Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
1 Mandatory File Locking For The Linux Operating System 2 3 Andy Walker <firstname.lastname@example.org> 4 5 15 April 1996 6 (Updated September 2007) 7 8 0. Why you should avoid mandatory locking 9 ----------------------------------------- 10 11 The Linux implementation is prey to a number of difficult-to-fix race 12 conditions which in practice make it not dependable: 13 14 - The write system call checks for a mandatory lock only once 15 at its start. It is therefore possible for a lock request to 16 be granted after this check but before the data is modified. 17 A process may then see file data change even while a mandatory 18 lock was held. 19 - Similarly, an exclusive lock may be granted on a file after 20 the kernel has decided to proceed with a read, but before the 21 read has actually completed, and the reading process may see 22 the file data in a state which should not have been visible 23 to it. 24 - Similar races make the claimed mutual exclusion between lock 25 and mmap similarly unreliable. 26 27 1. What is mandatory locking? 28 ------------------------------ 29 30 Mandatory locking is kernel enforced file locking, as opposed to the more usual 31 cooperative file locking used to guarantee sequential access to files among 32 processes. File locks are applied using the flock() and fcntl() system calls 33 (and the lockf() library routine which is a wrapper around fcntl().) It is 34 normally a process' responsibility to check for locks on a file it wishes to 35 update, before applying its own lock, updating the file and unlocking it again. 36 The most commonly used example of this (and in the case of sendmail, the most 37 troublesome) is access to a user's mailbox. The mail user agent and the mail 38 transfer agent must guard against updating the mailbox at the same time, and 39 prevent reading the mailbox while it is being updated. 40 41 In a perfect world all processes would use and honour a cooperative, or 42 "advisory" locking scheme. However, the world isn't perfect, and there's 43 a lot of poorly written code out there. 44 45 In trying to address this problem, the designers of System V UNIX came up 46 with a "mandatory" locking scheme, whereby the operating system kernel would 47 block attempts by a process to write to a file that another process holds a 48 "read" -or- "shared" lock on, and block attempts to both read and write to a 49 file that a process holds a "write " -or- "exclusive" lock on. 50 51 The System V mandatory locking scheme was intended to have as little impact as 52 possible on existing user code. The scheme is based on marking individual files 53 as candidates for mandatory locking, and using the existing fcntl()/lockf() 54 interface for applying locks just as if they were normal, advisory locks. 55 56 Note 1: In saying "file" in the paragraphs above I am actually not telling 57 the whole truth. System V locking is based on fcntl(). The granularity of 58 fcntl() is such that it allows the locking of byte ranges in files, in addition 59 to entire files, so the mandatory locking rules also have byte level 60 granularity. 61 62 Note 2: POSIX.1 does not specify any scheme for mandatory locking, despite 63 borrowing the fcntl() locking scheme from System V. The mandatory locking 64 scheme is defined by the System V Interface Definition (SVID) Version 3. 65 66 2. Marking a file for mandatory locking 67 --------------------------------------- 68 69 A file is marked as a candidate for mandatory locking by setting the group-id 70 bit in its file mode but removing the group-execute bit. This is an otherwise 71 meaningless combination, and was chosen by the System V implementors so as not 72 to break existing user programs. 73 74 Note that the group-id bit is usually automatically cleared by the kernel when 75 a setgid file is written to. This is a security measure. The kernel has been 76 modified to recognize the special case of a mandatory lock candidate and to 77 refrain from clearing this bit. Similarly the kernel has been modified not 78 to run mandatory lock candidates with setgid privileges. 79 80 3. Available implementations 81 ---------------------------- 82 83 I have considered the implementations of mandatory locking available with 84 SunOS 4.1.x, Solaris 2.x and HP-UX 9.x. 85 86 Generally I have tried to make the most sense out of the behaviour exhibited 87 by these three reference systems. There are many anomalies. 88 89 All the reference systems reject all calls to open() for a file on which 90 another process has outstanding mandatory locks. This is in direct 91 contravention of SVID 3, which states that only calls to open() with the 92 O_TRUNC flag set should be rejected. The Linux implementation follows the SVID 93 definition, which is the "Right Thing", since only calls with O_TRUNC can 94 modify the contents of the file. 95 96 HP-UX even disallows open() with O_TRUNC for a file with advisory locks, not 97 just mandatory locks. That would appear to contravene POSIX.1. 98 99 mmap() is another interesting case. All the operating systems mentioned 100 prevent mandatory locks from being applied to an mmap()'ed file, but HP-UX 101 also disallows advisory locks for such a file. SVID actually specifies the 102 paranoid HP-UX behaviour. 103 104 In my opinion only MAP_SHARED mappings should be immune from locking, and then 105 only from mandatory locks - that is what is currently implemented. 106 107 SunOS is so hopeless that it doesn't even honour the O_NONBLOCK flag for 108 mandatory locks, so reads and writes to locked files always block when they 109 should return EAGAIN. 110 111 I'm afraid that this is such an esoteric area that the semantics described 112 below are just as valid as any others, so long as the main points seem to 113 agree. 114 115 4. Semantics 116 ------------ 117 118 1. Mandatory locks can only be applied via the fcntl()/lockf() locking 119 interface - in other words the System V/POSIX interface. BSD style 120 locks using flock() never result in a mandatory lock. 121 122 2. If a process has locked a region of a file with a mandatory read lock, then 123 other processes are permitted to read from that region. If any of these 124 processes attempts to write to the region it will block until the lock is 125 released, unless the process has opened the file with the O_NONBLOCK 126 flag in which case the system call will return immediately with the error 127 status EAGAIN. 128 129 3. If a process has locked a region of a file with a mandatory write lock, all 130 attempts to read or write to that region block until the lock is released, 131 unless a process has opened the file with the O_NONBLOCK flag in which case 132 the system call will return immediately with the error status EAGAIN. 133 134 4. Calls to open() with O_TRUNC, or to creat(), on a existing file that has 135 any mandatory locks owned by other processes will be rejected with the 136 error status EAGAIN. 137 138 5. Attempts to apply a mandatory lock to a file that is memory mapped and 139 shared (via mmap() with MAP_SHARED) will be rejected with the error status 140 EAGAIN. 141 142 6. Attempts to create a shared memory map of a file (via mmap() with MAP_SHARED) 143 that has any mandatory locks in effect will be rejected with the error status 144 EAGAIN. 145 146 5. Which system calls are affected? 147 ----------------------------------- 148 149 Those which modify a file's contents, not just the inode. That gives read(), 150 write(), readv(), writev(), open(), creat(), mmap(), truncate() and 151 ftruncate(). truncate() and ftruncate() are considered to be "write" actions 152 for the purposes of mandatory locking. 153 154 The affected region is usually defined as stretching from the current position 155 for the total number of bytes read or written. For the truncate calls it is 156 defined as the bytes of a file removed or added (we must also consider bytes 157 added, as a lock can specify just "the whole file", rather than a specific 158 range of bytes.) 159 160 Note 3: I may have overlooked some system calls that need mandatory lock 161 checking in my eagerness to get this code out the door. Please let me know, or 162 better still fix the system calls yourself and submit a patch to me or Linus. 163 164 6. Warning! 165 ----------- 166 167 Not even root can override a mandatory lock, so runaway processes can wreak 168 havoc if they lock crucial files. The way around it is to change the file 169 permissions (remove the setgid bit) before trying to read or write to it. 170 Of course, that might be a bit tricky if the system is hung :-(