mounted_linux.go

Documentation: github.com/moby/sys/mountinfo

     1  package mountinfo
     2  
     3  import (
     4  	"os"
     5  	"path/filepath"
     6  
     7  	"golang.org/x/sys/unix"
     8  )
     9  
    10  // MountedFast is a method of detecting a mount point without reading
    11  // mountinfo from procfs. A caller can only trust the result if no error
    12  // and sure == true are returned. Otherwise, other methods (e.g. parsing
    13  // /proc/mounts) have to be used. If unsure, use Mounted instead (which
    14  // uses MountedFast, but falls back to parsing mountinfo if needed).
    15  //
    16  // If a non-existent path is specified, an appropriate error is returned.
    17  // In case the caller is not interested in this particular error, it should
    18  // be handled separately using e.g. errors.Is(err, fs.ErrNotExist).
    19  //
    20  // This function is only available on Linux. When available (since kernel
    21  // v5.6), openat2(2) syscall is used to reliably detect all mounts. Otherwise,
    22  // the implementation falls back to using stat(2), which can reliably detect
    23  // normal (but not bind) mounts.
    24  func MountedFast(path string) (mounted, sure bool, err error) {
    25  	// Root is always mounted.
    26  	if path == string(os.PathSeparator) {
    27  		return true, true, nil
    28  	}
    29  
    30  	path, err = normalizePath(path)
    31  	if err != nil {
    32  		return false, false, err
    33  	}
    34  	mounted, sure, err = mountedFast(path)
    35  	return
    36  }
    37  
    38  // mountedByOpenat2 is a method of detecting a mount that works for all kinds
    39  // of mounts (incl. bind mounts), but requires a recent (v5.6+) linux kernel.
    40  func mountedByOpenat2(path string) (bool, error) {
    41  	dir, last := filepath.Split(path)
    42  
    43  	dirfd, err := unix.Openat2(unix.AT_FDCWD, dir, &unix.OpenHow{
    44  		Flags: unix.O_PATH | unix.O_CLOEXEC,
    45  	})
    46  	if err != nil {
    47  		return false, &os.PathError{Op: "openat2", Path: dir, Err: err}
    48  	}
    49  	fd, err := unix.Openat2(dirfd, last, &unix.OpenHow{
    50  		Flags:   unix.O_PATH | unix.O_CLOEXEC | unix.O_NOFOLLOW,
    51  		Resolve: unix.RESOLVE_NO_XDEV,
    52  	})
    53  	_ = unix.Close(dirfd)
    54  	switch err { //nolint:errorlint // unix errors are bare
    55  	case nil: // definitely not a mount
    56  		_ = unix.Close(fd)
    57  		return false, nil
    58  	case unix.EXDEV: // definitely a mount
    59  		return true, nil
    60  	}
    61  	// not sure
    62  	return false, &os.PathError{Op: "openat2", Path: path, Err: err}
    63  }
    64  
    65  // mountedFast is similar to MountedFast, except it expects a normalized path.
    66  func mountedFast(path string) (mounted, sure bool, err error) {
    67  	// Root is always mounted.
    68  	if path == string(os.PathSeparator) {
    69  		return true, true, nil
    70  	}
    71  
    72  	// Try a fast path, using openat2() with RESOLVE_NO_XDEV.
    73  	mounted, err = mountedByOpenat2(path)
    74  	if err == nil {
    75  		return mounted, true, nil
    76  	}
    77  
    78  	// Another fast path: compare st.st_dev fields.
    79  	mounted, err = mountedByStat(path)
    80  	// This does not work for bind mounts, so false negative
    81  	// is possible, therefore only trust if return is true.
    82  	if mounted && err == nil {
    83  		return true, true, nil
    84  	}
    85  
    86  	return
    87  }
    88  
    89  func mounted(path string) (bool, error) {
    90  	path, err := normalizePath(path)
    91  	if err != nil {
    92  		return false, err
    93  	}
    94  	mounted, sure, err := mountedFast(path)
    95  	if sure && err == nil {
    96  		return mounted, nil
    97  	}
    98  
    99  	// Fallback to parsing mountinfo.
   100  	return mountedByMountinfo(path)
   101  }
   102
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