On Fri, 17 Jul 2020, Josh Poimboeuf wrote:
> Use of the new -flive-patching flag was introduced with the following
> commit:
>
> 43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled")
>
> This flag has several drawbacks:
>
> - It disables some optimizations, so it can have a negative effect on
> performance.
>
> - According to the GCC documentation it's not compatible with LTO, which
> will become a compatibility issue as LTO support gets upstreamed in
> the kernel.
>
> - It was intended to be used for source-based patch generation tooling,
> as opposed to binary-based patch generation tooling (e.g.,
> kpatch-build). It probably should have at least been behind a
> separate config option so as not to negatively affect other livepatch
> users.
>
> - Clang doesn't have the flag, so as far as I can tell, this method of
> generating patches is incompatible with Clang, which like LTO is
> becoming more mainstream.
>
> - It breaks GCC's implicit noreturn detection for local functions. This
> is the cause of several "unreachable instruction" objtool warnings.
>
> - The broken noreturn detection is an obvious GCC regression, but we
> haven't yet gotten GCC developers to acknowledge that, which doesn't
> inspire confidence in their willingness to keep the feature working as
> optimizations are added or changed going forward.
>
> - While there *is* a distro which relies on this flag for their distro
> livepatch module builds, there's not a publicly documented way to
> create safe livepatch modules with it. Its use seems to be based on
> tribal knowledge. It serves no benefit to those who don't know how to
> use it.
>
> (In fact, I believe the current livepatch documentation and samples
> are misleading and dangerous, and should be corrected. Or at least
> amended with a disclaimer. But I don't feel qualified to make such
> changes.)
>
> Also, we have an idea for using objtool to detect function changes,
> which could potentially obsolete the need for this flag anyway.
>
> At this point the flag has no benefits for upstream which would
> counteract the above drawbacks. Revert it until it becomes more ready.
>
> This reverts commit 43bd3a95c98e1a86b8b55d97f745c224ecff02b9.
>
> Fixes: 43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled")
> Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Acked-by: Miroslav Benes <mbenes@xxxxxxx>
M
