Emerging description of DDLP

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Thomas Schmitt 2007-04-16 19:45:11 +00:00
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Delicate Device Locking Protocol
(a joint sub project of cdrkit and libburnia)
DDLP shall help to avoid collisions between programs in the process of
recording to a CD or DVD drive and other programs which access that drive.
The protocol intends to provide advisory locking. So any good-willing program
has to take some extra precautions to participate.
If a program does not feel vulnerable to disturbance, then the precautions
impose much less effort than if the program feels the need for protection.
Two levels are specified:
DDLP-A operates on device files only. It is very Linux specific.
DDLP-B adds proxy lock files, inspired by FHS /var/lock standard.
This protocol relies on the hardly documented feature open(O_EXCL | O_RDWR)
with Linux device files and on POSIX compliant fcntl(F_SETLK).
Other than the original meaning of O_EXCL with creating regular files, the
effect on device files is mutual exclusion of access. I.e. if one
filedescriptor is open on that combination of major-minor device number, then
no other open(O_EXCL) will succeed. But open() without O_EXCL would succeed.
So this is advisory and exclusive locking.
With kernel 2.6 it seems to work on all device drivers which might get used
to access a CD/DVD drive.
The vulnerable programs do not start their operation before they occupied a
wide collection of drive representations.
Non-vulnerable programs take care to detect the occupation of _one_ such
So for Friendly Programs
A program which does not feel vulnerable to disturbance is urged to access
CD/DVD drives by opening a file descriptor which will uphold the lock
as long as it does not get closed. There are two alternative ways to achieve
Very reliable is
open( some_path , O_EXCL | O_RDWR | ...)
Regrettably, O_EXCL | O_RDONLY did not always succeed with our tests on
kernel 2.6 systems.
So it may be necessary to use a cautious open() without O_EXCL and to aquire
a POSIX lock via fcntl().
With this gesture it is important to use the standard paths because
fcntl(F_SETLK) does not lock the device but only a device-inode.
std_path = one of the standard device files:
/dev/sr[0..255] /dev/scd[0..255] /dev/sg[0..255] /dev/hd[a..z]
or a symbolic link pointing to one of them.
open( std_path , ... | O_NDELAY)
fcntl(F_SETLK) and close() on failure
... eventually disable O_NDELAY by fcntl(F_SETFL) ...
There is a pitfall mentioned in man 2 fcntl :
"locks are automatically released [...] if it closes any file descriptor
referring to a file on which locks are held. This is bad [...]"
So you may have to re-lock after some temporary fd got closed.
Vulnerable Programs
For programs which do feel vulnerable, O_EXCL would suffice for the /dev/hd*
device file family and their driver. But USB and SATA recorders appear with
at least two different major-minor combinations simultaneously.
One as /dev/sr* alias /dev/scd*, the other as /dev/sg*.
The same is true for ide-scsi or recorders attached to SCSI controllers.
So, in order to lock any access to the recorder, one has to open(O_EXCL)
not only the device file that is intended for accessing the recorder but also
a device file of any other major-minor representation of the recorder.
This is done via the SCSI address parameter vector (Host,Channel,Id,Lun)
and a search on standard device file paths /dev/sr* /dev/scd* /dev/sg*.
In this text the alternative device representations are called "siblings".
For finding them, it is necessary to apply open() to many device files which
might be occupied by delicate operations. On the other hand it is very
important to occupy all reasonable representations of the drive.
So the reading of the (Host,Channel,Id,Lun) parameters demands an
open(O_RDONLY | O_NDELAY) _without_ fcntl() in order to find the outmost
number of representations among the standard device files. Only ioctls
Hopefully this gesture is unable to cause harmful side effects on kernel 2.6.
At least one file of each class sr, scd and sg should be found to regard
the occupation as satisfying. Thus corresponding sr-scd-sg triplets should have
matching ownerships and access permissions.
One will have to help the sysadmins to find those triplets.
A spicy detail is that sr and scd may be distinct device files for the same
major-minor combination. In this case fcntl() locks on both are needed
but O_EXCL can only be applied to one of them.
An open and free implementation ddlpa.[ch] is provided as
The current version of this text is
Put ddlpa.h and ddlpa.c into the same directory and compile as test program by
cc -g -Wall -DDDLPA_C_STANDALONE -o ddlpa ddlpa.c
Use it to occupy a drive's representations for a given number of seconds
./ddlpa /dev/sr0 300
It should do no harm to any of your running activities.
If it does: Please, please alert us.
Your own programs should not be able to circumvent the occupation if the
obey the rules for Friendly Programs.
Of course ./ddlpa should be unable to circumvent itself.
A successfull occupation looks like
DDLPA_DEBUG: ddlpa_std_by_rdev("/dev/scd0") = "/dev/sr0"
DDLPA_DEBUG: ddlpa_collect_siblings() found "/dev/sr0"
DDLPA_DEBUG: ddlpa_collect_siblings() found "/dev/scd0"
DDLPA_DEBUG: ddlpa_collect_siblings() found "/dev/sg0"
DDLPA_DEBUG: ddlpa_occupy() : '/dev/scd0'
DDLPA_DEBUG: ddlpa_occupy() O_EXCL : '/dev/sg0'
DDLPA_DEBUG: ddlpa_occupy() O_EXCL : '/dev/sr0'
---------------------------------------------- Lock gained
ddlpa: opened /dev/sr0
ddlpa: opened siblings: /dev/scd0 /dev/sg0
slept 1 seconds of 300
Now an attempt via device file alias /dev/NEC must fail:
DDLPA_DEBUG: ddlpa_std_by_rdev("/dev/NEC") = "/dev/sg0"
DDLPA_DEBUG: ddlpa_collect_siblings() found "/dev/sr0"
DDLPA_DEBUG: ddlpa_collect_siblings() found "/dev/scd0"
DDLPA_DEBUG: ddlpa_collect_siblings() found "/dev/sg0"
Cannot exclusively open '/dev/sg0'
Reason given : Failed to open O_RDWR | O_NDELAY | O_EXCL : '/dev/sr0'
Error condition : 16 'Device or resource busy'
With hdc, of course, things are trivial
DDLPA_DEBUG: ddlpa_std_by_rdev("/dev/hdc") = "/dev/hdc"
DDLPA_DEBUG: ddlpa_occupy() O_EXCL : '/dev/hdc'
---------------------------------------------- Lock gained
ddlpa: opened /dev/hdc
slept 1 seconds of 1
>>> Proxy lock file protocol embedded in DDLP-A which itself might be a
>>> generic dummy on non-Linux systems (i.e. without O_EXCL or SCSI sibling
>>> search).
>>> Definition to come soon. Implementation possibly to come never.