From: Josh Poimboeuf jpoimboe@redhat.com
[ Upstream commit 318af7b80b6a6751520cf2b71edb8c45abb9d9d8 ]
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@infradead.org Signed-off-by: Josh Poimboeuf jpoimboe@redhat.com Acked-by: Miroslav Benes mbenes@suse.cz Signed-off-by: Petr Mladek pmladek@suse.com Link: https://lore.kernel.org/r/696262e997359666afa053fe7d1a9fb2bb373964.159501049... Signed-off-by: Sasha Levin sashal@kernel.org --- Makefile | 4 ---- 1 file changed, 4 deletions(-)
diff --git a/Makefile b/Makefile index 36eab48d1d4a6..3299cc45c1dd0 100644 --- a/Makefile +++ b/Makefile @@ -876,10 +876,6 @@ KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections LDFLAGS_vmlinux += --gc-sections endif
-ifdef CONFIG_LIVEPATCH -KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone) -endif - ifdef CONFIG_SHADOW_CALL_STACK CC_FLAGS_SCS := -fsanitize=shadow-call-stack KBUILD_CFLAGS += $(CC_FLAGS_SCS)