[PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[Jiri Slaby (SUSE)]

From: Andi Kleen <andi@firstfloor.org>

It is not supported by gcc 12 so far, so it causes compiler "sorry"
messages.

Other than the compiler support, there shouldn't be any barriers for
live patching LTOed kernels, although it might be more difficult to
create patches for larger functions.

Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: live-patching@vger.kernel.org
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Martin Liska <mliska@suse.cz>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 kernel/livepatch/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/livepatch/Kconfig b/kernel/livepatch/Kconfig
index 53d51ed619a3..22699adc39a6 100644
--- a/kernel/livepatch/Kconfig
+++ b/kernel/livepatch/Kconfig
@@ -12,6 +12,7 @@ config LIVEPATCH
 	depends on KALLSYMS_ALL
 	depends on HAVE_LIVEPATCH
 	depends on !TRIM_UNUSED_KSYMS
+	depends on !LTO_GCC # not supported in gcc
 	help
 	  Say Y here if you want to support kernel live patching.
 	  This option has no runtime impact until a kernel "patch"
-- 
2.38.1

Re: [PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[Josh Poimboeuf]

On Mon, Nov 14, 2022 at 12:43:38PM +0100, Jiri Slaby (SUSE) wrote:
> From: Andi Kleen <andi@firstfloor.org>
> 
> It is not supported by gcc 12 so far, so it causes compiler "sorry"
> messages.

What specifically is not supported by GCC 12?  What are the "sorry"
messages?

> Other than the compiler support, there shouldn't be any barriers for
> live patching LTOed kernels, although it might be more difficult to
> create patches for larger functions.

This seems to conflict with the documentation.

> Cc: Josh Poimboeuf <jpoimboe@kernel.org>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Miroslav Benes <mbenes@suse.cz>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Joe Lawrence <joe.lawrence@redhat.com>
> Cc: live-patching@vger.kernel.org
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Martin Liska <mliska@suse.cz>
> Signed-off-by: Jiri Slaby <jslaby@suse.cz>
> ---
>  kernel/livepatch/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/livepatch/Kconfig b/kernel/livepatch/Kconfig
> index 53d51ed619a3..22699adc39a6 100644
> --- a/kernel/livepatch/Kconfig
> +++ b/kernel/livepatch/Kconfig
> @@ -12,6 +12,7 @@ config LIVEPATCH
>  	depends on KALLSYMS_ALL
>  	depends on HAVE_LIVEPATCH
>  	depends on !TRIM_UNUSED_KSYMS
> +	depends on !LTO_GCC # not supported in gcc

The comment doesn't help.

-- 
Josh

Re: [PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[Andi Kleen]

On Mon, Nov 14, 2022 at 11:07:42AM -0800, Josh Poimboeuf wrote:
> On Mon, Nov 14, 2022 at 12:43:38PM +0100, Jiri Slaby (SUSE) wrote:
> > From: Andi Kleen <andi@firstfloor.org>
> > 
> > It is not supported by gcc 12 so far, so it causes compiler "sorry"
> > messages.
> 
> What specifically is not supported by GCC 12? 

-fwhole-program and the live patching options are mutually exclusive.
Okay I suppose it could be handled by disabling -fwhole-program, although 
that might limit some optimizations.

> What are the "sorry" messages?

It's an error message from the compiler telling you that something is
not implemented.


-Andi

Re: [PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[Josh Poimboeuf]

On Mon, Nov 14, 2022 at 12:28:09PM -0800, Andi Kleen wrote:
> On Mon, Nov 14, 2022 at 11:07:42AM -0800, Josh Poimboeuf wrote:
> > On Mon, Nov 14, 2022 at 12:43:38PM +0100, Jiri Slaby (SUSE) wrote:
> > > From: Andi Kleen <andi@firstfloor.org>
> > > 
> > > It is not supported by gcc 12 so far, so it causes compiler "sorry"
> > > messages.
> > 
> > What specifically is not supported by GCC 12? 
> 
> -fwhole-program and the live patching options are mutually exclusive.

What live patching options are you referring to?

-- 
Josh

Re: [PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[Martin Liška]

On 11/14/22 23:00, Josh Poimboeuf wrote:
> On Mon, Nov 14, 2022 at 12:28:09PM -0800, Andi Kleen wrote:
>> On Mon, Nov 14, 2022 at 11:07:42AM -0800, Josh Poimboeuf wrote:
>>> On Mon, Nov 14, 2022 at 12:43:38PM +0100, Jiri Slaby (SUSE) wrote:
>>>> From: Andi Kleen <andi@firstfloor.org>
>>>>
>>>> It is not supported by gcc 12 so far, so it causes compiler "sorry"
>>>> messages.
>>>
>>> What specifically is not supported by GCC 12? 
>>
>> -fwhole-program and the live patching options are mutually exclusive.
> 
> What live patching options are you referring to?
> 

As mentioned in the reply to the next email, we speak about:
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-flive-patching
option:

gcc -flto -flive-patching=inline-clone a.c
cc1: sorry, unimplemented: live patching is not supported with LTO

Cheers,
Martin

Re: [PATCH 40/46] x86/livepatch, lto: Disable live patching with gcc LTO

[Song Liu]

On Mon, Nov 14, 2022 at 3:48 AM Jiri Slaby (SUSE) <jirislaby@kernel.org> wrote:
>
> From: Andi Kleen <andi@firstfloor.org>
>
> It is not supported by gcc 12 so far, so it causes compiler "sorry"
> messages.
>
> Other than the compiler support, there shouldn't be any barriers for
> live patching LTOed kernels, although it might be more difficult to
> create patches for larger functions.

A loosely related question: does livepatch work with CLANG LTO?
AFAICT, kpatch-build doesn't support it. But the kernel side should
work just fine?

Thanks,
Song

[...]