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Magic groups in 2.6Magic groups in 2.6Posted May 13, 2004 17:33 UTC (Thu) by elanthis (subscriber, #6227)In reply to: Magic groups in 2.6 by rjw Parent article: Magic groups in 2.6 In order to open a file descriptor, you have to access the device node. This results already in a system call just to do this. You have to locate and read each component of the device path. So if you have even just a basic /dev/syscall file descriptor, that's three accesses including lookup (including querying the hard-disk if it's not in a file cache) for /, then /dev, and then finally /dev/syscall. So that's several syscalls, possible hard-drive access, and several access checks just to invoke a single other syscall. Yuck.
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Magic groups in 2.6 Posted May 13, 2004 21:10 UTC (Thu) by rjw (guest, #10415) [Link] When you wish to *obtain* access to a new bit of functionality, you go andopen a file descriptior to whatever path - this is a ONE OFF cost. And it is certainly cheaper than the other one off costs that almost all processes incur - notably, mapping all their libraries. After that, any calls to the functionality will be ONE syscall, which just has to check that the fd number you passed is in the set of fds that your process has open, and then follow a pointer to get to the file operations structure. Do you have a solution that allows you to access privileged functionality without syscalls? If so, I have a bridge I would like to sell you. Or do you believe that permissions are rechecked every time a file desciptor is used? They are not. That is the whole damn point of them. eg: big_map_cap = open("/dev/caps/big_map"); //one off cost of a syscall foreach(big_map_that_i_want){ address = do_me_a_big_map_syscall(big_map_cap, size); // oh my god, it is a syscall! } So in fact, this is far cheaper than all these ridiculous system call checkers than context switch to user space to a policy agent if the decision isn't cached or has been thrown away.
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