代码.%.*s 不是一个令人满意的解决方案，因为:

The two expressions you gave are not equivalent: sprintf takes no argument specifying the maximum number of bytes to write; it simply takes a destination buffer, a format string, and a bunch of arguments. Therefore, it may write more bytes than your buffer has space for, and in so doing write arbitrary code. The %.*s is not a satisfactory solution because:

1. 当格式说明符指的是长度时，它指的是相当于strlen；这是字符串中字符数的度量，而不是它在内存中的长度(即它不计算空终止符).
2. 格式字符串的任何更改(例如，添加换行符)都将更改 sprintf 版本在缓冲区溢出方面的行为.使用 snprintf，无论格式字符串或输入类型如何变化，都会设置一个固定的、清晰的最大值.

1. When the format specifier refers to length, it's referring to the equivalent of strlen; this is a measure of the number of characters in the string, not its length in memory (i.e. it doesn't count the null terminator).
2. Any change in the format string (adding a newline, for example) will change the behavior of the sprintf version with respect to buffer overflows. With snprintf, a fixed, clear maximum is set regardless of changes in the format string or input types.

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