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runtime: avoid write barrier in cgo mmap code
Tested by hand with a runtime/cgo modified to return an mmap failure after 10 calls. This is an interim patch. For 1.7 we should fix mmap properly to avoid using the same value as both a pointer and an errno value. Fixes #14149. Change-Id: I8f2bbd47d711e283001ba73296f1c34a26c59241 Reviewed-on: https://go-review.googlesource.com/19084 TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Russ Cox <rsc@golang.org>
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@ -15,12 +15,19 @@ import "unsafe"
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//go:linkname _cgo_mmap _cgo_mmap
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var _cgo_mmap unsafe.Pointer
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func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) (ret unsafe.Pointer) {
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func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer {
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if _cgo_mmap != nil {
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// Make ret a uintptr so that writing to it in the
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// function literal does not trigger a write barrier.
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// A write barrier here could break because of the way
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// that mmap uses the same value both as a pointer and
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// an errno value.
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// TODO: Fix mmap to return two values.
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var ret uintptr
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systemstack(func() {
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ret = callCgoMmap(addr, n, prot, flags, fd, off)
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})
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return
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return unsafe.Pointer(ret)
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}
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return sysMmap(addr, n, prot, flags, fd, off)
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}
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@ -31,4 +38,4 @@ func sysMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32)
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// cgoMmap calls the mmap function in the runtime/cgo package on the
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// callCgoMmap calls the mmap function in the runtime/cgo package
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// using the GCC calling convention. It is implemented in assembly.
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func callCgoMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer
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func callCgoMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) uintptr
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