LibC: Avoid silent truncation after overlong realpath

The realpath syscall can attempt to return arbitrarily long paths, in particular
paths that are longer than PATH_MAX. The only way to detect this case is
checking whether 'rc', the true length of the returned path including NUL byte,
exceeds our buffer length. In such a case, the buffer contains invalid data.

All Serenity code calls LibC's realpath() with a nullptr buffer, meaning that
realpath is supposed to allocate memory on its own. All Serenity code can handle
arbitrarily long paths returned by LibC's realpath, so it is safe to "do the
dance" and repeat the syscall with a new buffer.

Ports are likely to be graceful in this regard, too. If a Port calls realpath()
with a pre-allocated buffer, however, there is nothing better we can do than
return a truncated buffer.

Userland/Libraries/LibC/stdlib.cpp

@@ -1051,6 +1051,8 @@ char* realpath(const char* pathname, char* buffer)
     size_t size = PATH_MAX;
     bool self_allocated = false;
     if (buffer == nullptr) {
+        // Since we self-allocate, try to sneakily use a smaller buffer instead, in an attempt to use less memory.
+        size = 64;
         buffer = (char*)malloc(size);
         self_allocated = true;
     }
@@ -1062,6 +1064,29 @@ char* realpath(const char* pathname, char* buffer)
         errno = -rc;
         return nullptr;
     }
+    if (self_allocated && static_cast<size_t>(rc) > size) {
+        // There was silent truncation, *and* we can simply retry without the caller noticing.
+        free(buffer);
+        size = static_cast<size_t>(rc);
+        buffer = (char*)malloc(size);
+        params.buffer = { buffer, size };
+        rc = syscall(SC_realpath, &params);
+        if (rc < 0) {
+            // Can only happen if we lose a race. Let's pretend we lost the race in the first place.
+            free(buffer);
+            errno = -rc;
+            return nullptr;
+        }
+        size_t new_size = static_cast<size_t>(rc);
+        if (new_size < size) {
+            // If we're here, the symlink has become longer while we were looking at it.
+            // There's not much we can do, unless we want to loop endlessly
+            // in this case. Let's leave it up to the caller whether to loop.
+            free(buffer);
+            errno = EAGAIN;
+            return nullptr;
+        }
+    }
     errno = 0;
     return buffer;
 }