mkdir: do not error out with -EEXIST for rancing MkdirAlls

If two programs are doing MkdirAll, the previous logic would return an
error if a directory already existed once we got into the "mkdir"
portion of the creation.

Since we already have to accept that an attacker can swap the inode with
a different directory, returning -EEXIST from mkdirat(2) just causes
spurrious errors. All we care about is that we open a directory.

Signed-off-by: Aleksa Sarai <[email protected]>

CHANGELOG.md

@@ -5,6 +5,11 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased] ##
+### Fixed ###
+- `MkdirAll` will now no longer return an `EEXIST` error if two racing
+  processes are creating the same directory. We will still verify that the path
+  is a directory, but this will avoid spurious errors when multiple threads or
+  programs are trying to `MkdirAll` the same path. opencontainers/runc#4543
 
 ## [0.3.4] - 2024-10-09 ##
 

mkdir_linux.go

@@ -119,7 +119,12 @@ func MkdirAllHandle(root *os.File, unsafePath string, mode int) (_ *os.File, Err
 		// NOTE: mkdir(2) will not follow trailing symlinks, so we can safely
 		// create the final component without worrying about symlink-exchange
 		// attacks.
-		if err := unix.Mkdirat(int(currentDir.Fd()), part, uint32(mode)); err != nil {
+		//
+		// If we get -EEXIST, it's possible that another program created the
+		// directory at the same time as us. In that case, just continue on as
+		// if we created it (if the created inode is not a directory, the
+		// following open call will fail).
+		if err := unix.Mkdirat(int(currentDir.Fd()), part, uint32(mode)); err != nil && !errors.Is(err, unix.EEXIST) {
 			err = &os.PathError{Op: "mkdirat", Path: currentDir.Name() + "/" + part, Err: err}
 			// Make the error a bit nicer if the directory is dead.
 			if err2 := isDeadInode(currentDir); err2 != nil {

mkdir_linux_test.go

@@ -149,14 +149,14 @@ func testMkdirAll_Basic(t *testing.T, mkdirAll mkdirAllFunc) {
 			"nondir-symlink-dotdot":   {unsafePath: "b-file/../d", expectedErr: unix.ENOTDIR},
 			"nondir-symlink-subdir":   {unsafePath: "b-file/subdir", expectedErr: unix.ENOTDIR},
 			// Dangling symlinks are not followed.
-			"dangling1-trailing": {unsafePath: "a-fake1", expectedErr: unix.EEXIST},
-			"dangling1-basic":    {unsafePath: "a-fake1/foo", expectedErr: unix.EEXIST},
+			"dangling1-trailing": {unsafePath: "a-fake1", expectedErr: unix.ENOTDIR},
+			"dangling1-basic":    {unsafePath: "a-fake1/foo", expectedErr: unix.ENOTDIR},
 			"dangling1-dotdot":   {unsafePath: "a-fake1/../bar/baz", expectedErr: unix.ENOENT},
-			"dangling2-trailing": {unsafePath: "a-fake2", expectedErr: unix.EEXIST},
-			"dangling2-basic":    {unsafePath: "a-fake2/foo", expectedErr: unix.EEXIST},
+			"dangling2-trailing": {unsafePath: "a-fake2", expectedErr: unix.ENOTDIR},
+			"dangling2-basic":    {unsafePath: "a-fake2/foo", expectedErr: unix.ENOTDIR},
 			"dangling2-dotdot":   {unsafePath: "a-fake2/../bar/baz", expectedErr: unix.ENOENT},
-			"dangling3-trailing": {unsafePath: "a-fake3", expectedErr: unix.EEXIST},
-			"dangling3-basic":    {unsafePath: "a-fake3/foo", expectedErr: unix.EEXIST},
+			"dangling3-trailing": {unsafePath: "a-fake3", expectedErr: unix.ENOTDIR},
+			"dangling3-basic":    {unsafePath: "a-fake3/foo", expectedErr: unix.ENOTDIR},
 			"dangling3-dotdot":   {unsafePath: "a-fake3/../bar/baz", expectedErr: unix.ENOENT},
 			// Non-lexical symlinks should work.
 			"nonlexical-basic":           {unsafePath: "target/foo"},
@@ -171,11 +171,11 @@ func testMkdirAll_Basic(t *testing.T, mkdirAll mkdirAllFunc) {
 			"nonlexical-level3-abs":      {unsafePath: "link3/target_abs/foo"},
 			"nonlexical-level3-rel":      {unsafePath: "link3/target_rel/foo"},
 			// But really tricky dangling symlinks should fail.
-			"dangling-tricky1-trailing": {unsafePath: "link3/deep_dangling1", expectedErr: unix.EEXIST},
-			"dangling-tricky1-basic":    {unsafePath: "link3/deep_dangling1/foo", expectedErr: unix.EEXIST},
+			"dangling-tricky1-trailing": {unsafePath: "link3/deep_dangling1", expectedErr: unix.ENOTDIR},
+			"dangling-tricky1-basic":    {unsafePath: "link3/deep_dangling1/foo", expectedErr: unix.ENOTDIR},
 			"dangling-tricky1-dotdot":   {unsafePath: "link3/deep_dangling1/../bar", expectedErr: unix.ENOENT},
-			"dangling-tricky2-trailing": {unsafePath: "link3/deep_dangling2", expectedErr: unix.EEXIST},
-			"dangling-tricky2-basic":    {unsafePath: "link3/deep_dangling2/foo", expectedErr: unix.EEXIST},
+			"dangling-tricky2-trailing": {unsafePath: "link3/deep_dangling2", expectedErr: unix.ENOTDIR},
+			"dangling-tricky2-basic":    {unsafePath: "link3/deep_dangling2/foo", expectedErr: unix.ENOTDIR},
 			"dangling-tricky2-dotdot":   {unsafePath: "link3/deep_dangling2/../bar", expectedErr: unix.ENOENT},
 			// And trying to mkdir inside a loop should fail.
 			"loop-trailing": {unsafePath: "loop/link", expectedErr: unix.ELOOP},
@@ -348,7 +348,7 @@ func TestMkdirAllHandle_RacingRename(t *testing.T) {
 			{"trailing", "target/a/b/c/d/e", "swapdir-nonempty1", "target/a/b/c/d/e", nil},
 			{"partial", "target/a/b/c/d/e", "swapdir-nonempty1", "target/a/b/c/d/e/f/g/h/i/j/k", nil},
 			{"trailing", "target/a/b/c/d/e", "swapdir-nonempty2", "target/a/b/c/d/e", nil},
-			{"partial", "target/a/b/c/d/e", "swapdir-nonempty2", "target/a/b/c/d/e/f/g/h/i/j/k", []error{unix.EEXIST}},
+			{"partial", "target/a/b/c/d/e", "swapdir-nonempty2", "target/a/b/c/d/e/f/g/h/i/j/k", []error{unix.ENOTDIR}},
 			{"trailing", "target/a/b/c/d/e", "swapfile", "target/a/b/c/d/e", []error{unix.ENOTDIR}},
 			{"partial", "target/a/b/c/d/e", "swapfile", "target/a/b/c/d/e/f/g/h/i/j/k", []error{unix.ENOTDIR}},
 		}