kernel+libc: Add the O_TMPFILE open flag and the tmpfile() function

kernel/src/sys/open.cpp

@@ -28,6 +28,12 @@ Result<u64> sys_openat(Registers*, SyscallArgs args)
         if ((flags & O_WRONLY) || (flags & O_CREAT)) return err(EINVAL);
     }
 
+    if (flags & O_TMPFILE)
+    {
+        if (!(flags & O_WRONLY)) return err(EINVAL);
+        if (flags & O_CREAT) return err(EINVAL);
+    }
+
     int error;
     SharedPtr<VFS::Inode> parent_inode;
     bool ok = current->resolve_atfile(dirfd, path, false, &parent_inode).try_set_value_or_error(inode, error);
@@ -50,6 +56,14 @@ Result<u64> sys_openat(Registers*, SyscallArgs args)
         if ((flags & O_WRONLY) && !VFS::can_write(inode, current->auth)) return err(EACCES);
     }
 
+    if (flags & O_TMPFILE)
+    {
+        if (inode->type() != VFS::InodeType::Directory) return err(EINVAL);
+        inode = TRY(inode->fs().create_file_inode());
+        inode->chmod(mode);
+        inode->chown(current->auth.euid, current->auth.egid);
+    }
+
     if (inode->type() != VFS::InodeType::Directory && (flags & O_DIRECTORY)) return err(ENOTDIR);
 
     if ((flags & O_WRONLY) && (flags & O_TRUNC)) inode->truncate(0);

libc/include/bits/open-flags.h

@@ -13,6 +13,7 @@
 #define O_NONBLOCK 64
 #define O_CLOEXEC 128
 #define O_DIRECTORY 256
+#define O_TMPFILE 512
 
 #define O_ACCMODE O_RDWR
 

libc/include/stdio.h

@@ -91,7 +91,7 @@ extern "C"
     int getc(FILE* stream);
 
     /* Read a character from standard input. */
-    int getchar();
+    int getchar(void);
 
     /* Read a line from stream. */
     char* fgets(char* buf, size_t size, FILE* stream);
@@ -134,6 +134,9 @@ extern "C"
     /* Remove a file from the filesystem. */
     int remove(const char* path);
 
+    /* Create a unique temporary file. */
+    FILE* tmpfile(void);
+
 #ifdef __cplusplus
 }
 #endif

libc/src/stdio.cpp

@@ -341,6 +341,17 @@ extern "C"
         // On Luna, unlink() allows removal of directories.
         return unlink(path);
     }
+
+    FILE* tmpfile()
+    {
+        // FIXME: use /tmp as the directory when the tmpfs is mounted only there.
+        int fd = open("/", O_RDWR | O_TMPFILE, 0600);
+        if (fd < 0) return nullptr;
+
+        FILE* f = fdopen(fd, "w+b");
+        if (!f) close(fd);
+        return f;
+    }
 }
 
 void debug_log_impl(const char* format, va_list ap)