Re: [PATCH v7 00/10] livepatch: klp-convert tool

Joe Lawrence
2023-03-17T20:31:59Z

On Tue, Mar 14, 2023 at 05:23:56PM -0300, Marcos Paulo de Souza wrote:
> On Mon, Mar 06, 2023 at 09:08:14AM -0500, Joe Lawrence wrote:
> > Summary
> > -------
> >
> > Livepatches may use symbols which are not contained in its own scope,
> > and, because of that, may end up compiled with relocations that will
> > only be resolved during module load. Yet, when the referenced symbols
> > are not exported, solving this relocation requires information on the
> > object that holds the symbol (either vmlinux or modules) and its
> > position inside the object, as an object may contain multiple symbols
> > with the same name. Providing such information must be done accordingly
> > to what is specified in Documentation/livepatch/module-elf-format.txt.
> >
> > Currently, there is no trivial way to embed the required information as
> > requested in the final livepatch elf object. klp-convert solves this
> > problem in two different forms: (i) by relying on a symbol map, which is
> > built during kernel compilation, to automatically infer the relocation
> > targeted symbol, and, when such inference is not possible (ii) by using
> > annotations in the elf object to convert the relocation accordingly to
> > the specification, enabling it to be handled by the livepatch loader.
> >
> > Given the above, add support for symbol mapping in the form of a
> > symbols.klp file; add klp-convert tool; integrate klp-convert tool into
> > kbuild; make livepatch modules discernible during kernel compilation
> > pipeline; add data-structure and macros to enable users to annotate
> > livepatch source code; make modpost stage compatible with livepatches;
> > update livepatch-sample and update documentation.
> >
> > The patch was tested under three use-cases:
> >
> > use-case 1: There is a relocation in the lp that can be automatically
> > resolved by klp-convert. For example. see the saved_command_line
> > variable in lib/livepatch/test_klp_convert2.c.
> >
> > use-case 2: There is a relocation in the lp that cannot be automatically
> > resolved, as the name of the respective symbol appears in multiple
> > objects. The livepatch contains an annotation to enable a correct
> > relocation. See the KLP_MODULE_RELOC / KLP_SYMPOS annotation sections
> > in lib/livepatch/test_klp_convert{1,2}.c.
> >
> > use-case 3: There is a relocation in the lp that cannot be automatically
> > resolved similarly as 2, but no annotation was provided in the
> > livepatch, triggering an error during compilation. Reproducible by
> > removing the KLP_MODULE_RELOC / KLP_SYMPOS annotation sections in
> > lib/livepatch/test_klp_convert{1,2}.c.
> >
> > Selftests have been added to exercise these klp-convert use-cases
> > through several tests.
> >
> >
> > Testing
> > -------
> >
> > The patchset selftests build and execute on x86_64, s390x, and ppc64le
> > for both default config (with added livepatch dependencies) and a larger
> > RHEL-9-ish config.
> >
> > Using the Intel's Linux Kernel Performance tests's make.cross,
> > klp-convert builds and processes livepatch .ko's for x86_64 ppc64le
> > ppc32 s390 arm64 arches.
> >
> >
> > Summary of changes in v7
> > ------------------------
> >
> > - rebase for v6.2
> > - combine ("livepatch: Add klp-convert tool") with ("livepatch: Add
> > klp-convert annotation helpers")
> > - combine ("kbuild: Support for symbols.klp creation") with ("modpost:
> > Integrate klp-convert") to simplify Kbuild magic [Petr, Nicolas]
> > - klp-convert: add safe_snprintf() (-Wsign-compare)
> > - klp-convert: fix -Wsign-compare warnings
> > - klp-convert: use calloc() where appropriate
> > - klp-convert: copy ELF e_flags
> > - selftests: fix various build warnings
> > - klp-convert: WARN msg simplification, failed sanity checks, and sympos
> > comment [Marcos]
> > - klp-convert: fix elf_write_file() error paths [Petr]
>
> Thanks for the new version Joe. I've run the ksefltests on my x86 laptop, and it
> succeed as expected, so
>
> Tested-by: Marcos Paulo de Souza <mpdesouza@suse.com>
>

Thanks for the testing and reviews, Marcos.

The selftests are the first level of testing... we should probably
tackle a real or simulated CVE fix to see how well the tooling fits
larger livepatches.

One complication that I can envision is symbol positioning. Currently,
the klp-convert annotations are a direct mirror of the kernel's
<obj,symbol,pos> tuple. It should be possible to make this a bit more
user friendly for the livepatch developer if the annotations were
<obj,file,symbol>, as derived from the vmlinux / module.tmp.ko symbol
tables.

For example, the following code:

KLP_MODULE_RELOC(test_klp_convert_mod, test_klp_convert_mod_b.c) test_klp_convert_mod_relocs_b[] = {
KLP_SYMPOS(homonym_string),
KLP_SYMPOS(get_homonym_string),
};

could generate the following relocations:

Relocation section '.rela.klp.module_relocs.test_klp_convert_mod.test_klp_convert_mod_b.c' at offset 0x1dc0 contains 2 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000000000 0000003f00000001 R_X86_64_64 0000000000000000 homonym_string + 0
0000000000000008 0000004900000001 R_X86_64_64 0000000000000000 get_homonym_string + 0

for which klp-convert looks up in symbols.klp:

klp-convert-symbol-data.0.2
*vmlinux
...
*test_klp_convert_mod
-test_klp_convert_mod_a.c << added filenames to the format
test_klp_get_driver_name
driver_name
get_homonym_string << sympos = 1
homonym_string << sympos = 1
...
-test_klp_convert_mod_b.c
get_homonym_string << sympos = 2
homonym_string << sympos = 2
...

and then generates the usual klp-relocations as currently defined.

(Unfortunately full pathnames are not saved in the STT_FILE symbol table
entries, so there will be a few non-unique <obj,file,symbol> entries. I
believe the last time this was discussed, we found that there were a
relatively small number of such symbols.)

Have you tried retrofitting klp-convert into any real-world livepatch?
I'm curious as to your observations on the overall experience, or
thoughts on the sympos annotation style noted above.

Regards,

-- Joe