OS161中vfs_setbootfs函数冒号校验逻辑的技术疑问

Here's the relevant code from OS161's vfs_setbootfs function:

/*
 * Set bootfs_vnode.
 *
 * Bootfs_vnode is the vnode used for beginning path translation of
 * pathnames starting with /.
 *
 * It is also incidentally the system's first current directory.
 */
int vfs_setbootfs(const char *fsname) {
    char tmp[NAME_MAX+1];
    char *s;
    int result;
    struct vnode *newguy;

    vfs_biglock_acquire();
    snprintf(tmp, sizeof(tmp)-1, "%s", fsname);
    s = strchr(tmp, ':');
    if (s) {
        /* If there's a colon, it must be at the end */
        if (strlen(s)>0) {
            vfs_biglock_release();
            return EINVAL;
        }
    } else {
        strcat(tmp, ":");
    }

    result = vfs_chdir(tmp);
    if (result) {
        vfs_biglock_release();
        return result;
    }

    result = vfs_getcurdir(&newguy);
    if (result) {
        vfs_biglock_release();
        return result;
    }

    change_bootfs(newguy);
    vfs_biglock_release();
    return 0;
}

The Question #

The comment states that if a colon exists, it must be at the end of the string. But why does the code return EINVAL when strlen(s) > 0? If the colon is at the end, strlen(s) would be 1 (not 0), which would trigger this error—this seems contradictory to the comment's intent.

The Explanation #

Let's unpack this clearly, because there's a clear disconnect between the comment's intended behavior and the code's current condition:

First, remember what strchr(tmp, ':') does: it returns a pointer to the first occurrence of the colon in tmp. Let's look at two key scenarios:

• Valid scenario (trailing colon): If fsname is "bootfs:", s points directly to that trailing colon. The substring starting at s is just ":", so strlen(s) equals 1.
• Invalid scenario (colon with text after): If fsname is "bootfs:extra", s points to the colon after "bootfs", and the substring starting at s is ":extra"—so strlen(s) equals 6.

The comment says the colon must be at the end (no characters after it), but the code checks if (strlen(s) > 0) and errors out. This is a bug in the condition logic:

• The check strlen(s) > 0 is always true if a colon exists (since the colon itself is one character), which would incorrectly reject valid inputs that end with a colon.
• The intended check should verify that there are no characters after the colon. The correct conditions would be either:
  1. if (*(s + 1) != '\0'): Directly check if the character right after the colon isn't the end of the string (meaning extra text exists).
  2. if (strlen(s) > 1): Since strlen(s) counts the colon plus any characters after it; a value greater than 1 means there's extra text following the colon.

Likely Root Cause #

This is almost certainly a typo or logic mix-up by the developer. It's possible they intended to check for extra characters after the colon but accidentally used strlen(s) > 0 instead of strlen(s) > 1, or confused the pointer returned by strchr with the position relative to the end of the string.

内容的提问来源于stack exchange，提问作者Osama Ahmad