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

Miroslav Benes
2019-04-03T12:48:50Z

> > Hmm, maybe I spoke too soon.
> >
> > I am having issues if I have a two-object livepatch module in which each
> > object file needs to specify its own KLP_MODULE_RELOC for the same
> > symbol name.
> >
> > For example: I have test_klp_convert.ko which is comprised of
> > test_klp_convert_a.o. which needs to refer to state_show,1 and
> > test_klp_convert_b.o. which needs to refer to state_show,2.
> >
> > % make
> > ...
> > KLP lib/livepatch/test_klp_convert.ko
> > klp-convert: Conflicting KLP_SYMPOS definition: vmlinux.state_show,0 vs. vmlinux.state_show,1.
> > klp-convert: Unable to load user-provided sympos
> > make[2]: *** [scripts/Makefile.modpost:148: lib/livepatch/test_klp_convert.ko] Error 255
> > make[1]: *** [/home/cloud-user/disk/linux/Makefile:1282: modules] Error 2
> > make: *** [Makefile:170: sub-make] Error 2
> >
> > I take a closer look next week, but in the meantime, see the source
> > files below.
>
> Okay, with a fresh set of eyes today, I think the issue can be
> summarized as:
>
> * "Special" livepatch symbols, as processed by klp-convert, require
> external linkage, otherwise a new local storage instance will be
> created and miss klp-convert altogether
>
> * When linking together two object files, their combined symbol table
> will include a de-duped list of uniquly named global symbols
>
> So if we are to run klp-convert on the combined module object file (as
> per v2 plus suggested changes in this thread), we are going to run into
> problems if ...
>
>
> Example
> =======
>
> (quoted in previous reply), test_klp_convert_a and test_klp_convert_b
> have their own "state_show" variable in which each wishes to resolve to
> unique symbol positions (2 and 3 accordingly).
>
>
> test_klp_convert_a
> ------------------
>
> We get one symbol table entry and one relocation as expected.
>
> % eu-readelf --symbols lib/livepatch/test_klp_convert_a.o
>
> Symbol table [27] '.symtab' contains 38 entries:
> 29 local symbols String table: [28] '.strtab'
> Num: Value Size Type Bind Vis Ndx Name
> ...
> 30: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
> ...
>
> % eu-readelf --relocs lib/livepatch/test_klp_convert_a.o
>
> Relocation section [12] '.rela.klp.module_relocs.vmlinux' for section [11] '.klp.module_relocs.vmlinux' at offset 0x4b8 contains 1 entry:
> Offset Type Value Addend Name
> 000000000000000000 X86_64_64 000000000000000000 +0 state_show
>
>
> test_klp_convert_b
> ------------------
>
> Just like the other object file, one symbol table entry and one
> relocation:
>
> % eu-readelf --symbols lib/livepatch/test_klp_convert_a.o
> Symbol table [24] '.symtab' contains 23 entries:
> 19 local symbols String table: [25] '.strtab'
> Num: Value Size Type Bind Vis Ndx Name
> ...
> 20: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
> ...
>
> % eu-readelf --relocs lib/livepatch/test_klp_convert_b.o
>
> Relocation section [ 9] '.rela.klp.module_relocs.vmlinux' for section [ 8] '.klp.module_relocs.vmlinux' at offset 0x118 contains 1 entry:
> Offset Type Value Addend Name
> 000000000000000000 X86_64_64 000000000000000000 +0 state_show
>
>
> test_klp_convert
> ----------------
>
> But the combined test_klp_convert.klp.o file has only a single
> unresolved "state_show" symbol in its symbol table:
>
> % eu-readelf --symbols lib/livepatch/test_klp_convert.klp.o
>
> Symbol table [35] '.symtab' contains 57 entries:
> 47 local symbols String table: [36] '.strtab'
> Num: Value Size Type Bind Vis Ndx Name
> ...
> 52: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
> ...
>
> though the .rela.klp.module_relocs.vmlinux relocation section has two
> entries:
>
> % eu-readelf --relocs lib/livepatch/test_klp_convert.klp.o
>
> Relocation section [17] '.rela.klp.module_relocs.vmlinux' for section [16] '.klp.module_relocs.vmlinux' at offset 0x446c8 contains 2 entries:
> Offset Type Value Addend Name
> 000000000000000000 X86_64_64 000000000000000000 +0 state_show
> 0x0000000000000010 X86_64_64 000000000000000000 +0 state_show
>
> 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.

I think it should not be a big problem to fix it though (famous last
words). The information is there, so klp-convert with a changed annotation
macro should deal with it.

> Note: I'm inclined to defer work on resolving this limitation to v4. I
> would like v3 to focus on collecting and squashing the feedback up to
> now on v2. There are a few other outstanding issues that I have run
> across while testing this patchset, so I feel that it would be clearer
> for folks to base comments on those off a clean v3 slate.

Makes sense to me.

Miroslav