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on the epc instruction. The epc instruction, given the permissions of the page in which the epc is located, allows the privilege level to be increased with little or no overhead. The previous privilege level is recorded in the current frame marker and is restored by a regular (function) return. Since the epc instruction has to live in a page with non-standard properties, we hardwire a "gateway" page in the address space. The address of the gateway page is exported to userland in ar.k7. This allows us to rewire the page without breaking the ABI. The syscall stubs in libc are regular function calls that slightly differ from the normal runtime. The difference is mostly to simplify the stubs themselves by by moving some of the logic to the kernel. The libc stubs call into the gateway page (offset 0), from where the kernel trampolines to the code that sets up a minimal trapframe and arranges to execute from the kernel stack. The way back is basicly the same. The kernel returns to the gateway page, whereby privilege is dropped, and jumps back to the syscall stub. Only the special registers are saved in the trapframe. None of the scratch registers are preserved and since the kernel follows the same runtime model, none of the preserved registers are saved. Future enhancements can include the implementation of lightweight syscalls, where kernel functions are performed without setting up a trapframe. Good candidates are the *context syscalls for example. Now that there's a gateway page from which code can be executed in a non-privileged context, we also have the ideal place to put the signal trampolines. By moving the signal trampolines from the user stack to the gateway page, we open up the doors to unexecutable stacks. The gateway page contains signal trampolines for both the "legacy" break-based syscall code and the new and improved epc- based syscall code. Approved: re@ (blanket) |
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