fseek的使用和FTELL确定文件的大小有漏洞？ [英] Using fseek and ftell to determine the size of a file has a vulnerability?

问题描述

我读过的帖子显示如何使用fseek的和FTELL确定文件的大小。

I've read posts that show how to use fseek and ftell to determine the size of a file.

FILE *fp;
long file_size;
char *buffer;

fp = fopen("foo.bin", "r");
if (NULL == fp) {
 /* Handle Error */
}

if (fseek(fp, 0 , SEEK_END) != 0) {
  /* Handle Error */
}

file_size = ftell(fp);
buffer = (char*)malloc(file_size);
if (NULL == buffer){
  /* handle error */
}

我正要使用这种技术，但后来我遇到了这个<一个href=\"https://www.securecoding.cert.org/confluence/display/sec$c$c/FIO19-C.+Do+not+use+fseek%28%29+and+ftell%28%29+to+compute+the+size+of+a+file\">link描述一个潜在的漏洞。

I was about to use this technique but then I ran into this link that describes a potential vulnerability.

链接推荐使用FSTAT来代替。任何人都可以对此有何评论？

The link recommends using fstat instead. Can anyone comment on this?

推荐答案

链接是C从CERT编码咨询很多无厘头的作品之一。他们的理由是基于自由的C标准允许实现需要，但它们的不是POSIX允许的，因而无关紧要的，你必须所有情况下 FSTAT 作为替代。

The link is one of the many nonsensical pieces of C coding advice from CERT. Their justification is based on liberties the C standard allows an implementation to take, but which are not allowed by POSIX and thus irrelevant in all cases where you have fstat as an alternative.

POSIX要求：

1. 的B修改为的fopen 没有任何效果，即文本和二进制模式相同的行为。这意味着他们对文本文件调用UB关注是无稽之谈。

1. that the "b" modifier for fopen have no effect, i.e. that text and binary mode behave identically. This means their concern about invoking UB on text files is nonsense.

这文件有一个字节分辨率大小通过写操作设置和截断操作。这意味着他们对空字节的随机数在文件的最后关注的是废话。

that files have a byte-resolution size set by write operations and truncate operations. This means their concern about random numbers of null bytes at the end of the file is nonsense.

不幸的是所有这样他们出版的废话，这是很难知道认真对待这CERT的出版物。这是一种耻辱，因为他们很多是严重的。

Sadly with all the nonsense like this they publish, it's hard to know which CERT publications to take seriously. Which is a shame, because lots of them are serious.

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