Source: signal_darwin_amd64.go in package runtime


package runtime

import "unsafe"

type sigctxt struct {
	info *siginfo
	ctxt unsafe.Pointer
}

go:nosplitgo:nowritebarrierrec

func (c *sigctxt) regs() *regs64 { return &(*ucontext)(c.ctxt).uc_mcontext.ss }

func (c *sigctxt) rax() uint64 { return c.regs().rax }
func (c *sigctxt) rbx() uint64 { return c.regs().rbx }
func (c *sigctxt) rcx() uint64 { return c.regs().rcx }
func (c *sigctxt) rdx() uint64 { return c.regs().rdx }
func (c *sigctxt) rdi() uint64 { return c.regs().rdi }
func (c *sigctxt) rsi() uint64 { return c.regs().rsi }
func (c *sigctxt) rbp() uint64 { return c.regs().rbp }
func (c *sigctxt) rsp() uint64 { return c.regs().rsp }
func (c *sigctxt) r8() uint64  { return c.regs().r8 }
func (c *sigctxt) r9() uint64  { return c.regs().r9 }
func (c *sigctxt) r10() uint64 { return c.regs().r10 }
func (c *sigctxt) r11() uint64 { return c.regs().r11 }
func (c *sigctxt) r12() uint64 { return c.regs().r12 }
func (c *sigctxt) r13() uint64 { return c.regs().r13 }
func (c *sigctxt) r14() uint64 { return c.regs().r14 }
func (c *sigctxt) r15() uint64 { return c.regs().r15 }

go:nosplitgo:nowritebarrierrec

func (c *sigctxt) rip() uint64 { return c.regs().rip }

func (c *sigctxt) rflags() uint64  { return c.regs().rflags }
func (c *sigctxt) cs() uint64      { return c.regs().cs }
func (c *sigctxt) fs() uint64      { return c.regs().fs }
func (c *sigctxt) gs() uint64      { return c.regs().gs }
func (c *sigctxt) sigcode() uint64 { return uint64(c.info.si_code) }
func (c *sigctxt) sigaddr() uint64 { return c.info.si_addr }

func (c *sigctxt) set_rip(x uint64)     { c.regs().rip = x }
func (c *sigctxt) set_rsp(x uint64)     { c.regs().rsp = x }
func (c *sigctxt) set_sigcode(x uint64) { c.info.si_code = int32(x) }
func (c *sigctxt) set_sigaddr(x uint64) { c.info.si_addr = x }

go:nosplit

func (c *sigctxt) fixsigcode(sig uint32) {
	switch sig {

OS X sets c.sigcode() == TRAP_BRKPT unconditionally for all SIGTRAPs, leaving no way to distinguish a breakpoint-induced SIGTRAP from an asynchronous signal SIGTRAP. They all look breakpoint-induced by default. Try looking at the code to see if it's a breakpoint. The assumption is that we're very unlikely to get an asynchronous SIGTRAP at just the moment that the PC started to point at unmapped memory.

OS X will leave the pc just after the INT 3 instruction. INT 3 is usually 1 byte, but there is a 2-byte form.

		code := (*[2]byte)(unsafe.Pointer(pc - 2))

SIGTRAP on something other than INT 3.

			c.set_sigcode(_SI_USER)
		}

x86-64 has 48-bit virtual addresses. The top 16 bits must echo bit 47. The hardware delivers a different kind of fault for a malformed address than it does for an attempt to access a valid but unmapped address. OS X 10.9.2 mishandles the malformed address case, making it look like a user-generated signal (like someone ran kill -SEGV ourpid). We pass user-generated signals to os/signal, or else ignore them. Doing that here - and returning to the faulting code - results in an infinite loop. It appears the best we can do is rewrite what the kernel delivers into something more like the truth. The address used below has very little chance of being the one that caused the fault, but it is malformed, it is clearly not a real pointer, and if it does get printed in real life, people will probably search for it and find this code. There are no Google hits for b01dfacedebac1e or 0xb01dfacedebac1e as I type this comment.

		if c.sigcode() == _SI_USER {
			c.set_sigcode(_SI_USER + 1)
			c.set_sigaddr(0xb01dfacedebac1e)
		}
	}