runtime: check only up to ptrdata bytes for pointers

Fixes #14508.

Change-Id: I237d0c5a79a73e6c97bdb2077d8ede613128b978
Reviewed-on: https://go-review.googlesource.com/23224
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

misc/cgo/errors/ptr.go

@@ -290,6 +290,30 @@ var ptrTests = []ptrTest{
 		},
 		fail: true,
 	},
+	{
+		// Don't check non-pointer data.
+		// Uses unsafe code to get a pointer we shouldn't check.
+		// Although we use unsafe, the uintptr represents an integer
+		// that happens to have the same representation as a pointer;
+		// that is, we are testing something that is not unsafe.
+		name: "ptrdata1",
+		c: `#include <stdlib.h>
+                    void f(void* p) {}`,
+		imports: []string{"unsafe"},
+		support: `type S struct { p *int; a [8*8]byte; u uintptr }`,
+		body:    `i := 0; p := &S{u:uintptr(unsafe.Pointer(&i))}; q := (*S)(C.malloc(C.size_t(unsafe.Sizeof(*p)))); *q = *p; C.f(unsafe.Pointer(q))`,
+		fail:    false,
+	},
+	{
+		// Like ptrdata1, but with a type that uses a GC program.
+		name: "ptrdata2",
+		c: `#include <stdlib.h>
+                    void f(void* p) {}`,
+		imports: []string{"unsafe"},
+		support: `type S struct { p *int; a [32769*8]byte; q *int; u uintptr }`,
+		body:    `i := 0; p := S{u:uintptr(unsafe.Pointer(&i))}; q := (*S)(C.malloc(C.size_t(unsafe.Sizeof(p)))); *q = p; C.f(unsafe.Pointer(q))`,
+		fail:    false,
+	},
 }
 
 func main() {

src/runtime/cgocheck.go

@@ -94,6 +94,14 @@ func cgoCheckSliceCopy(typ *_type, dst, src slice, n int) {
 //go:nosplit
 //go:nowritebarrier
 func cgoCheckTypedBlock(typ *_type, src unsafe.Pointer, off, size uintptr) {
+	// Anything past typ.ptrdata is not a pointer.
+	if typ.ptrdata <= off {
+		return
+	}
+	if ptrdataSize := typ.ptrdata - off; size > ptrdataSize {
+		size = ptrdataSize
+	}
+
 	if typ.kind&kindGCProg == 0 {
 		cgoCheckBits(src, typ.gcdata, off, size)
 		return
@@ -184,7 +192,7 @@ func cgoCheckBits(src unsafe.Pointer, gcbits *byte, off, size uintptr) {
 
 // cgoCheckUsingType is like cgoCheckTypedBlock, but is a last ditch
 // fall back to look for pointers in src using the type information.
-// We only this when looking at a value on the stack when the type
+// We only use this when looking at a value on the stack when the type
 // uses a GC program, because otherwise it's more efficient to use the
 // GC bits. This is called on the system stack.
 //go:nowritebarrier
@@ -193,6 +201,15 @@ func cgoCheckUsingType(typ *_type, src unsafe.Pointer, off, size uintptr) {
 	if typ.kind&kindNoPointers != 0 {
 		return
 	}
+
+	// Anything past typ.ptrdata is not a pointer.
+	if typ.ptrdata <= off {
+		return
+	}
+	if ptrdataSize := typ.ptrdata - off; size > ptrdataSize {
+		size = ptrdataSize
+	}
+
 	if typ.kind&kindGCProg == 0 {
 		cgoCheckBits(src, typ.gcdata, off, size)
 		return