[RFC] Kernel livepatching support in GCC

[libin]

在 2015/5/28 16:39, Maxim Kuvyrkov 写道:
> Hi,
>
> Akashi-san and I have been discussing required GCC changes to make kernel's livepatching work for AArch64 and other architectures.  At the moment livepatching is supported for x86[_64] using the following options: "-pg -mfentry -mrecord-mcount -mnop-mcount" which is geek-speak for "please add several NOPs at the very beginning of each function, and make a section with addresses of all those NOP pads".
>
> The above long-ish list of options is a historical artifact of how livepatching support evolved for x86.  The end result is that for livepatching (or ftrace, or possible future kernel features) to work compiler needs to generate a little bit of empty code space at the beginning of each function.  Kernel can later use that space to insert call sequences for various hooks.
>
> Our proposal is that instead of adding -mfentry/-mnop-count/-mrecord-mcount options to other architectures, we should implement a target-independent option -fprolog-pad=N, which will generate a pad of N nops at the beginning of each function and add a section entry describing the pad similar to -mrecord-mcount [1].
>
> Since adding NOPs is much less architecture-specific then outputting call instruction sequences, this option can be handled in a target-independent way at least for some/most architectures.
>
> Comments?
>
> As I found out today, the team from Huawei has implemented [2], which follows x86 example of -mfentry option generating a hard-coded call sequence.  I hope that this proposal can be easily incorporated into their work since most of the livepatching changes are in the kernel.
>

Thanks very much for your effort for this, and the arch-independed implementation
is very good to me, but only have one question that how to enture the atomic
replacement of multi instructions in kernel side?

And before this arch-independed option, can we consider the arch-depended -mfentry
implemention for arm64 like arch x86 firstly? I will post it soon.

livepatch for arm64 based on this arm64 -mfentry feature on github:
https://github.com/libin2015/livepatch-for-arm64.git  master

discussions on this topic:
https://lkml.org/lkml/2015/5/28/54

Thanks,
Li Bin

> [1] Technically, generating a NOP pad and adding a section entry in .__mcount_loc are two separate actions, so we may want to have a -fprolog-pad-record option.  My instinct is to stick with a single option for now, since we can always add more later.
>
> [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-May/346905.html
>
> --
> Maxim Kuvyrkov
> www.linaro.org
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