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Use correct LOOP_CTL_GET_FREE API in pkg/loopback

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The `ioctl` interface for the `LOOP_CTL_GET_FREE` request on
`/dev/loop-control` is a little different from what `unix.IoctlGetInt`
expects: the first index is the returned status in `r1`, not an `int`
pointer as the first parameter.

Unfortunately we have to go a little lower level to get the appropriate
loop device index out, using `unix.Syscall` directly to read from
`r1`. Internally, the index is returned as a signed integer to match the
internal `ioctl` expectations of interpreting a negative signed integer
as an error at the userspace ABI boundary, so the direct interface of
`ioctlLoopCtlGetFree` can remain as-is.

[@kolyshkin: it still worked before this fix because of
/dev scan fallback in ioctlLoopCtlGetFree()]

Signed-off-by: Daniel Sweet <danieljsweet@icloud.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
This commit is contained in:
Daniel Sweet 2019-04-21 21:38:40 -04:00 committed by Kir Kolyshkin
parent 5c0d487416
commit db2bc43017
1 changed files with 7 additions and 3 deletions
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10
pkg/loopback/ioctl.go
View file

@ -9,11 +9,15 @@ import (
)
func ioctlLoopCtlGetFree(fd uintptr) (int, error) {
index, err := unix.IoctlGetInt(int(fd), LoopCtlGetFree)
if err != nil {
// The ioctl interface for /dev/loop-control (since Linux 3.1) is a bit
// off compared to what you'd expect: instead of writing an integer to a
// parameter pointer like unix.IoctlGetInt() expects, it returns the first
// available loop device index directly.
ioctlReturn, _, err := unix.Syscall(unix.SYS_IOCTL, fd, LoopCtlGetFree, 0)
if err != 0 {
return 0, err
}
return index, nil
return int(ioctlReturn), nil
}
func ioctlLoopSetFd(loopFd, sparseFd uintptr) error {