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Diffstat (limited to 'internal/syscallcompat/sys_linux.go')
-rw-r--r--internal/syscallcompat/sys_linux.go35
1 files changed, 14 insertions, 21 deletions
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index e662001..625e136 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -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)
}