diff --git a/libc/src/stdio.cpp b/libc/src/stdio.cpp
index bc5cc32e..42c2b1c5 100644
--- a/libc/src/stdio.cpp
+++ b/libc/src/stdio.cpp
@@ -29,6 +29,22 @@ static int fopen_parse_mode(const char* mode)
     return result;
 }
 
+static int fdopen_check_compatible_mode(int fd, int new_flags)
+{
+    int old_flags = fcntl(fd, F_GETFL);
+    if (old_flags < 0) return -1;
+
+    int old_mode = old_flags & O_ACCMODE;
+    int new_mode = new_flags & O_ACCMODE;
+    if ((old_mode & new_mode) != new_mode)
+    {
+        errno = EINVAL;
+        return -1;
+    }
+
+    return 0;
+}
+
 extern "C"
 {
     FILE* fopen(const char* path, const char* mode)
@@ -59,9 +75,7 @@ extern "C"
 
         if ((flags = fopen_parse_mode(mode)) < 0) return nullptr;
 
-        // FIXME: We do verify that fd is valid, but not that the mode is compatible.
-        long rc = lseek(fd, 0, SEEK_CUR);
-        if (rc < 0) return nullptr;
+        if (fdopen_check_compatible_mode(fd, flags) < 0) return nullptr;
 
         FILE* f = (FILE*)malloc(sizeof(FILE));
         if (!f) { return nullptr; }