Re: [PATCH] arc4random: fix getrandom fallback to /dev/urandom

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 03/04/24 13:55, Anthony Iliopoulos wrote:
> arc4random_buf relies on the errno of getrandom_nocancel to fallback to
> /dev/urandom, but getrandom_nocancel returns a status code instead of
> the syscall errno (-ENOSYS) so it breaks the expectation and thus the
> fallback in cases where a kernel does not support the getrandom syscall.
> 
> Commit 609c9d0951da ("malloc: Do not clobber errno on __getrandom_nocancel
> (BZ #29624)") changed __getrandom_nocancel from INLINE_SYSCALL_CALL to
> INTERNAL_SYSCALL_CALL and modified arc4random_buf to rely on the return
> status instead of errno.
> 
> Commit 5a85786a9005 ("Make __getrandom_nocancel set errno and add a
> _nostatus version") changed __getrandom_nocancel back to
> INLINE_SYSCALL_CALL and added a __getrandom_nocancel_nostatus variant
> that calls via INTERNAL_SYSCALL_CALL, but this broke the fallback of
> arc4random on kernels where the getrandom syscall is not available.
> 
> Fix it by calling __getrandom_nocancel_nostatus from arc4random_buf so
> that the fallback works again.
> 
> Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
> Fixes: 5a85786a9005 ("Make __getrandom_nocancel set errno and add a _nostatus version")

It is a user-visible change, could you open a bug report so we can backport it
2.39? The patch looks good to me.

> ---
>  stdlib/arc4random.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/stdlib/arc4random.c b/stdlib/arc4random.c
> index 3ae8fc130234..9b6a6ee85150 100644
> --- a/stdlib/arc4random.c
> +++ b/stdlib/arc4random.c
> @@ -42,7 +42,7 @@ __arc4random_buf (void *p, size_t n)
>  
>    for (;;)
>      {
> -      l = TEMP_FAILURE_RETRY (__getrandom_nocancel (p, n, 0));
> +      l = TEMP_FAILURE_RETRY (__getrandom_nocancel_nostatus (p, n, 0));
>        if (l > 0)
>  	{
>  	  if ((size_t) l == n)