libc: Check for a compatible mode in fdopen()

kernel/src/sys/file.cpp

@@ -113,6 +113,25 @@ Result<u64> sys_fcntl(Registers*, SyscallArgs args)
 
         return (u64)new_fd;
     }
+    case F_GETFD: return (u64) !!(descriptor.flags & O_CLOEXEC);
+    case F_SETFD: {
+        int arg = (int)args[2];
+        if (arg == FD_CLOEXEC) descriptor.flags |= O_CLOEXEC;
+        else
+            descriptor.flags &= ~O_CLOEXEC;
+        return 0;
+    }
+    case F_GETFL: return (u64)(descriptor.flags & ~O_CLOEXEC);
+    case F_SETFL: {
+        int arg = (int)args[2];
+
+        descriptor.flags &= ~(O_APPEND | O_NONBLOCK);
+        arg &= (O_APPEND | O_NONBLOCK);
+
+        descriptor.flags |= arg;
+
+        return 0;
+    }
     default: return err(EINVAL);
     }
 }

libc/include/bits/fcntl.h

@@ -5,5 +5,11 @@
 
 #define F_DUPFD 0
 #define F_DUPFD_CLOEXEC 1
+#define F_GETFD 2
+#define F_SETFD 3
+#define F_SETFL 4
+#define F_GETFL 5
+
+#define FD_CLOEXEC 1
 
 #endif

libc/include/bits/open-flags.h

@@ -13,4 +13,6 @@
 #define O_NONBLOCK 64
 #define O_CLOEXEC 128
 
+#define O_ACCMODE O_RDWR
+
 #endif

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; }