Re: [PATCH V5 00/22] IOREQ feature (+ virtio-mmio) on Arm

[Julien Grall <julien@xxxxxxx>]

On 27/01/2021 10:51, Jan Beulich wrote:
> On 27.01.2021 11:13, Oleksandr wrote:
> > On 26.01.21 02:14, Oleksandr wrote:
> > > On 26.01.21 01:20, Julien Grall wrote:
> > > > On Mon, 25 Jan 2021 at 20:56, Stefano Stabellini
> > > > <sstabellini@xxxxxxxxxx> wrote:
> > > > > This seems to be an arm randconfig failure:
> > > > >
> > > > > https://gitlab.com/xen-project/patchew/xen/-/pipelines/246632953
> > > > > https://gitlab.com/xen-project/patchew/xen/-/jobs/985455044
> > > > Thanks! The error is:
> > > >
> > > > #'target_mem_ref' not supported by expression#'memory.c: In function
> Btw, I found the first part of this line pretty confusing, to a
> degree that when seeing it initially I thought this must be some
> odd tool producing the odd error. But perhaps this is just
> unfortunate output ordering from different tools running in
> parallel.
This message is actually coming from GCC. There are quite a few reports 
online.
Although, I am not sure whether this was intended.

> > > > 'do_memory_op':
> > > > memory.c:1210:18: error:  may be used uninitialized in this function
> > > > [-Werror=maybe-uninitialized]
> > > >    1210 |             rc = set_foreign_p2m_entry(currd, d, gfn_list[i],
> > > >         | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > >    1211 | _mfn(mfn_list[i]));
> > > >         | ~~~~~~~~~~~~~~~~~~
> > > >
> > > > I found a few references online of the error message, but it is not
> > > > clear what it means. From a quick look at Oleksandr's branch, I also
> > > > can't spot anything unitialized. Any ideas?
> > > It seems that error happens if *both* CONFIG_GRANT_TABLE and
> > > CONFIG_IOREQ_SERVER are disabled. Looks like that mfn_list is
> > > initialized either in acquire_grant_table() or in acquire_ioreq_server().
> > > If these options disabled then corresponding helpers are just stubs,
> > > so indeed that mfn_list gets uninitialized. But, I am not sure why gcc
> > > complains about it as set_foreign_p2m_entry() is *not* going to be
> > > called in that case???
> > This weird build error goes away if I simply add:
> >
> > diff --git a/xen/common/memory.c b/xen/common/memory.c
> > index 33296e6..d1bd57b 100644
> > --- a/xen/common/memory.c
> > +++ b/xen/common/memory.c
> > @@ -1136,7 +1136,7 @@ static int acquire_resource(
> >         * moment since they are small, but if they need to grow in future
> >         * use-cases then per-CPU arrays or heap allocations may be required.
> >         */
> > -    xen_pfn_t mfn_list[32];
> > +    xen_pfn_t mfn_list[32] = {0};
> >        int rc;
> >
> >        if ( !arch_acquire_resource_check(currd) )
> >
> >
> > Shall I make the corresponding patch?
> I'd prefer if we could find another solution, avoiding this
> pointless writing of 256 bytes of zeros (and really to be on the
> safe side I think it should rather be ~0 that gets put in there).
> Could you check whether clearing the array along the lines of
> this
>
>      default:
>          memset(mfn_list, ~0, sizeof(mfn_list));
>          rc = -EOPNOTSUPP;
>          break;
>
> helps (avoiding the writes in all normal cases)? Of course this
> wouldn't be a guarantee that another compiler (version) won't
> warn yet again. But the only other alternative I can think of
> without having the writes on the common path would be something
> along the lines of older Linux'es uninitialized_var(). Maybe
> someone else has a better idea ...
So GCC is complaining specifically about mfn_list[0]. For instance, I 
wrote the following:
diff --git a/xen/common/memory.c b/xen/common/memory.c
index 33296e65cb04..81b4645047e7 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1139,6 +1139,8 @@ static int acquire_resource(
     xen_pfn_t mfn_list[32];
     int rc;

+    mfn_list[0] = 0;
+
     if ( !arch_acquire_resource_check(currd) )
         return -EACCES;

It will silence GCC. But the follwing will not:

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 33296e65cb04..cf558a6b9fa4 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1139,6 +1139,8 @@ static int acquire_resource(
     xen_pfn_t mfn_list[32];
     int rc;

+    mfn_list[1] = 0;
+
     if ( !arch_acquire_resource_check(currd) )
         return -EACCES;

I also try to set mfn_list[0] to 0 in different part of the switch. It 
doesn't help, if I add it in the default. But it does, if I put in the 
grant table path.
So it looks like the grant table path is the issue.

I can confirm that your solution will silenece GCC, but I am a bit worry 
that this may only hide a different bug because the failure seems to be 
widespread on arm (gitlab reported the error with GCC 9.2.1 and I have 
managed to reproduced on GCC 7.5.0).
Given this is a randconfig where CONFIG_GRANT_TABLE is disabled (we 
technically don't grant table disabled), I would rather take a bit more 
time to understand the problem rather than rushing it.
Cheers,

--
Julien Grall