fs,permission: make handling of buffers consistent

Commit 2000c26 added explicit handling
of Buffers to fs.symlink, but not to fs.symlinkSync or
fs.promises.symlink. This change adapts the latter two functions to
behave like fs.symlink.

Refs: nodejs#49156
Refs: nodejs#51212

lib/fs.js

@@ -1793,7 +1793,11 @@ function symlinkSync(target, path, type) {
   if (permission.isEnabled()) {
     // The permission model's security guarantees fall apart in the presence of
     // relative symbolic links. Thus, we have to prevent their creation.
-    if (typeof target !== 'string' || !isAbsolute(toPathIfFileURL(target))) {
+    if (BufferIsBuffer(target)) {
+      if (!isAbsolute(BufferToString(target))) {
+        throw new ERR_ACCESS_DENIED('relative symbolic link target');
+      }
+    } else if (typeof target !== 'string' || !isAbsolute(toPathIfFileURL(target))) {
       throw new ERR_ACCESS_DENIED('relative symbolic link target');
     }
   }

lib/internal/fs/promises.js

@@ -17,6 +17,7 @@ const {
   SymbolAsyncDispose,
   Uint8Array,
   FunctionPrototypeBind,
+  uncurryThis,
 } = primordials;
 
 const { fs: constants } = internalBinding('constants');
@@ -30,6 +31,8 @@ const {
 
 const binding = internalBinding('fs');
 const { Buffer } = require('buffer');
+const { isBuffer: BufferIsBuffer } = Buffer;
+const BufferToString = uncurryThis(Buffer.prototype.toString);
 
 const {
   codes: {
@@ -985,7 +988,11 @@ async function symlink(target, path, type_) {
   if (permission.isEnabled()) {
     // The permission model's security guarantees fall apart in the presence of
     // relative symbolic links. Thus, we have to prevent their creation.
-    if (typeof target !== 'string' || !isAbsolute(toPathIfFileURL(target))) {
+    if (BufferIsBuffer(target)) {
+      if (!isAbsolute(BufferToString(target))) {
+        throw new ERR_ACCESS_DENIED('relative symbolic link target');
+      }
+    } else if (typeof target !== 'string' || !isAbsolute(toPathIfFileURL(target))) {
       throw new ERR_ACCESS_DENIED('relative symbolic link target');
     }
   }