syscallcompat: add thread_credentials.go & friends

Private copies of per-thread Setreuid/Setegid/Setgroups.

https://github.com/rfjakob/gocryptfs/issues/893
https://github.com/rfjakob/gocryptfs/issues/892

1 files changed, 32 insertions, 0 deletions

diff --git a/internal/syscallcompat/thread_credentials.go b/internal/syscallcompat/thread_credentials.go
new file mode 100644
index 0000000..3ef5233
--- /dev/null
+++ b/internal/syscallcompat/thread_credentials.go
@@ -0,0 +1,32 @@
+//go:build linux
+
+// golang.org/x/sys/unix commit
+// https://github.com/golang/sys/commit/d0df966e6959f00dc1c74363e537872647352d51
+// changed unix.Setreuid/unix.Setregid functions to affect the whole thread, which is
+// what gocryptfs does NOT want (https://github.com/rfjakob/gocryptfs/issues/893).
+// The functions Setreuid/Setegid are copy-pasted from one commit before
+// (9e1f76180b77a12eb07c82eb8e1ea8a7f8d202e7).
+//
+// Looking at the diff at https://github.com/golang/sys/commit/d0df966e6959f00dc1c74363e537872647352d51
+// we see that only two architectures, 386 and arm, use SYS_SETREUID32/SYS_SETREGID32
+// (see "man 2 setreuid" for why).
+// All the others architectures use SYS_SETREUID/SYS_SETREGID.
+//
+// As of golang.org/x/sys/unix v0.30.0, Setgroups/setgroups is still per-thread, but
+// it is likely that this will change, too. Setgroups/setgroups are copy-pasted from
+// v0.30.0. The SYS_SETGROUPS32/SYS_SETGROUPS split is the same as for Setreuid.
+//
+// Note: _Gid_t is always uint32 on linux, so we can directly use uint32 for setgroups.
+package syscallcompat
+
+func Setgroups(gids []int) (err error) {
+	if len(gids) == 0 {
+		return setgroups(0, nil)
+	}
+
+	a := make([]uint32, len(gids))
+	for i, v := range gids {
+		a[i] = uint32(v)
+	}
+	return setgroups(len(a), &a[0])
+}