Messages in this thread | | | Date | Fri, 18 May 2018 01:31:32 -0600 | From | "Jan Beulich" <> | Subject | Re: [PATCH v3 1/2] xen/PVH: Set up GS segment for stack canary |
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>>> On 17.05.18 at 19:47, <boris.ostrovsky@oracle.com> wrote: > On 05/17/2018 11:02 AM, Jan Beulich wrote: >>>>> On 17.05.18 at 16:47, <boris.ostrovsky@oracle.com> wrote: >>> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen) >>> mov %eax,%es >>> mov %eax,%ss >>> >>> + mov $PVH_CANARY_SEL,%eax >>> + mov %eax,%gs >> I doubt this is needed for 64-bit (you could equally well load zero or leave >> in place what's there in that case), > > I don't understand this.
The actual selector value doesn't matter on 64-bit. Hence you could omit the load altogether, or you could use plain zero. No need for the (non-zero) selector, or (by implication) the GDT descriptor.
>> and loading the selector before setting >> the base address in the descriptor won't have the intended effect. > > > I wasn't sure about this either but then I noticed that > secondary_startup_64() does it in the same order (although not using the > MSR).
Well, for one they load a null selector, which is independent of setting up any GDT descriptors. I also don't understand why you say "although not using the MSR" when they clearly do. And then, as said above (and also in a comment in secondary_startup_64()), the actual selector value (and when / if at all it is loaded) doesn't matter on 64-bit. The ordering does matter on 32-bit though.
Jan
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