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[RFC] Kernel livepatching support in GCC


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.


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.


Maxim Kuvyrkov

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