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+ Mandatory File Locking For The Linux Operating System
+
+ Andy Walker <andy@lysaker.kvaerner.no>
+
+ 15 April 1996
+ (Updated September 2007)
+
+0. Why you should avoid mandatory locking
+-----------------------------------------
+
+The Linux implementation is prey to a number of difficult-to-fix race
+conditions which in practice make it not dependable:
+
+ - The write system call checks for a mandatory lock only once
+ at its start. It is therefore possible for a lock request to
+ be granted after this check but before the data is modified.
+ A process may then see file data change even while a mandatory
+ lock was held.
+ - Similarly, an exclusive lock may be granted on a file after
+ the kernel has decided to proceed with a read, but before the
+ read has actually completed, and the reading process may see
+ the file data in a state which should not have been visible
+ to it.
+ - Similar races make the claimed mutual exclusion between lock
+ and mmap similarly unreliable.
+
+1. What is mandatory locking?
+------------------------------
+
+Mandatory locking is kernel enforced file locking, as opposed to the more usual
+cooperative file locking used to guarantee sequential access to files among
+processes. File locks are applied using the flock() and fcntl() system calls
+(and the lockf() library routine which is a wrapper around fcntl().) It is
+normally a process' responsibility to check for locks on a file it wishes to
+update, before applying its own lock, updating the file and unlocking it again.
+The most commonly used example of this (and in the case of sendmail, the most
+troublesome) is access to a user's mailbox. The mail user agent and the mail
+transfer agent must guard against updating the mailbox at the same time, and
+prevent reading the mailbox while it is being updated.
+
+In a perfect world all processes would use and honour a cooperative, or
+"advisory" locking scheme. However, the world isn't perfect, and there's
+a lot of poorly written code out there.
+
+In trying to address this problem, the designers of System V UNIX came up
+with a "mandatory" locking scheme, whereby the operating system kernel would
+block attempts by a process to write to a file that another process holds a
+"read" -or- "shared" lock on, and block attempts to both read and write to a
+file that a process holds a "write " -or- "exclusive" lock on.
+
+The System V mandatory locking scheme was intended to have as little impact as
+possible on existing user code. The scheme is based on marking individual files
+as candidates for mandatory locking, and using the existing fcntl()/lockf()
+interface for applying locks just as if they were normal, advisory locks.
+
+Note 1: In saying "file" in the paragraphs above I am actually not telling
+the whole truth. System V locking is based on fcntl(). The granularity of
+fcntl() is such that it allows the locking of byte ranges in files, in addition
+to entire files, so the mandatory locking rules also have byte level
+granularity.
+
+Note 2: POSIX.1 does not specify any scheme for mandatory locking, despite
+borrowing the fcntl() locking scheme from System V. The mandatory locking
+scheme is defined by the System V Interface Definition (SVID) Version 3.
+
+2. Marking a file for mandatory locking
+---------------------------------------
+
+A file is marked as a candidate for mandatory locking by setting the group-id
+bit in its file mode but removing the group-execute bit. This is an otherwise
+meaningless combination, and was chosen by the System V implementors so as not
+to break existing user programs.
+
+Note that the group-id bit is usually automatically cleared by the kernel when
+a setgid file is written to. This is a security measure. The kernel has been
+modified to recognize the special case of a mandatory lock candidate and to
+refrain from clearing this bit. Similarly the kernel has been modified not
+to run mandatory lock candidates with setgid privileges.
+
+3. Available implementations
+----------------------------
+
+I have considered the implementations of mandatory locking available with
+SunOS 4.1.x, Solaris 2.x and HP-UX 9.x.
+
+Generally I have tried to make the most sense out of the behaviour exhibited
+by these three reference systems. There are many anomalies.
+
+All the reference systems reject all calls to open() for a file on which
+another process has outstanding mandatory locks. This is in direct
+contravention of SVID 3, which states that only calls to open() with the
+O_TRUNC flag set should be rejected. The Linux implementation follows the SVID
+definition, which is the "Right Thing", since only calls with O_TRUNC can
+modify the contents of the file.
+
+HP-UX even disallows open() with O_TRUNC for a file with advisory locks, not
+just mandatory locks. That would appear to contravene POSIX.1.
+
+mmap() is another interesting case. All the operating systems mentioned
+prevent mandatory locks from being applied to an mmap()'ed file, but HP-UX
+also disallows advisory locks for such a file. SVID actually specifies the
+paranoid HP-UX behaviour.
+
+In my opinion only MAP_SHARED mappings should be immune from locking, and then
+only from mandatory locks - that is what is currently implemented.
+
+SunOS is so hopeless that it doesn't even honour the O_NONBLOCK flag for
+mandatory locks, so reads and writes to locked files always block when they
+should return EAGAIN.
+
+I'm afraid that this is such an esoteric area that the semantics described
+below are just as valid as any others, so long as the main points seem to
+agree.
+
+4. Semantics
+------------
+
+1. Mandatory locks can only be applied via the fcntl()/lockf() locking
+ interface - in other words the System V/POSIX interface. BSD style
+ locks using flock() never result in a mandatory lock.
+
+2. If a process has locked a region of a file with a mandatory read lock, then
+ other processes are permitted to read from that region. If any of these
+ processes attempts to write to the region it will block until the lock is
+ released, unless the process has opened the file with the O_NONBLOCK
+ flag in which case the system call will return immediately with the error
+ status EAGAIN.
+
+3. If a process has locked a region of a file with a mandatory write lock, all
+ attempts to read or write to that region block until the lock is released,
+ unless a process has opened the file with the O_NONBLOCK flag in which case
+ the system call will return immediately with the error status EAGAIN.
+
+4. Calls to open() with O_TRUNC, or to creat(), on a existing file that has
+ any mandatory locks owned by other processes will be rejected with the
+ error status EAGAIN.
+
+5. Attempts to apply a mandatory lock to a file that is memory mapped and
+ shared (via mmap() with MAP_SHARED) will be rejected with the error status
+ EAGAIN.
+
+6. Attempts to create a shared memory map of a file (via mmap() with MAP_SHARED)
+ that has any mandatory locks in effect will be rejected with the error status
+ EAGAIN.
+
+5. Which system calls are affected?
+-----------------------------------
+
+Those which modify a file's contents, not just the inode. That gives read(),
+write(), readv(), writev(), open(), creat(), mmap(), truncate() and
+ftruncate(). truncate() and ftruncate() are considered to be "write" actions
+for the purposes of mandatory locking.
+
+The affected region is usually defined as stretching from the current position
+for the total number of bytes read or written. For the truncate calls it is
+defined as the bytes of a file removed or added (we must also consider bytes
+added, as a lock can specify just "the whole file", rather than a specific
+range of bytes.)
+
+Note 3: I may have overlooked some system calls that need mandatory lock
+checking in my eagerness to get this code out the door. Please let me know, or
+better still fix the system calls yourself and submit a patch to me or Linus.
+
+6. Warning!
+-----------
+
+Not even root can override a mandatory lock, so runaway processes can wreak
+havoc if they lock crucial files. The way around it is to change the file
+permissions (remove the setgid bit) before trying to read or write to it.
+Of course, that might be a bit tricky if the system is hung :-(
+