Re: [PATCH] tools/nolibc: stackprotector: Avoid stalling program startup if crng is not init yet

[Willy Tarreau <w@1wt.eu>]

Hi Daniel!

On Fri, May 22, 2026 at 06:07:26PM +0900, Daniel Palmer wrote:
> We are using the getrandom syscall to get a random seed for the
> stack protector canary but we are calling it with no flags which means
> it'll block until there is some real randomness to return.
> 
> This means that if the crng is not ready yet program startup will
> block and if you are unlucky that could be for a long time and
> look like the program has crashed.
> 
> There is a comment in the code about mixing in the pid to make
> sure the canary isn't 0 even if getrandom fails so it seems ok
> to pass the non-blocking and insecure flags so it doesn't block
> and potentially return something even if the crng is not init
> yet.
> 
> Fixes: 7188d4637e95 ("tools/nolibc: add support for stack protector")
> Signed-off-by: Daniel Palmer <daniel@thingy.jp>

Acked-by: Willy Tarreau <w@1wt.eu>

> ---
> 
> The insecure flag is apparently from 5.6, I think Willy said before
> we are trying to keep nolibc working on the oldest LTS kernel.
> That seems to be 5.10 so I think its ok?

Sounds reasonable. We could also condition the flag to its existence
if it causes issues.

> Anyhow, I switched compilers for my nommu target and everything
> stopped working, tracked it down to this. my other compiler must
> have not supported the stack protector.

Possible, indeed.

Thanks!
Willy

> 
>  tools/include/nolibc/stackprotector.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/include/nolibc/stackprotector.h b/tools/include/nolibc/stackprotector.h
> index e11c20c75465..916a92062ba0 100644
> --- a/tools/include/nolibc/stackprotector.h
> +++ b/tools/include/nolibc/stackprotector.h
> @@ -42,7 +42,8 @@ uintptr_t __stack_chk_guard;
>  
>  static __nolibc_no_stack_protector void __stack_chk_init(void)
>  {
> -	__nolibc_syscall3(__NR_getrandom, &__stack_chk_guard, sizeof(__stack_chk_guard), 0);
> +	__nolibc_syscall3(__NR_getrandom, &__stack_chk_guard, sizeof(__stack_chk_guard),
> +			  GRND_INSECURE | GRND_NONBLOCK);
>  	/* a bit more randomness in case getrandom() fails, ensure the guard is never 0 */
>  	if (__stack_chk_guard != (uintptr_t) &__stack_chk_guard)
>  		__stack_chk_guard ^= (uintptr_t) &__stack_chk_guard;
> -- 
> 2.53.0
>