What version of Go are you using (go version)?

$ go version

go version go1.15.3 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""

GOARCH="amd64"

GOBIN=""

GOCACHE="/root/.cache/go-build"

GOENV="/root/.config/go/env"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOINSECURE=""

GOMODCACHE="/go/path/pkg/mod"

GONOPROXY=""

GONOSUMDB=""

GOOS="linux"

GOPATH="/go/path"

GOPRIVATE=""

GOPROXY="https://goproxy.io"

GOROOT="/go/root/go"

GOSUMDB="sum.golang.org"

GOTMPDIR=""

GOTOOLDIR="/go/root/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

AR="ar"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD="/code/goldeneyes/common/go.mod"

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build111842700=/tmp/go-build -gno-record-gcc-switches"

run at CentOS Linux release 7.8.2003 (Kernel 3.10.0-1127.el7.x86_64)

What did you do?

I set up a samba service , share a directory, allow user to read and create and write.

Then, mount the share directory at CentOS 7 /mnt/test

build a example:

func main() {
	fmt.Println(CopyFile(os.Args[1], os.Args[2]))
}

func CopyFile(src string, dst string) (int64, error) {
	srcFile, err := os.Open(src)
	if err != nil {
		return -1, err
	}
	defer srcFile.Close()

	dstFile, err := os.OpenFile(dst, os.O_RDWR|os.O_CREATE|os.O_TRUNC, os.ModePerm)
	if err != nil {
		return -1, err
	}
	defer dstFile.Close()
	return io.Copy(dstFile, srcFile)
}

go build -o gocopy
Run gocopy on CentOS7 :

[root@localhost ~]# ./gocopy 1.txt 2.txt
18 <nil>
[root@localhost ~]# ./gocopy /mnt/test/1.txt 1.txt
18 <nil>
[root@localhost ~]# ./gocopy 1.txt /mnt/test/2.txt
18 <nil>
[root@localhost ~]# ./gocopy /mnt/test/1.txt /mnt/test/2.txt
0 write /mnt/test/2.txt: copy_file_range: input/output error

Follow the source code，found：

On success case, copyFileRange return ENOSYS . But when copy two file on cifs will return EIO

What did you expect to see?

copy success