Re: [PATCH v1 4/7] xen_pvdev: Do not assume Dom0 when creating a directrory

From: David Woodhouse <dwmw2@infradead.org>
To: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Anthony Perard <anthony.perard@citrix.com>,
	Paul Durrant <paul@xen.org>,
	"open list:X86 Xen CPUs" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v1 4/7] xen_pvdev: Do not assume Dom0 when creating a directrory
Date: Sun, 12 Nov 2023 16:12:47 -0500	[thread overview]
Message-ID: <851f9138ccc9c4a9ec1c8f7f6c2cc57c652f2b96.camel@infradead.org> (raw)
In-Reply-To: <20231110204207.2927514-5-volodymyr_babchuk@epam.com>

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On Fri, 2023-11-10 at 20:42 +0000, Volodymyr Babchuk wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> Instead of forcing the owner to domid 0, use XS_PRESERVE_OWNER to save
> the previous owner of the directory.
> 

You're missing the words "... if it already exists" from that sentence.

If the directory *didn't* already exist, it gets created with dom0 as
the owner still, right? Assuming XenStore allows QEMU to do that.

Strictly, the node gets created (if permitted) and *then*
xs_set_permissions() attempts to set dom0 as the owner (if permitted).

> Note that for other than Dom0 domain (non toolstack domain) the
> "driver_domain" property should be set in domain config file for the
> toolstack to create required directories in advance.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> ---
>  hw/xen/xen_pvdev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/xen/xen_pvdev.c b/hw/xen/xen_pvdev.c
> index c5ad71e8dc..42bdd4f6c8 100644
> --- a/hw/xen/xen_pvdev.c
> +++ b/hw/xen/xen_pvdev.c
> @@ -60,7 +60,8 @@ void xen_config_cleanup(void)
>  
>  int xenstore_mkdir(char *path, int p)
>  {
> -    if (!qemu_xen_xs_create(xenstore, 0, 0, xen_domid, p, path)) {
> +    if (!qemu_xen_xs_create(xenstore, 0, XS_PRESERVE_OWNER,
> +                            xen_domid, p, path)) {
>          xen_pv_printf(NULL, 0, "xs_mkdir %s: failed\n", path);
>          return -1;
>      }

Why bother with xenstore_mkdir()? AFAICT it's *only* used from the
legacy XenLegacyDevice stuff, and can't we just finish the job of
moving from that to the XenDevice model? I've done console and net
recently; want to keep going?

And even then... the xenstore_mkdir() function is called twice from
xen_config_dev_dirs() in hw/xen/xen_devconfig.c to create the frontend
and backend directories — which is what the rest of your patch series
is trying to eliminate because a driver domain doesn't have permissions
to do that anyway.

It's also called from xen_be_register() in hw/xen/xen_devconfig.c to
create device-model/${GUEST_DOMID}/backends/${DEVICE_TYPE} (using a
relative path, so in the driver domain's XenStore). That one presumably
*won't* exist already, and so XS_PRESERVE_OWNER won't even have any
effect?

What practical difference does this even make? Am I missing something?

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