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Re: [RFC] Kernel livepatching support in GCC


On May 28, 2015 10:39:27 AM GMT+02:00, Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org> wrote:
>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?

Maybe follow s390 -mhotpatch instead?

>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.
>
>[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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