unix: add namespaced versions of Listxattr/Flistxattr/Llistxattr on *BSD

Current extended attributes API mixes together attributes from different
namespaces without a way to separate them. Add versions of *listxattr
functions that return attributes only from the specified namespace.

For golang/go#54357

Change-Id: I605d677dd2f2a17541bc02f3146a7509c69269c9
Reviewed-on: https://go-review.googlesource.com/c/sys/+/430215
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Dmitri Goutnik <dgoutnik@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>

Author: dmgk

unix/xattr_bsd.go

@@ -160,13 +160,12 @@ func Lremovexattr(link string, attr string) (err error) {
 }
 
 func Listxattr(file string, dest []byte) (sz int, err error) {
-	d := initxattrdest(dest, 0)
 	destsiz := len(dest)
 
 	// FreeBSD won't allow you to list xattrs from multiple namespaces
-	s := 0
+	s, pos := 0, 0
 	for _, nsid := range [...]int{EXTATTR_NAMESPACE_USER, EXTATTR_NAMESPACE_SYSTEM} {
-		stmp, e := ExtattrListFile(file, nsid, uintptr(d), destsiz)
+		stmp, e := ListxattrNS(file, nsid, dest[pos:])
 
 		/* Errors accessing system attrs are ignored so that
 		 * we can implement the Linux-like behavior of omitting errors that
@@ -175,66 +174,102 @@ func Listxattr(file string, dest []byte) (sz int, err error) {
 		 * Linux will still error if we ask for user attributes on a file that
 		 * we don't have read permissions on, so don't ignore those errors
 		 */
-		if e != nil && e == EPERM && nsid != EXTATTR_NAMESPACE_USER {
-			continue
-		} else if e != nil {
+		if e != nil {
+			if e == EPERM && nsid != EXTATTR_NAMESPACE_USER {
+				continue
+			}
 			return s, e
 		}
 
 		s += stmp
-		destsiz -= s
-		if destsiz < 0 {
-			destsiz = 0
+		pos = s
+		if pos > destsiz {
+			pos = destsiz
 		}
-		d = initxattrdest(dest, s)
 	}
 
 	return s, nil
 }
 
-func Flistxattr(fd int, dest []byte) (sz int, err error) {
+func ListxattrNS(file string, nsid int, dest []byte) (sz int, err error) {
 	d := initxattrdest(dest, 0)
 	destsiz := len(dest)
 
-	s := 0
+	s, e := ExtattrListFile(file, nsid, uintptr(d), destsiz)
+	if e != nil {
+		return 0, err
+	}
+
+	return s, nil
+}
+
+func Flistxattr(fd int, dest []byte) (sz int, err error) {
+	destsiz := len(dest)
+
+	s, pos := 0, 0
 	for _, nsid := range [...]int{EXTATTR_NAMESPACE_USER, EXTATTR_NAMESPACE_SYSTEM} {
-		stmp, e := ExtattrListFd(fd, nsid, uintptr(d), destsiz)
-		if e != nil && e == EPERM && nsid != EXTATTR_NAMESPACE_USER {
-			continue
-		} else if e != nil {
+		stmp, e := FlistxattrNS(fd, nsid, dest[pos:])
+
+		if e != nil {
+			if e == EPERM && nsid != EXTATTR_NAMESPACE_USER {
+				continue
+			}
 			return s, e
 		}
 
 		s += stmp
-		destsiz -= s
-		if destsiz < 0 {
-			destsiz = 0
+		pos = s
+		if pos > destsiz {
+			pos = destsiz
 		}
-		d = initxattrdest(dest, s)
 	}
 
 	return s, nil
 }
 
-func Llistxattr(link string, dest []byte) (sz int, err error) {
+func FlistxattrNS(fd int, nsid int, dest []byte) (sz int, err error) {
 	d := initxattrdest(dest, 0)
 	destsiz := len(dest)
 
-	s := 0
+	s, e := ExtattrListFd(fd, nsid, uintptr(d), destsiz)
+	if e != nil {
+		return 0, err
+	}
+
+	return s, nil
+}
+
+func Llistxattr(link string, dest []byte) (sz int, err error) {
+	destsiz := len(dest)
+
+	s, pos := 0, 0
 	for _, nsid := range [...]int{EXTATTR_NAMESPACE_USER, EXTATTR_NAMESPACE_SYSTEM} {
-		stmp, e := ExtattrListLink(link, nsid, uintptr(d), destsiz)
-		if e != nil && e == EPERM && nsid != EXTATTR_NAMESPACE_USER {
-			continue
-		} else if e != nil {
+		stmp, e := LlistxattrNS(link, nsid, dest[pos:])
+
+		if e != nil {
+			if e == EPERM && nsid != EXTATTR_NAMESPACE_USER {
+				continue
+			}
 			return s, e
 		}
 
 		s += stmp
-		destsiz -= s
-		if destsiz < 0 {
-			destsiz = 0
+		pos = s
+		if pos > destsiz {
+			pos = destsiz
 		}
-		d = initxattrdest(dest, s)
+	}
+
+	return s, nil
+}
+
+func LlistxattrNS(link string, nsid int, dest []byte) (sz int, err error) {
+	d := initxattrdest(dest, 0)
+	destsiz := len(dest)
+
+	s, e := ExtattrListLink(link, nsid, uintptr(d), destsiz)
+	if e != nil {
+		return 0, err
 	}
 
 	return s, nil