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 Code Example

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

char buf[1024];
ssize_t len = readlink("/usr/bin/perl", buf, sizeof(buf));
buf[len] = '\0';

An incorrect solution to this problem is to try to make buf large enough that it can always hold the result.

long symlink_max;
size_t bufsize;
char *buf;
ssize_t len;
errno = 0;

symlink_max = pathconf("/usr/bin/", _PC_SYMLINK_MAX);
if (errno != 0) {
    /* handle error condition */
}
if (symlink_max == -1) {
    bufsize = 10000;
}
else {
    bufsize = symlink_max+1;
}

buf = (char *)malloc(bufsize);
if (buf == NULL) {
    /* handle error condition */
}

len = readlink("/usr/bin/perl", buf, bufsize);
buf[len] = '\0';

This modification incorrectly assumes that the symbolic link cannot be longer than the value of SYMLINK_MAX returned by pathconf(). However, the value returned by pathconf() is out-of-date by the time readlink() is called, and so the off-by-one buffer overflow risk is still present because in between the two calls, the location of /usr/bin/perl could change to a file system with a larger SYMLINK_MAX value. Also, if SYMLINK_MAX is indeterminate (that is, if pathconf() returned -1 without setting errno), the code uses an arbitrary large buffer size (10000) that it hopes will be sufficient, but there is a small chance that readlink() could return exactly this size.

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 = 1024 };
char buf[BUFFERSIZE];
ssize_t len = readlink("/usr/bin/perl", buf, sizeof(buf)-1);

if (len != -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]]

POS02-A. Follow the principle of least privilege      50. POSIX (POS)       POS31-C. Do not unlock or destroy another thread's mutex