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runtime: avoid write barrier in cgo mmap code

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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>
This commit is contained in:
Ian Lance Taylor 2016-01-29 15:43:24 -08:00
parent d37d76af43
commit b3c05f08a9
1 changed files with 10 additions and 3 deletions
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13
src/runtime/cgo_mmap.go
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@ -15,12 +15,19 @@ import "unsafe"
//go:linkname _cgo_mmap _cgo_mmap
var _cgo_mmap unsafe.Pointer
func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) (ret unsafe.Pointer) {
func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer {
if _cgo_mmap != nil {
// Make ret a uintptr so that writing to it in the
// function literal does not trigger a write barrier.
// A write barrier here could break because of the way
// that mmap uses the same value both as a pointer and
// an errno value.
// TODO: Fix mmap to return two values.
var ret uintptr
systemstack(func() {
ret = callCgoMmap(addr, n, prot, flags, fd, off)
})
return
return unsafe.Pointer(ret)
}
return sysMmap(addr, n, prot, flags, fd, off)
}
@ -31,4 +38,4 @@ func sysMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32)
// cgoMmap calls the mmap function in the runtime/cgo package on the
// callCgoMmap calls the mmap function in the runtime/cgo package
// using the GCC calling convention. It is implemented in assembly.
func callCgoMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer
func callCgoMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) uintptr