mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-30 08:06:49 +07:00
1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
153 lines
7.1 KiB
Plaintext
153 lines
7.1 KiB
Plaintext
Mandatory File Locking For The Linux Operating System
|
|
|
|
Andy Walker <andy@lysaker.kvaerner.no>
|
|
|
|
15 April 1996
|
|
|
|
|
|
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 :-(
|
|
|