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package mountinfo
import (
"os"
"path/filepath"
"golang.org/x/sys/unix"
)
// MountedFast is a method of detecting a mount point without reading
// mountinfo from procfs. A caller can only trust the result if no error
// and sure == true are returned. Otherwise, other methods (e.g. parsing
// /proc/mounts) have to be used. If unsure, use Mounted instead (which
// uses MountedFast, but falls back to parsing mountinfo if needed).
//
// If a non-existent path is specified, an appropriate error is returned.
// In case the caller is not interested in this particular error, it should
// be handled separately using e.g. errors.Is(err, fs.ErrNotExist).
//
// This function is only available on Linux. When available (since kernel
// v5.6), openat2(2) syscall is used to reliably detect all mounts. Otherwise,
// the implementation falls back to using stat(2), which can reliably detect
// normal (but not bind) mounts.
func MountedFast(path string) (mounted, sure bool, err error) {
// Root is always mounted.
if path == string(os.PathSeparator) {
return true, true, nil
}
path, err = normalizePath(path)
if err != nil {
return false, false, err
}
mounted, sure, err = mountedFast(path)
return
}
// mountedByOpenat2 is a method of detecting a mount that works for all kinds
// of mounts (incl. bind mounts), but requires a recent (v5.6+) linux kernel.
func mountedByOpenat2(path string) (bool, error) {
dir, last := filepath.Split(path)
dirfd, err := unix.Openat2(unix.AT_FDCWD, dir, &unix.OpenHow{
Flags: unix.O_PATH | unix.O_CLOEXEC,
})
if err != nil {
return false, &os.PathError{Op: "openat2", Path: dir, Err: err}
}
fd, err := unix.Openat2(dirfd, last, &unix.OpenHow{
Flags: unix.O_PATH | unix.O_CLOEXEC | unix.O_NOFOLLOW,
Resolve: unix.RESOLVE_NO_XDEV,
})
_ = unix.Close(dirfd)
switch err { //nolint:errorlint // unix errors are bare
case nil: // definitely not a mount
_ = unix.Close(fd)
return false, nil
case unix.EXDEV: // definitely a mount
return true, nil
}
// not sure
return false, &os.PathError{Op: "openat2", Path: path, Err: err}
}
// mountedFast is similar to MountedFast, except it expects a normalized path.
func mountedFast(path string) (mounted, sure bool, err error) {
// Root is always mounted.
if path == string(os.PathSeparator) {
return true, true, nil
}
// Try a fast path, using openat2() with RESOLVE_NO_XDEV.
mounted, err = mountedByOpenat2(path)
if err == nil {
return mounted, true, nil
}
// Another fast path: compare st.st_dev fields.
mounted, err = mountedByStat(path)
// This does not work for bind mounts, so false negative
// is possible, therefore only trust if return is true.
if mounted && err == nil {
return true, true, nil
}
return
}
func mounted(path string) (bool, error) {
path, err := normalizePath(path)
if err != nil {
return false, err
}
mounted, sure, err := mountedFast(path)
if sure && err == nil {
return mounted, nil
}
// Fallback to parsing mountinfo.
return mountedByMountinfo(path)
}
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