Added basic support for FreeBSD.

Freebsd-support: Change bash shebang to use /usr/bin/env

Freebsd-support: Fix go vet "undefined" fixes when running make ci

freebsd: stub xattr functions

/proc/PID/fd does not exist on freebsd.

freebsd-support: modify FchmodatNofollow for FreeBSD

FreeBSD supports the Fchmodat system call, with the AT_SYMLINK_NOFOLLOW
flag. FchmodatNofollow has been modified to use this system call and
flag.

freebsd-support: PR changes and fixes

* Functions in fusefrontend_reverse/node_xattr_freebsd.go have been
  stubbed for now.

* asuser_freebsd.go updated to only run f() when context is nil;
  otherwise log a warning and return an error.

* emulate.go build flags updated, and FreeBSD specific version added.

* sys_freebsd.go bug in Renameat2 with RENAME_EXCHANGE flag fixed.
  FreeBSD does not support atomic file swapping, so this flag now
  returns an error.

* unix2syscall and atime is identical between FreeBSD and Darwin,
  updated filenames so Go will build the file for FreeBSD and Mac OS.

freebsd-support: Addressed more PR comments and fixed build tags

1 files changed, 24 insertions, 0 deletions

diff --git a/internal/syscallcompat/asuser_freebsd.go b/internal/syscallcompat/asuser_freebsd.go
new file mode 100644
index 0000000..dfa8e18
--- /dev/null
+++ b/internal/syscallcompat/asuser_freebsd.go
@@ -0,0 +1,24 @@
+package syscallcompat
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/hanwen/go-fuse/v2/fuse"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/tlog"
+)
+
+// asUser runs `f()` under the effective uid, gid, groups specified
+// in `context`.
+//
+// If `context` is nil, `f()` is executed directly without switching user id.
+//
+// FreeBSD does not support changing uid/gid per thread. If context is not nil,
+// an error is returned.
+func asUser(f func() (int, error), context *fuse.Context) (int, error) {
+	if context == nil {
+		return f()
+	}
+	tlog.Warn.Printf("asUser: error, only nil context is supported\n")
+	return 0, unix.EOPNOTSUPP
+}