syscallcompat: fix build contraints on thread_credentials_linux_368_arm.go

This file was only compiled for arm because
(  https://pkg.go.dev/cmd/go#hdr-Build_constraints ):

	If a file's
	name, after stripping the extension and a possible _test suffix,
	matches any of the following patterns:

	*_GOOS
	*_GOARCH
	*_GOOS_GOARCH

	(example: source_windows_amd64.go) where GOOS and GOARCH
	represent any known operating system and architecture values
	respectively, then the file is considered to have an implicit
	build constraint requiring those terms (in addition to any
	explicit constraints in the file).

Error was:

	+ GOOS=linux
	+ GOARCH=386
	+ build
	+ go build -tags without_openssl -o /dev/null
	# github.com/rfjakob/gocryptfs/v2/internal/syscallcompat
	Error: internal/syscallcompat/asuser_linux.go:41:8: undefined: Setregid
	Error: internal/syscallcompat/asuser_linux.go:47:8: undefined: Setreuid
	Error: internal/syscallcompat/thread_credentials_linux.go:29:10: undefined: setgroups
	Error: internal/syscallcompat/thread_credentials_linux.go:36:9: undefined: setgroups
	Error: internal/syscallcompat/thread_credentials_linux.go:49:9: undefined: Setregid
	Error: internal/syscallcompat/thread_credentials_linux.go:57:9: undefined: Setreuid

Rename the file to fix the problem. And add a comment about why this file exists.

Fixes https://github.com/rfjakob/gocryptfs/issues/907

1 files changed, 40 insertions, 0 deletions

diff --git a/internal/syscallcompat/thread_credentials_linux_32.go b/internal/syscallcompat/thread_credentials_linux_32.go
new file mode 100644
index 0000000..69fffca
--- /dev/null
+++ b/internal/syscallcompat/thread_credentials_linux_32.go
@@ -0,0 +1,40 @@
+//go:build (linux && 386) || (linux && arm)
+
+// Linux on i386 and 32-bit ARM has SYS_SETREUID and friends returning 16-bit values.
+// We need to use SYS_SETREUID32 instead.
+
+package syscallcompat
+
+import (
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// See thread_credentials_linux.go for docs
+
+// Setreuid is like setreuid(2) but affects only the current thread
+func Setreuid(ruid int, euid int) (err error) {
+	_, _, e1 := unix.RawSyscall(unix.SYS_SETREUID32, uintptr(ruid), uintptr(euid), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// Setreuid is like setregid(2) but affects only the current thread
+func Setregid(rgid int, egid int) (err error) {
+	_, _, e1 := unix.RawSyscall(unix.SYS_SETREGID32, uintptr(rgid), uintptr(egid), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+func setgroups(n int, list *uint32) (err error) {
+	_, _, e1 := unix.RawSyscall(unix.SYS_SETGROUPS32, uintptr(n), uintptr(unsafe.Pointer(list)), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}