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

Miroslav Benes
2019-04-04T09:14:57Z

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