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