x/sys/unix: the Linux ioctl FIDEDUPERANGE wrapper is implemented wrong

[viric]

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

I use this version below, but I want to talk about code in master (that came from x/sys/unix).

$ go version
go version go1.15.2 linux/amd64

Does this issue reproduce with the latest release?

Yes, a ioctl implementation doesn't match the linux manual.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/viric/.cache/go-build"
GOENV="/home/viric/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/viric/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/viric/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/nix/store/dz12rdqlp94rqinvcmk6knia6cw5s8ra-go-1.15.2/share/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/nix/store/dz12rdqlp94rqinvcmk6knia6cw5s8ra-go-1.15.2/share/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build423744141=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I want to use the (x/)sys/unix IoctlFileDedupeRange function

What did you expect to see?

It should ressemble the ioctl described in the linux manual:

int ioctl(int src_fd, FIDEDUPERANGE, struct file_dedupe_range *arg);

           struct file_dedupe_range {
               __u64 src_offset;
               __u64 src_length;
               __u16 dest_count;
               __u16 reserved1;
               __u32 reserved2;
               struct file_dedupe_range_info info[0];
           };

           struct file_dedupe_range_info {
               __s64 dest_fd;
               __u64 dest_offset;
               __u64 bytes_deduped;
               __s32 status;
               __u32 reserved;
           };

What did you see instead?

This is the ioctl in sys/unix (golang master or x/sys/unix) is like this: https://pkg.go.dev/golang.org/x/sys/unix#IoctlFileDedupeRange

destFd should be srcFd, and the 'info' member is not there, making the call unusable.

func IoctlFileDedupeRange(destFd int, value *FileDedupeRange) error

type FileDedupeRange struct {
    Src_offset uint64
    Src_length uint64
    Dest_count uint16
    Reserved1  uint16
    Reserved2  uint32
}

[ianlancetaylor]

CC @tklauser

[KyleSanderson]

Funny enough I'm looking to use this as well, then saw the incorrect and incomplete definition in golang 😕. Google lead me here, which was nice.

Go code cannot refer to zero-sized fields that occur at the end of non-empty C structs. To get the address of such a field (which is the only operation you can do with a zero-sized field) you must take the address of the struct and add the size of the struct.

Specifically the bug here is file_dedupe_range_info is not implemented, which allows the syscall to still happen, but it doesn't do any work because Dest_count can only be 0 due to the bug.

[gopherbot]

Change https://golang.org/cl/284352 mentions this issue: sys/unix: fix and test the FIDEDUPERANGE Linux ioctl