runtime: intercept munmap as we do mmap

For cgo programs on linux-amd64 we call the C function mmap.
This supports programs such as the C memory sanitizer that need to
intercept all calls to mmap. It turns out that there are programs that
intercept both mmap and munmap, or that at least expect that if they
intercept mmap, they also intercept munmap. So, if we permit mmap
to be intercepted, also permit munmap to be intercepted.

No test, as it requires two odd things: a C program that intercepts
mmap and munmap, and a Go program that calls munmap.

Change-Id: Iec33f47d59f70dbb7463fd12d30728c24cd4face
Reviewed-on: https://go-review.googlesource.com/45016
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/runtime/cgo_mmap.go b/src/runtime/cgo_mmap.go
index 5a2a1a2c37..aa531b9020 100644
--- a/src/runtime/cgo_mmap.go
+++ b/src/runtime/cgo_mmap.go
@@ -15,6 +15,11 @@ import "unsafe"
 //go:linkname _cgo_mmap _cgo_mmap
 var _cgo_mmap unsafe.Pointer
 
+// _cgo_munmap is filled in by runtime/cgo when it is linked into the
+// program, so it is only non-nil when using cgo.
+//go:linkname _cgo_munmap _cgo_munmap
+var _cgo_munmap 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
@@ -32,9 +37,24 @@ func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) uns
 	return sysMmap(addr, n, prot, flags, fd, off)
 }
 
+func munmap(addr unsafe.Pointer, n uintptr) {
+	if _cgo_munmap != nil {
+		systemstack(func() { callCgoMunmap(addr, n) })
+		return
+	}
+	sysMunmap(addr, n)
+}
+
 // sysMmap calls the mmap system call. It is implemented in assembly.
 func sysMmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer
 
 // 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) uintptr
+
+// sysMunmap calls the munmap system call. It is implemented in assembly.
+func sysMunmap(addr unsafe.Pointer, n uintptr)
+
+// callCgoMunmap calls the munmap function in the runtime/cgo package
+// using the GCC calling convention. It is implemented in assembly.
+func callCgoMunmap(addr unsafe.Pointer, n uintptr)