Re: [Xen-devel] [PATCH] tools/livepatch: recognize functions in livepatch.ignore.functions

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 12/12/2017 05:54 AM, elena.ufimtseva@xxxxxxxxxx wrote:
> From: Elena Ufimtseva <elena.ufimtseva@xxxxxxxxxx>
>
> It is expected that the symbol has type STT_FUNC in livpatch.ignore.functions
> sections, but it is incorrect and results in functions not to be ignored.
> To actually ignore functions in livepatch.ignore.functions section, attempt to
> find the symbol of type STT_FUNC by its name.
Hi Elena,

I suspect you might have got the wrong idea about how it is meant to be 
used (or I misunderstood your patch). It is expected that the relocation 
points to a symbol that has type STT_FUNC. This is subtly different from 
having a symbol with type STT_FUNC in .livepatch.ignore.functions.
The correct way to use it is to declare a pointer stored in 
.livepatch.ignore.functions that points to the function you want to ignore.
For example, place this at the end of arch/x86/mm/p2m.c to ignore 
changes to map_domain_gfn:
void *__lp_ignore_func_map_domain_gfn 
__section(.livepatch.ignore.functions) = map_domain_gfn;
The code in livepatch-build-tools is based on kpatch and they have a 
macro to do this in a more friendly way [1]. If you want, it would be 
great if you could port this to Xen LivePatch.
[1] 
https://github.com/dynup/kpatch/blob/f4c0f3209e8e856d93622344560c8794cd8d8a45/kmod/patch/kpatch-macros.h#L39
As an aside, much of the code in livepatch-build-tools comes from 
kpatch, so patches are ideally sent there first. We do intend eventually 
to merge the tools to avoid code duplication and kpatch's build tool was 
recently refactored to make this easier.
Cheers,
--
Ross Lagerwall

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