syscallcompat: rename thread_credentials files to include "linux"

As with the other files, include "linux" because the code only
builds on linux

renamed:    internal/syscallcompat/thread_credentials.go -> internal/syscallcompat/thread_credentials_linux.go
renamed:    internal/syscallcompat/thread_credentials_368_arm.go -> internal/syscallcompat/thread_credentials_linux_368_arm.go
renamed:    internal/syscallcompat/thread_credentials_other.go -> internal/syscallcompat/thread_credentials_linux_other.go

1 files changed, 60 insertions, 0 deletions

diff --git a/internal/syscallcompat/thread_credentials_linux.go b/internal/syscallcompat/thread_credentials_linux.go
new file mode 100644
index 0000000..8cfb703
--- /dev/null
+++ b/internal/syscallcompat/thread_credentials_linux.go
@@ -0,0 +1,60 @@
+//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
+
+import (
+	"log"
+)
+
+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])
+}
+
+// SetgroupsPanic calls Setgroups and panics on error
+func SetgroupsPanic(gids []int) {
+	err := Setgroups(gids)
+	if err != nil {
+		log.Panic(err)
+	}
+}
+
+// SetregidPanic calls Setregid and panics on error
+func SetregidPanic(rgid int, egid int) {
+	err := Setregid(rgid, egid)
+	if err != nil {
+		log.Panic(err)
+	}
+}
+
+// SetreuidPanic calls Setreuid and panics on error
+func SetreuidPanic(ruid int, euid int) {
+	err := Setreuid(ruid, euid)
+	if err != nil {
+		log.Panic(err)
+	}
+}