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syscallcompat: rework Fchmodat to FchmodatNofollow

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We never want Fchmodat to follow symlinks, so follow what
Qemu does, and call our function FchmodatNofollow.
This commit is contained in:
Jakob Unterwurzacher 2019-01-14 21:54:16 +01:00
parent a9d8eb49ef
commit a7d59032d3
5 changed files with 22 additions and 36 deletions
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2
internal/fusefrontend/fs.go
View File

@ -290,7 +290,7 @@ func (fs *FS) Chmod(path string, mode uint32, context *fuse.Context) (code fuse.
defer syscall.Close(dirfd)
// os.Chmod goes through the "syscallMode" translation function that messes
// up the suid and sgid bits. So use a syscall directly.
err = syscallcompat.Fchmodat(dirfd, cName, mode, unix.AT_SYMLINK_NOFOLLOW)
err = syscallcompat.FchmodatNofollow(dirfd, cName, mode)
return fuse.ToStatus(err)
}

4
internal/fusefrontend/fs_dir.go
View File

@ -164,7 +164,7 @@ func (fs *FS) Rmdir(relPath string, context *fuse.Context) (code fuse.Status) {
}
// This cast is needed on Darwin, where st.Mode is uint16.
origMode := uint32(st.Mode)
err = syscallcompat.Fchmodat(parentDirFd, cName, origMode|0700, unix.AT_SYMLINK_NOFOLLOW)
err = syscallcompat.FchmodatNofollow(parentDirFd, cName, origMode|0700)
if err != nil {
tlog.Debug.Printf("Rmdir: Fchmodat failed: %v", err)
return fuse.ToStatus(err)
@ -175,7 +175,7 @@ func (fs *FS) Rmdir(relPath string, context *fuse.Context) (code fuse.Status) {
// Undo the chmod if removing the directory failed
defer func() {
if code != fuse.OK {
err = syscallcompat.Fchmodat(parentDirFd, cName, origMode, unix.AT_SYMLINK_NOFOLLOW)
err = syscallcompat.FchmodatNofollow(parentDirFd, cName, origMode)
if err != nil {
tlog.Warn.Printf("Rmdir: Chmod rollback failed: %v", err)
}

6
internal/syscallcompat/sys_common_test.go
View File

@ -175,7 +175,7 @@ func TestFchmodat(t *testing.T) {
defer syscall.Close(dirfd)
// Check that chmod on a regular file works ok
err = Fchmodat(dirfd, regular, 0111, 0)
err = FchmodatNofollow(dirfd, regular, 0111, 0)
if err != nil {
t.Fatal(err)
}
@ -185,7 +185,7 @@ func TestFchmodat(t *testing.T) {
if st.Mode != 0111 {
t.Errorf("wrong mode: %#0o", st.Mode)
}
err = Fchmodat(dirfd, regular, 0000, 0)
err = FchmodatNofollow(dirfd, regular, 0000, 0)
if err != nil {
t.Error(err)
}
@ -196,7 +196,7 @@ func TestFchmodat(t *testing.T) {
}
// Check what happens on a symlink
err = Fchmodat(dirfd, symlink, 0333, 0)
err = FchmodatNofollow(dirfd, symlink, 0333, 0)
if err == nil {
syscall.Lstat(tmpDir+"/"+symlink, &st)
st.Mode &= 0777

11
internal/syscallcompat/sys_darwin.go
View File

@ -8,8 +8,6 @@ import (
"golang.org/x/sys/unix"
"github.com/hanwen/go-fuse/fuse"
"github.com/rfjakob/gocryptfs/internal/tlog"
)
const (
@ -93,13 +91,8 @@ func MknodatUser(dirfd int, path string, mode uint32, dev int, context *fuse.Con
return Mknodat(dirfd, path, mode, dev)
}
func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
// Why would we ever want to call this without AT_SYMLINK_NOFOLLOW?
if flags&unix.AT_SYMLINK_NOFOLLOW == 0 {
tlog.Warn.Printf("Fchmodat: adding missing AT_SYMLINK_NOFOLLOW flag")
flags |= unix.AT_SYMLINK_NOFOLLOW
}
return unix.Fchmodat(dirfd, path, mode, flags)
func FchmodatNofollow(dirfd int, path string, mode uint32) (err error) {
return unix.Fchmodat(dirfd, path, mode, unix.AT_SYMLINK_NOFOLLOW)
}
func SymlinkatUser(oldpath string, newdirfd int, newpath string, context *fuse.Context) (err error) {

35
internal/syscallcompat/sys_linux.go
View File

@ -113,20 +113,17 @@ func Dup3(oldfd int, newfd int, flags int) (err error) {
return syscall.Dup3(oldfd, newfd, flags)
}
// Fchmodat syscall.
func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
// Linux does not support passing flags to Fchmodat! From the man page:
// AT_SYMLINK_NOFOLLOW ... This flag is not currently implemented.
// Linux ignores any flags, but Go stdlib rejects them with EOPNOTSUPP starting
// with Go 1.11. See https://github.com/golang/go/issues/20130 for more info.
// TODO: Use fchmodat2 once available on Linux.
// Why would we ever want to call this without AT_SYMLINK_NOFOLLOW?
if flags&unix.AT_SYMLINK_NOFOLLOW == 0 {
tlog.Warn.Printf("Fchmodat: adding missing AT_SYMLINK_NOFOLLOW flag")
}
// FchmodatNofollow is like Fchmodat but never follows symlinks.
//
// This should be handled by the AT_SYMLINK_NOFOLLOW flag, but Linux
// does not implement it, so we have to perform an elaborate dance
// with O_PATH and /proc/self/fd.
//
// See also: Qemu implemented the same logic as fchmodat_nofollow():
// https://git.qemu.org/?p=qemu.git;a=blob;f=hw/9pfs/9p-local.c#l335
func FchmodatNofollow(dirfd int, path string, mode uint32) (err error) {
// Open handle to the filename (but without opening the actual file).
// This succeeds even when we don't have read permissions to the file.
fd, err := syscall.Openat(dirfd, path, syscall.O_NOFOLLOW|O_PATH, 0)
if err != nil {
return err
@ -134,22 +131,18 @@ func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
defer syscall.Close(fd)
// Now we can check the type without the risk of race-conditions.
// Return syscall.ELOOP if it is a symlink.
var st syscall.Stat_t
err = syscall.Fstat(fd, &st)
if err != nil {
return err
}
// Return syscall.ELOOP if path refers to a symlink.
var a fuse.Attr
a.FromStat(&st)
if a.IsSymlink() {
if st.Mode&syscall.S_IFMT == syscall.S_IFLNK {
return syscall.ELOOP
}
// Change mode of the actual file. Note that we can neither use
// Fchmodat (since fd is not necessarily a directory) nor Fchmod
// (since we are using O_PATH).
// Change mode of the actual file. Fchmod does not work with O_PATH,
// but Chmod via /proc/self/fd works.
procPath := fmt.Sprintf("/proc/self/fd/%d", fd)
return syscall.Chmod(procPath, mode)
}