1 files changed, 80 insertions, 0 deletions
diff --git a/internal/syscallcompat/asuser_linux.go b/internal/syscallcompat/asuser_linux.go
new file mode 100644
index 0000000..804a898
--- /dev/null
+++ b/internal/syscallcompat/asuser_linux.go
@@ -0,0 +1,80 @@
+package syscallcompat
+
+import (
+	"fmt"
+	"io/ioutil"
+	"runtime"
+	"strconv"
+	"strings"
+
+	"github.com/hanwen/go-fuse/v2/fuse"
+)
+
+// asUser runs `f()` under the effective uid, gid, groups specified
+// in `context`.
+//
+// If `context` is nil, `f()` is executed directly without switching user id.
+func asUser(f func() (int, error), context *fuse.Context) (int, error) {
+	if context == nil {
+		return f()
+	}
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	// Since go1.16beta1 (commit d1b1145cace8b968307f9311ff611e4bb810710c ,
+	// https://go-review.googlesource.com/c/go/+/210639 )
+	// syscall.{Setgroups,Setregid,Setreuid} affects all threads, which
+	// is exactly what we not want.
+	//
+	// And unix.{Setgroups,Setregid,Setreuid} also changed to this behavoir in
+	// v0.1.0 (commit d0df966e6959f00dc1c74363e537872647352d51 ,
+	// https://go-review.googlesource.com/c/sys/+/428174 ), so we use
+	// our own syscall wrappers.
+
+	err := Setgroups(getSupplementaryGroups(context.Pid))
+	if err != nil {
+		return -1, err
+	}
+	defer SetgroupsPanic(nil)
+
+	err = Setregid(-1, int(context.Owner.Gid))
+	if err != nil {
+		return -1, err
+	}
+	defer SetregidPanic(-1, 0)
+
+	err = Setreuid(-1, int(context.Owner.Uid))
+	if err != nil {
+		return -1, err
+	}
+	defer SetreuidPanic(-1, 0)
+
+	return f()
+}
+
+func getSupplementaryGroups(pid uint32) (gids []int) {
+	procPath := fmt.Sprintf("/proc/%d/task/%d/status", pid, pid)
+	blob, err := ioutil.ReadFile(procPath)
+	if err != nil {
+		return nil
+	}
+
+	lines := strings.Split(string(blob), "\n")
+	for _, line := range lines {
+		if strings.HasPrefix(line, "Groups:") {
+			f := strings.Fields(line[7:])
+			gids = make([]int, len(f))
+			for i := range gids {
+				val, err := strconv.ParseInt(f[i], 10, 32)
+				if err != nil {
+					return nil
+				}
+				gids[i] = int(val)
+			}
+			return gids
+		}
+	}
+
+	return nil
+}