SunOS 4 protection module
|copyright||See Copyright and License.|
|index terms||pair: SunOS 4; protection interface design pair: SunOS 4 protection interface; design|
As of 2013-05-26, the MPS is no longer supported on SunOS, so this document is only of historical interest.
.readership: Any MPS developer.
.intro: This is the design of the SunOS 4 implementation of the protection module. It is intended to be used only in SunOS 4 (os.su). It makes use of various services provided by SunOS 4.
.req.general: Required to implement the general protection interface defined in design.mps.prot.if.*.
.improve.sig-stack: Currently we do not handle signals on a separate signal stack. If we handled signals on our own stack then we could guarantee not to run out of stack while we were handling the signal. This would be useful (it may even be required). We would have to use sigvec(2) rather than signal(3) (set the SV_ONSTACK flag and use sigstack(2)). This has drawbacks as the signal stack is not grown automatically, so we would have to to frig the stacks back if we wanted to pass on the signal to some other handler as that handler may require arbitrary amounts of stack.
.improve.sigvec: Note 1 of ProtSetup() notes that we can't honour the sigvec(2) entries of the next handler in the chain. What if when we want to pass on the signal instead of calling the handler we call sigvec() with the old entry and use kill to send the signal to ourselves and then restore our handler using sigvec again.
.data.signext: This is static. Because that is the only communications channel available to signal handlers. [write a little more here]
.fun.setup: ProtSetup(). The setup involves installing a signal handler for the signal SIGSEGV to catch and handle protection faults (this handler is the function sigHandle(), see `.fun.sighandle`_). The previous handler is recorded (in the variable sigNext, see .data.signext) so that it can be reached from sigHandle() if it fails to handle the fault.
The problem with this approach is that we can't honor the wishes of the sigvec(2) entry for the previous handler (in terms of masks in particular).
Obviously it would be okay to always chain the previous signal handler onto sigNext, however in the case where the previous handler is the one we've just installed (that is, sigHandle) then it is not necessary to chain the handler, so we don't.
.fun.set.convert: The requested protection (which is expressed in the mode parameter, see design.mps.prot.if.set) is translated into an operating system protection. If read accesses are to be forbidden then all accesses are forbidden, this is done by setting the protection of the page to PROT_NONE. If write access are to be forbidden (and not read accesses) then write accesses are forbidden and read accesses are allowed, this is done by setting the protection of the page to PROT_READ | PROT_EXEC. Otherwise (all access are okay), the protection is set to PROT_READ | PROT_WRITE | PROT_EXEC.
.fun.set.assume.mprotect: We assume that the call to mprotect() always succeeds. This is because we should always call the function with valid arguments (aligned, references to mapped pages, and with an access that is compatible with the access of the underlying object).
.fun.sync: ProtSync(). This does nothing in this implementation as ProtSet sets the protection without any delay.
.fun.tramp: ProtTramp(). The protection trampoline is trivial under SunOS, as there is nothing that needs to be done in the dynamic context of the mutator in order to catch faults. (Contrast this with Win32 Structured Exception Handling.)
Copyright and License
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