exec: only build read_code when needed

Only build read_code when binary formats that use it are built into the
kernel.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Link: http://lkml.kernel.org/r/20200515143646.3857579-26-hch@lst.deSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1053,6 +1053,8 @@ int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, loff_t max_size,
 }
 EXPORT_SYMBOL_GPL(kernel_read_file_from_fd);
 
+#if defined(CONFIG_HAVE_AOUT) || defined(CONFIG_BINFMT_FLAT) || \
+    defined(CONFIG_BINFMT_ELF_FDPIC)
 ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
 {
 	ssize_t res = vfs_read(file, (void __user *)addr, len, &pos);
@@ -1061,6 +1063,7 @@ ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
 	return res;
 }
 EXPORT_SYMBOL(read_code);
+#endif
 
 /*
  * Maps the mm_struct mm into the current task struct.