Messages in this thread | | | Subject | Re: [PATCH 0/3] Unexport kallsyms_lookup_name() and kallsyms_on_each_symbol() | From | Joe Lawrence <> | Date | Tue, 25 Feb 2020 10:00:45 -0500 |
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On 2/25/20 7:11 AM, Petr Mladek wrote: > On Tue 2020-02-25 11:05:39, Miroslav Benes wrote: >> CC live-patching ML, because this could affect many of its users... >> >> On Fri, 21 Feb 2020, Will Deacon wrote: >> >>> Hi folks, >>> >>> Despite having just a single modular in-tree user that I could spot, >>> kallsyms_lookup_name() is exported to modules and provides a mechanism >>> for out-of-tree modules to access and invoke arbitrary, non-exported >>> kernel symbols when kallsyms is enabled. > > Just to explain how this affects livepatching users. > > Livepatch is a module that inludes fixed copies of functions that > are buggy in the running kernel. These functions often > call functions or access variables that were defined static in > the original source code. There are two ways how this is currently > solved. > > Some livepatch authors use kallsyms_lookup_name() to locate the > non-exported symbols in the running kernel and then use these > address in the fixed code. >
FWIW, kallsyms was historically used by the out-of-tree kpatch support module to resolve external symbols as well as call set_memory_r{w,o}() API. All of that support code has been merged upstream, so modern kpatch modules* no longer leverage kallsyms by default.
* That said, there are still some users who still use the deprecated support module with newer kernels, but that is not officially supported by the project.
> Another possibility is to used special relocation sections, > see Documentation/livepatch/module-elf-format.rst > > The problem with the special relocations sections is that the support > to generate them is not ready yet. The main piece would klp-convert > tool. Its development is pretty slow. The last version can be > found at > https://lkml.kernel.org/r/20190509143859.9050-1-joe.lawrence@redhat.com > > I am not sure if this use case is enough to keep the symbols exported. > Anyway, there are currently some out-of-tree users. >
Another (temporary?) klp-relocation issue is that binutils has limited support for them as currently implemented:
https://sourceware.org/ml/binutils/2020-02/msg00317.html
For example, try running strip or objcopy on a .ko that includes them and you may find surprising results :(
As far as the klp-convert patchset goes, I forget whether or not we tied its use case to source-based livepatch creation. If kallsyms goes unexported, perhaps it finds more immediate users.
However since klp-convert provides nearly the same functionality as kallsyms, i.e. both can be used to circumvent symbol export licensing -- one could make similar arguments against its inclusion.
If there is renewed (or greater, to be more accurate) interest in the klp-convert patchset, we can dust it off and see what's left. AFAIK it was blocked on arch-specific klp-relocations and whether per-object livepatch modules would remove that requirement.
-- Joe
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