Re: [patch][version 6] add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

[Kees Cook <keescook@chromium.org>]

On Tue, Jul 27, 2021 at 03:26:00AM +0000, Qing Zhao wrote:
> This is the 6th version of the patch for the new security feature for GCC.
> 
> I have tested it with bootstrap on both x86 and aarch64, regression testing on both x86 and aarch64.
> Also compile CPU2017 (running is ongoing), without any issue. (With the fix to bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101586).
> 
> Please take a look and let me know any issue.

Good news, this passes all my initialization tests in the kernel. Yay! :)

However, I see an unexpected side-effect from some static initializations:

net/core/sock.c: In function 'sock_no_sendpage':
net/core/sock.c:2849:23: warning: 'msg' is used uninitialized [-Wuninitialized]
 2849 |         struct msghdr msg = {.msg_flags = flags};
      |                       ^~~           

It seems like -Wuninitialized has suddenly stopped noticing explicit
static initializers when there are bit fields in the struct. Here's a
minimized case:

$ cat init.c
struct weird {
        int bit : 1;
        int val;
};

int func(int val)
{
        struct weird obj = { .val = val };
        return obj.val;
}

$ gcc -c -o init.o -Wall -O2 -ftrivial-auto-var-init=zero init.c
init.c: In function ‘func’:
init.c:8:22: warning: ‘obj’ is used uninitialized [-Wuninitialized]
    8 |         struct weird obj = { .val = val };
      |                      ^~~
init.c:8:22: note: ‘obj’ declared here
    8 |         struct weird obj = { .val = val };
      |                      ^~~



-- 
Kees Cook