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/45016Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/cgo/gcc_mmap.c

@@ -6,6 +6,7 @@
 
 #include <errno.h>
 #include <stdint.h>
+#include <stdlib.h>
 #include <sys/mman.h>
 
 #include "libcgo.h"
@@ -23,3 +24,16 @@ x_cgo_mmap(void *addr, uintptr_t length, int32_t prot, int32_t flags, int32_t fd
 	}
 	return p;
 }
+
+void
+x_cgo_munmap(void *addr, uintptr_t length) {
+	int r;
+
+	_cgo_tsan_acquire();
+	r = munmap(addr, length);
+	_cgo_tsan_release();
+	if (r < 0) {
+		/* The Go runtime is not prepared for munmap to fail.  */
+		abort();
+	}
+}

src/runtime/cgo/mmap.go

@@ -15,8 +15,17 @@ import _ "unsafe"
 // C/C++ code; this permits that code to see the Go code as normal
 // program addresses that have been initialized.
 
+// To support interceptors that look for both mmap and munmap,
+// also call the C library for munmap.
+
 //go:cgo_import_static x_cgo_mmap
 //go:linkname x_cgo_mmap x_cgo_mmap
 //go:linkname _cgo_mmap _cgo_mmap
 var x_cgo_mmap byte
 var _cgo_mmap = &x_cgo_mmap
+
+//go:cgo_import_static x_cgo_munmap
+//go:linkname x_cgo_munmap x_cgo_munmap
+//go:linkname _cgo_munmap _cgo_munmap
+var x_cgo_munmap byte
+var _cgo_munmap = &x_cgo_munmap

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)

src/runtime/mmap.go

@@ -17,3 +17,6 @@ import "unsafe"
 // assembly routine; the higher bits (if required), should be provided
 // by the assembly routine as 0.
 func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer
+
+// munmap calls the munmap system call. It is implemented in assembly.
+func munmap(addr unsafe.Pointer, n uintptr)

src/runtime/stubs2.go

@@ -18,8 +18,6 @@ func exit(code int32)
 func nanotime() int64
 func usleep(usec uint32)
 
-func munmap(addr unsafe.Pointer, n uintptr)
-
 //go:noescape
 func write(fd uintptr, p unsafe.Pointer, n int32) int32
 

src/runtime/sys_linux_amd64.s

@@ -393,7 +393,7 @@ TEXT runtime·callCgoMmap(SB),NOSPLIT,$16
 	MOVQ	AX, ret+32(FP)
 	RET
 
-TEXT runtime·munmap(SB),NOSPLIT,$0
+TEXT runtime·sysMunmap(SB),NOSPLIT,$0
 	MOVQ	addr+0(FP), DI
 	MOVQ	n+8(FP), SI
 	MOVQ	$11, AX	// munmap
@@ -403,6 +403,19 @@ TEXT runtime·munmap(SB),NOSPLIT,$0
 	MOVL	$0xf1, 0xf1  // crash
 	RET
 
+// Call the function stored in _cgo_munmap using the GCC calling convention.
+// This must be called on the system stack.
+TEXT runtime·callCgoMunmap(SB),NOSPLIT,$16-16
+	MOVQ	addr+0(FP), DI
+	MOVQ	n+8(FP), SI
+	MOVQ	_cgo_munmap(SB), AX
+	MOVQ	SP, BX
+	ANDQ	$~15, SP	// alignment as per amd64 psABI
+	MOVQ	BX, 0(SP)
+	CALL	AX
+	MOVQ	0(SP), SP
+	RET
+
 TEXT runtime·madvise(SB),NOSPLIT,$0
 	MOVQ	addr+0(FP), DI
 	MOVQ	n+8(FP), SI