Cygwin: aarch64: fix makecontext/setcontext and signal-handler asm#2
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mayankag-qti wants to merge 1 commit into
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Cygwin: aarch64: fix makecontext/setcontext and signal-handler asm#2mayankag-qti wants to merge 1 commit into
mayankag-qti wants to merge 1 commit into
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Three ARM64-specific problems in exceptions.cc prevented makecontext, setcontext and swapcontext from working and could corrupt state after a signal delivered on an alternate stack: * setcontext: RtlRestoreContext raises STATUS_ILLEGAL_INSTRUCTION when handed a synthetic context built by makecontext (as opposed to one captured by RtlCaptureContext/GetThreadContext) - it rejects an arbitrary PC and a stack outside the thread's registered range. Restore the registers by hand and branch to the saved PC instead, as glibc/musl do for aarch64. * __cont_link_context: the old trampoline copied the uc_link slot address into SP before reading it. That address is only 8-byte aligned when makecontext is called with an odd number of stack arguments, so the access through SP faulted. Read via x19 and mask into SP in one step. Tail-call setcontext/cygwin_exit with 'b' instead of 'bl' so the noreturn frame stays leaf for SEH unwinding, and load the exit code before the NULL-uc_link branch so cygwin_exit(0xff) is actually used. * call_signal_handler: the alternate-stack asm calls altstack_wrapper() but only declared x0-x7,x9,x10 clobbered. Declare every caller-saved register the call may touch (x8, x11-x17, v0-v7, v16-v31, cc); x18 (the Windows TEB pointer) and v8-v15 (callee-saved on Windows) are preserved and stay out of the list.
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Three ARM64-specific problems in exceptions.cc prevented makecontext, setcontext and swapcontext from working and could corrupt state after a signal delivered on an alternate stack:
setcontext: RtlRestoreContext raises STATUS_ILLEGAL_INSTRUCTION when handed a synthetic context built by makecontext (as opposed to one captured by RtlCaptureContext/GetThreadContext) - it rejects an arbitrary PC and a stack outside the thread's registered range. Restore the registers by hand and branch to the saved PC instead, as glibc/musl do for aarch64.
__cont_link_context: the old trampoline copied the uc_link slot address into SP before reading it. That address is only 8-byte aligned when makecontext is called with an odd number of stack arguments, so the access through SP faulted. Read via x19 and mask into SP in one step. Tail-call setcontext/cygwin_exit with 'b' instead of 'bl' so the noreturn frame stays leaf for SEH unwinding, and load the exit code before the NULL-uc_link branch so cygwin_exit(0xff) is actually used.
call_signal_handler: the alternate-stack asm calls altstack_wrapper() but only declared x0-x7,x9,x10 clobbered. Declare every caller-saved register the call may touch (x8, x11-x17, v0-v7, v16-v31, cc); x18 (the Windows TEB pointer) and v8-v15 (callee-saved on Windows) are preserved and stay out of the list.