linux-kernel - Re: [PATCH v2 2/8] kbuild: Support for Symbols.list creation

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Date:   Thu, 4 Apr 2019 11:14:51 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Joe Lawrence <joe.lawrence@...hat.com>
cc:     Joao Moreira <jmoreira@...e.de>, live-patching@...r.kernel.org,
        pmladek@...e.cz, jikos@...e.cz, nstange@...e.de,
        jpoimboe@...hat.com, khlebnikov@...dex-team.ru, jeyu@...nel.org,
        matz@...e.de, linux-kernel@...r.kernel.org,
        yamada.masahiro@...ionext.com, linux-kbuild@...r.kernel.org,
        michal.lkml@...kovi.net
Subject: Re: [PATCH v2 2/8] kbuild: Support for Symbols.list creation

On Wed, 3 Apr 2019, Joe Lawrence wrote:

> On 4/3/19 8:48 AM, Miroslav Benes wrote:
> > 
> >> and it looks like the combined KLP_MODULE_RELOC still contains the two
> >> unique symbol position values (2 and 3):
> >>
> >>    % objcopy -O binary --only-section=.klp.module_relocs.vmlinux lib/livepatch/test_klp_convert.klp.o >(hexdump -C)
> >>    00000000  00 00 00 00 00 00 00 00  02 00 00 00 00 00 00 00  |................|
> >>    00000010  00 00 00 00 00 00 00 00  03 00 00 00              |............|
> >>    0000001c
> > 
> > Nice :/
> >   
> >> Maybe we can work around this by modifying the annotation macros and/or
> >> klp-convert, or live with this for now.
> > 
> > The question is (and I'll check later. I cannot wrap my head around it
> > now) if it at least works if there are two references of the same symbol
> > in two different .o. It would be same state_show in this case and not two
> > different ones. If it works then I think we can live with it for a while,
> > because after all duplicate symbols are quite rare in the kernel.
> 
> Possibly, but in testing that scenario I found another issue.  Check out what
> happens to the combined .klp.module_relocs.vmlinux section for:
> 
>   test_klp_convert_a.c
>   KLP_MODULE_RELOC(vmlinux) vmlinux_relocs_a[] = {
>           KLP_SYMPOS(state_show, 2)
>           KLP_SYMPOS(joe, 10)
>           KLP_SYMPOS(joe2, 11)
>   };
> 
>   test_klp_convert_b.c
>   KLP_MODULE_RELOC(vmlinux) vmlinux_relocs_b[] = {
>           KLP_SYMPOS(state_show, 2)
>   };
> 
> The second file's klp_module_reloc are not aligned with the first,
> so I think there is additional padding to push the second set to a
> word boundary:
> 
>   % objcopy -O binary --only-section=.klp.module_relocs.vmlinux lib/livepatch/test_klp_convert.klp.o >(hexdump -C)
>   00000000  00 00 00 00 00 00 00 00  02 00 00 00 00 00 00 00
>             |-*sym----------------|  |--sympos-| |-*sym-----
> 
>   00000010  00 00 00 00 0a 00 00 00  00 00 00 00 00 00 00 00
>             ----------| |--sympos-|  |-*sym----------------|
> 
>   00000020  0b 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
>             |-sympos--|              |-*sym----------------|
> 
>                         ^^^^^^^^^^^
>                         padding
>   00000030  02 00 00 00
>             |-sympos--|
>
> 
> in this case, klp-convert thought the last symbol's sympos was
> incorrectly 0 and not 2.

Ugh. It's getting better and better.
 
> If the packed attribute is merely a space optimization, can we
> simply pull that (or can we specify slightly looser alignment to
> account for the padding)?

I think so.

> I'll continue working on putting together v3 and add this new item
> to the TODO list.

Great job btw. Thanks.

Miroslav