cmd/cgo: recognize that unsafe.StringData only points to a string

[ianlancetaylor]

After https://go.dev/cl/483815 this program fails when run with GOEXPERIMENT=boringcrypto:

package main

import (
	"crypto/sha1"
	"fmt"
	"io"
	"unsafe"
)

func hash(a, b string) []byte {
	h := sha1.New()
	h.(io.StringWriter).WriteString(a+b)
	return h.Sum(nil)
}

type S struct {
	buf [5]byte
	z   int
	p   *int
}

var x = S{p: new(int)}

func main() {
	x.buf[0] = 'a'
	s := unsafe.String(&x.buf[0], 1)
	fmt.Printf("%x\n", hash(s, ""))
}

Running this program normally works fine. With GOEXPERIMENT=boringcrypto it fails with

panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 1 [running]:
crypto/internal/boring.(*sha1Hash).WriteString.func1(0xc00009e000, {0x6fbcf0, 0x1})
	/home/iant/go/src/crypto/internal/boring/sha.go:149 +0x30
crypto/internal/boring.(*sha1Hash).WriteString(0xc0000061a0?, {0x6fbcf0?, 0x1})
	/home/iant/go/src/crypto/internal/boring/sha.go:149 +0x25
main.hash({0x6fbcf0, 0x1}, {0x0, 0x0})
	/home/iant/foo.go:12 +0x85
main.main()
	/home/iant/foo.go:27 +0x3d
exit status 2

The problem is that the boringcrypto code looks like this:

func (h *sha1Hash) WriteString(s string) (int, error) {
	if len(s) > 0 && C._goboringcrypto_SHA1_Update(h.noescapeCtx(), unsafe.Pointer(unsafe.StringData(s)), C.size_t(len(s))) == 0 {
		panic("boringcrypto: SHA1_Update failed")
	}
	return len(s), nil
}

The cgo command does not recognize that passing unsafe.Pointer(unsafe.StringData(s)) should only check the contents of the string. Since it doesn't recognize that, it winds up checking the entire contents of the struct that contains the string, leading to report an incorrect error.

While this test case is rather artificial, a more realistic test case can be constructed using arenas, in which the arena itself plays the part of the struct that may have associated pointers.

[gopherbot]

Change https://go.dev/cl/492355 mentions this issue: Revert "crypto/sha512: add WriteString and WriteByte method"

[gopherbot]

Change https://go.dev/cl/492375 mentions this issue: Revert "crypto/sha1: add WriteString and WriteByte method"

[gopherbot]

Change https://go.dev/cl/492356 mentions this issue: Revert "crypto/sha256: add WriteString and WriteByte method"

[gopherbot]

Change https://go.dev/cl/493556 mentions this issue: cmd/cgo: recognize unsafe.{StringData,SliceData}