POS30-C. Use the readlink() function properly

The readlink() function reads where a link points to. It makes no effort to null-terminate its second argument, buffer. Instead, it just returns the number of characters it has written.

Non-Compliant Coding Example

If len is equal to sizeof(buf), the null terminator will be written one byte past the end of buf.

char buf[256];
ssize_t len = readlink("/usr/bin/perl", buf, sizeof(buf));
buf[len] = '\0';

A simple (but incorrect) solution to this problem is to try to make buf large enough that it can always hold the result:

char buf[PATH_MAX+1];
ssize_t len = readlink("/usr/bin/perl", buf, sizeof(buf));
buf[len] = '\0';

This modification incorrectly assumes that PATH_MAX represents the longest possible path for a file in the filesystem. (PATH_MAX only bounds the longest possible relative path that can be passed to the kernel in a single call.) On most Unix and Linux systems, there is no easily-determined maximum length for a file path, and so the off-by-one buffer overflow risk is still present.

An additional issue is that readlink() can return -1 if it fails, causing an off-by-one underflow.

Compliant Solution

This example ensures there will be no overflow by only reading in sizeof(buf)-1 characters. It also properly checks to see if an error has occurred.

enum { BUFFERSIZE = 256 };
char buf[BUFFERSIZE];
ssize_t len;

if ((len = readlink("/usr/bin/perl", buf, sizeof(buf)-1)) != -1)
    buf[len] = '\0';
else {
   /* handle error condition */
}

Risk Assessment

Failing to properly terminate the result of readlink() can result in abnormal program termination.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ilja 06]]
[[Open Group 97]]
[[Open Group 04]]

POS01-A. Check for the existence of links      50. POSIX (POS)       POS31-C. Do not unlock or destroy another thread's mutex