"link" system call doesn't check whether the link count overflows

[Guy Harris]

Index:	sys/sys/ufs_syscalls.c sys/h/param.h 4.3BSD

Description:
	(Thanks to Don Cragun here for pointing this one out and providing
	the fix)
        Some system verification tests fail because the link() sytem call code
        in sys/ufs_syscalls.c does not check for the link count overflowing a
	signed short.  In fact, the code will allow 65535 links to be created,
	but none of the links can be removed after 32767 links have been
	created.  If 65535 links to a file exist and another link is made, the
	link will succeed, but all further attempts to use the file will fail
	because the link count will have wrapped to 0.
Repeat-By:
        You don't want to repeat this.  It will cause severe headaches while
        trying to patch the destroyed file system back to a sane state.  It
        could be repeated by using the following in the Bourne shell:
                i=2
                >1 
                while [ $i -lt 32769 ]
                do
                        ln 1 $i
                        i=`expr $i + 1`
                done
        Note that the C version of this will run for over 30 hours on a Sun-2,
	and probably for some similarly large time on other machines.
Fix:
	Put the "#define MAXLINK 32767" back in <sys/param.h> and change the
	following line of code in link():
                ip->i_nlink++;
        to:
                if (ip->i_nlink == MAXLINK) {
			u.u_error = EMLINK;
                        iput(ip);
                        return;
                }
                ip->i_nlink++;
	
	Note: You may want to define MAXLINK as some smaller value.  System
	V uses 1000.
-- 
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy at sun.com (or guy at sun.arpa)