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cmd/cgo: fix initialization of empty argument types

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CL 258938 changed the way C to Go calls work such that they now
construct a C struct on the C side for the arguments and space for the
results. Any pointers in the result space must be zeroed, so we just
zero the whole struct.

However, C makes it surprisingly hard to robustly zero any struct
type. We had used a "{0}" initializer, which works in the vast
majority of cases, but fails if the type is empty or effectively
empty.

This CL fixes this by changing how the cgo tool zero-initializes the
argument struct to be more robust.

Fixes #42495.

Change-Id: Id1749b9d751e59eb7a02a9d44fec0698a2bf63cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/269337
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Austin Clements 2020-11-11 15:33:39 -05:00
parent 35455fff0e
commit f423d616b1
2 changed files with 24 additions and 1 deletions
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15
misc/cgo/test/issue42495.go Normal file
View file

@ -0,0 +1,15 @@
// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package cgotest
// typedef struct { } T42495A;
// typedef struct { int x[0]; } T42495B;
import "C"
//export Issue42495A
func Issue42495A(C.T42495A) {}
//export Issue42495B
func Issue42495B(C.T42495B) {}

10
src/cmd/cgo/out.go
View file

@ -985,7 +985,15 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
// The results part of the argument structure must be
// initialized to 0 so the write barriers generated by
// the assignments to these fields in Go are safe.
fmt.Fprintf(fgcc, "\t%s %v _cgo_a = {0};\n", ctype, p.packedAttribute())
//
// We use a local static variable to get the zeroed
// value of the argument type. This avoids including
// string.h for memset, and is also robust to C++
// types with constructors. Both GCC and LLVM optimize
// this into just zeroing _cgo_a.
fmt.Fprintf(fgcc, "\ttypedef %s %v _cgo_argtype;\n", ctype, p.packedAttribute())
fmt.Fprintf(fgcc, "\tstatic _cgo_argtype _cgo_zero;\n")
fmt.Fprintf(fgcc, "\t_cgo_argtype _cgo_a = _cgo_zero;\n")
if gccResult != "void" && (len(fntype.Results.List) > 1 || len(fntype.Results.List[0].Names) > 1) {
fmt.Fprintf(fgcc, "\t%s r;\n", gccResult)
}