Re: [PATCH] libs/light: make it build without setresuid()

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Manuel Bouyer writes ("Re: [PATCH] libs/light: make it build without 
setresuid()"):
> On Wed, Jan 20, 2021 at 05:10:36PM +0000, Ian Jackson wrote:
> > My last mail had in it a thing that claims to be a proof that this is
> > not possible.
> 
> This code:
>         if (setreuid(375,0) < 0) {
>                 err(1, "setreuid");
>         }
>         if (seteuid(374) < 0) {
>                 err(1, "seteuid");
>         }
>         if (kill(-1, 9)) {
>                 err(1, "kill");
>         }
>         printf("kill done\n");
>         if (seteuid(0) < 0) {
>                 err(1, "setreuid2");
>         }
>         exit(0);
> 
> actually works on NetBSD. processes from 375 are killed, and the
> seteuid(0) call succeeds (showing that the saved used id is still 0).

I guess I must have been wrong.

> > What do you think ?
> 
> As this is supported by Xen, I hope I can make at last run qemu with a
> non-zero uid.

The logic for deciding what user to run qemu as, and whether to kill
by uid or by pid, is in libxl_dm.c, in the function
libxl__domain_get_device_model_uid.

The dm_restrict flag turns on various other things too.

Ian.