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runtime: set up read-only dummy TLS space for needm on Windows
On Windows, TLS is uninitialized for C threads calling into Go code. In this path, before calling into user Go code, we call into needm which runs without an m, but whose purpose is to pick one up. While in Go code, we may occasionally restore the G register from TLS for a number of reasons. Rather than try to flag all these cases, given that needm (and its callees) are already somewhat special, just set up a dummy TLS space for it that's read-only. If it ever actually tries to write to this space (it shouldn't), it will fail loudly. Otherwise, code that restores the G register will simply load a zero value, but that's OK since needm is careful never to require the G at any point, because it doesn't yet have a valid G. Furthermore, by the time needm returns, it will have set up TLS properly for a Windows C thread, so there's no need to do anything extra afterwards. For #40724. Change-Id: I34e8095059817e4ee663505e89cda8785b634b98 Reviewed-on: https://go-review.googlesource.com/c/go/+/307872 Trust: Michael Knyszek <mknyszek@google.com> Run-TryBot: Michael Knyszek <mknyszek@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Cherry Zhang <cherryyz@google.com>
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@ -802,6 +802,14 @@ nosave:
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MOVL AX, ret+16(FP)
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RET
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#ifdef GOOS_windows
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// Dummy TLS that's used on Windows so that we don't crash trying
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// to restore the G register in needm. needm and its callees are
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// very careful never to actually use the G, the TLS just can't be
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// unset since we're in Go code.
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GLOBL zeroTLS<>(SB),RODATA,$const_tlsSize
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#endif
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// func cgocallback(fn, frame unsafe.Pointer, ctxt uintptr)
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// See cgocall.go for more details.
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TEXT ·cgocallback(SB),NOSPLIT,$24-24
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@ -825,6 +833,15 @@ TEXT ·cgocallback(SB),NOSPLIT,$24-24
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MOVQ BX, savedm-8(SP) // saved copy of oldm
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JMP havem
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needm:
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#ifdef GOOS_windows
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// Set up a dummy TLS value. needm is careful not to use it,
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// but it needs to be there to prevent autogenerated code from
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// crashing when it loads from it.
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// We don't need to clear it or anything later because needm
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// will set up TLS properly.
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MOVQ $zeroTLS<>(SB), DI
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CALL runtime·settls(SB)
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#endif
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// On some platforms (Windows) we cannot call needm through
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// an ABI wrapper because there's no TLS set up, and the ABI
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// wrapper will try to restore the G register (R14) from TLS.
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@ -482,17 +482,24 @@ type g struct {
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gcAssistBytes int64
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}
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const (
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// tlsSlots is the number of pointer-sized slots reserved for TLS on some platforms,
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// like Windows.
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tlsSlots = 6
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tlsSize = tlsSlots * sys.PtrSize
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)
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type m struct {
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g0 *g // goroutine with scheduling stack
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morebuf gobuf // gobuf arg to morestack
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divmod uint32 // div/mod denominator for arm - known to liblink
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// Fields not known to debuggers.
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procid uint64 // for debuggers, but offset not hard-coded
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gsignal *g // signal-handling g
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goSigStack gsignalStack // Go-allocated signal handling stack
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sigmask sigset // storage for saved signal mask
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tls [6]uintptr // thread-local storage (for x86 extern register)
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procid uint64 // for debuggers, but offset not hard-coded
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gsignal *g // signal-handling g
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goSigStack gsignalStack // Go-allocated signal handling stack
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sigmask sigset // storage for saved signal mask
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tls [tlsSlots]uintptr // thread-local storage (for x86 extern register)
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mstartfn func()
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curg *g // current running goroutine
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caughtsig guintptr // goroutine running during fatal signal
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