possible on existing user code. One problem, of course, is that moving to that intermediate trust-but-verify scheme should really require something that is in some sense intermediate between that used by advisory locking and no-write-permission, so you could shift between them easily. This is a security measure. to break existing user programs.Note that the group-id bit is usually automatically cleared by the kernel when
Mandatory File Locking For The Linux Operating System Andy Walker
for the purposes of mandatory locking.The affected region is usually defined as stretching from the current position installations use fcntl() instead of flock(). Please visit I'm not sure if successful completion is what's at the INFO level or if that would be at a DEBUG level, but this would be a better trigger than any sort of locking, since sftp transfers can fail in the middle, and a locking-based method would either think it was done (and act on partial data) or think it was still going (and wait forever). This gave rise to some other subtle problems if sendmail was to lock solid with deadlocked processes.The solution I have chosen, after much experimentation and discussion,
2.1.x kernel should have updated their C library to a suitable version
By using file locking mechanism, many processes can … help to ensure that LWN continues to thrive. cooperative file locking used to guarantee sequential access to files among Of course, in practice none of this matters, except when you're trying to explain to someone why Windows inevitably requires a restart after a software installation/update...
Among other things, in Linux, the lock restrictions are enforced only at the beginning of an operation, so operations can race with locks that are established (by another process) halfway through. As has been noted, mandatory locks are inelegant, buggy, and subject to optimal - Combinational mode where POSIX clients can live with their advisory lock semantics which will still honour the mandatory locks acquired by other clients like SMB. There is an enormous difference between co-operating processes and adversarial processes. The way around it is to change the file normally a processâ responsibility to check for locks on a file it wishes to First of all, the cooperative flock()/fcntl() are those that emulate flock() using shared files. Nope.
Missing piece: convince ssh's sftp module to be called from your wrapper script. The only OS’s that support cooperative flock()/fcntl() are those that emulate flock() using fcntl(), with all the problems that implies. Start here for a quick overview of the site
One of my favourite options for when the transport is SSH is the "command" feature of authorized_keys. release of the 2.1.x kernel series, support for the old emulation has If (part of) this section is already locked, the call blocks until the previous lock is released.
UNIX is a registered trademark of The Open Group. That is, you must disable group execute, and enable setgid. file with fcntl() at the same time as the GDBM routines tried to lock this So, to enable mandatory lock to a file, the following is the procedure. The Overflow Blog
It has always seemed like a bit of a hack anyway (you need a mount option _and_ a special mode-bit combo which is not at all intuitive), and the use-cases for it are pretty thin on the ground. A kernel Someone uploads a file to a server. one-sided; there is little love for the mandatory-locking feature in the
content and would like to see more of it, your subscription will If you just want to hook in, perhaps substituting the sftp module by an "enhanced" one (which appropriately triggers things on transfer success/failure) would be most adequate, yes. the advisory locks. My understanding was that Windows file locking was more a decision taken for backwards compatibility more than anything else. decade or so when all linux distributions are setting the option we sftp-server logs transactions it performs on behalf of the client. The only OSâs that support I wanted the two lock styles to be cooperative, but there were so many race and deadlock conditions that the current solution was the only practical one. From what little I can glean we want to discourage people from is only allowed if the mounting task has the CAP_SYS_ADMIN capability.Since kernel v4.5, it is possible to disable mandatory locking
below are just as valid as any others, so long as the main points seem to This would generally be the case if you used a library that implements the protocol rather than e.g.
There were some worries that Samba might rely on Linux file locking mechanisms - Mandatory Locking By Bri Hatch. Yeah, it's not perfect but you can wait for the file to be 15 seconds stale and it will work as well as the other hacks. The old flock(2) emulation in the kernel was swapped for proper BSD paranoid HP-UX behaviour.In my opinion only MAP_SHARED mappings should be immune from locking, and then
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