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SubjectRe: [PATCH v3 0/9] klp-convert livepatch build tooling
On Wed, Apr 10, 2019 at 11:50:49AM -0400, Joe Lawrence wrote:
>
> [ ... snip ... ]
>
> TODO
> ----
>
> There are a few outstanding items that I came across while reviewing v2,
> but as changes started accumulating, it made sense to defer those to a
> later version. I'll reply to this thread individual topics to start
> discussion sub-threads for those.

Multiple object files
=====================

For v3, we tweaked the build scripts so that we could successfully build
a klp-converted livepatch module from multiple object files.

One interesting use-case is supporting two livepatch symbols of the same
name, but different object/position values.

An example target kernel module might be layed out like this:

test_klp_convert_mod.ko << target module is comprised of
two object files, each defining
test_klp_convert_mod_a.o their own local get_homonym_string()
get_homonym_string() function and homonym_string[]
homonym_string[] character arrays.

test_klp_convert_mod_b.o
get_homonym_string()
homonym_string[]


A look at interesting parts of the the module's symbol table:

% eu-readelf --symbols lib/livepatch/test_klp_convert_mod.ko | \
grep -e 'homonym' -e test_klp_convert_mod_

29: 0000000000000000 0 FILE LOCAL DEFAULT ABS test_klp_convert_mod_a.c
32: 0000000000000010 8 FUNC LOCAL DEFAULT 3 get_homonym_string
33: 0000000000000000 17 OBJECT LOCAL DEFAULT 5 homonym_string
37: 0000000000000000 0 FILE LOCAL DEFAULT ABS test_klp_convert_mod_b.c
38: 0000000000000020 8 FUNC LOCAL DEFAULT 3 get_homonym_string
39: 0000000000000000 17 OBJECT LOCAL DEFAULT 11 homonym_string

suggests that any livepatch module that wishes to resolve to
test_klp_convert_mod_a.c :: get_homonym_string() should include an
annotation like:

file_a.c:

KLP_MODULE_RELOC(test_klp_convert_mod) relocs_a[] = {
KLP_SYMPOS(get_homonym_string, 1),
};

and to resolve test_klp_convert_mod_b.c :: get_homonym_string():

file_b.c:

KLP_MODULE_RELOC(test_klp_convert_mod) relocs_b[] = {
KLP_SYMPOS(get_homonym_string, 2),
};


Unfortunately klp-convert v3 will fail to build such livepatch module,
regardless of sympos values:

klp-convert: Conflicting KLP_SYMPOS definition: vmlinux.state_show,2 vs. vmlinux.state_show,1.
klp-convert: Unable to load user-provided sympos
make[1]: *** [scripts/Makefile.modpost:148: lib/livepatch/test_klp_convert_multi_objs.ko] Error 255


This abort message can be traced to sympos_sanity_check() as it verifies
that there no duplicate symbol names found in its list of user specified
symbols (ie, those needing to be converted).


Common sympos
-------------

New test case and related fixup can be found here:

https://github.com/joe-lawrence/linux/commits/klp-convert-v4-multiple-objs

To better debug this issue, I added another selftest that demonstrates
this configuration in isolation. "show_state" is a popular kernel
function name (my VM reported 5 instances in kallsyms) so I chose that
symbol name.

Initially I specified the *same* symbol position (1) in both .c file
KLP_SYMPOS annotations. At the very least, we can trivially patch
klp-convert v3 to support annotations for the the same symbol <object,
name, position> value across object files.

Result: a small tweak to sympos_sanity_check() to relax its symbol
uniqueness verification: allow for duplicate <object, name, position>
instances. Now it will only complain when a supplied symbol references
the same <object, name> but a different <position>.

diff --git a/scripts/livepatch/klp-convert.c b/scripts/livepatch/klp-convert.c
index 82c27d219372..713835dfc9ec 100644
--- a/scripts/livepatch/klp-convert.c
+++ b/scripts/livepatch/klp-convert.c
@@ -194,7 +194,10 @@ static void clear_sympos_annontations(struct elf *klp_elf)
}
}

-/* Checks if two or more elements in usr_symbols have the same name */
+/*
+ * Checks if two or more elements in usr_symbols have the same
+ * object and name, but different symbol position
+ */
static bool sympos_sanity_check(void)
{
bool sane = true;
@@ -203,7 +206,9 @@ static bool sympos_sanity_check(void)
list_for_each_entry(sp, &usr_symbols, list) {
aux = list_next_entry(sp, list);
list_for_each_entry_from(aux, &usr_symbols, list) {
- if (strcmp(sp->symbol_name, aux->symbol_name) == 0) {
+ if (sp->pos != aux->pos &&
+ strcmp(sp->object_name, aux->object_name) == 0 &&
+ strcmp(sp->symbol_name, aux->symbol_name) == 0)
WARN("Conflicting KLP_SYMPOS definition: %s.%s,%d vs. %s.%s,%d.",
sp->object_name, sp->symbol_name, sp->pos,
aux->object_name, aux->symbol_name, aux->pos);

Unique sympos
-------------
But even with the above workaround, specifying unique symbol positions
will (and should) result in a klp-convert complaint.

When modifying the test module with unique symbol position annotation
values (1 and 2 respectively):

test_klp_convert_multi_objs_a.c:

extern void *state_show;
...
KLP_MODULE_RELOC(vmlinux) vmlinux_relocs_b[] = {
KLP_SYMPOS(state_show, 1)
};
test_klp_convert_multi_objs_b.c:

extern void *state_show;
...
KLP_MODULE_RELOC(vmlinux) vmlinux_relocs_b[] = {
KLP_SYMPOS(state_show, 2)
};

Each object file's symbol table contains a "state_show" instance:

% eu-readelf --symbols lib/livepatch/test_klp_convert_multi_objs_a.o | grep '\<state_show\>'
30: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
% eu-readelf --symbols lib/livepatch/test_klp_convert_multi_objs_b.o | grep '\<state_show\>'
20: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
and the intermediate test_klp_convert_multi_objs.klp.o file contains a
combined .klp.module_relocs.vmlinux relocation section with two
KLP_SYMPOS structures, each with a unique <sympos> value:

% objcopy -O binary --only-section=.klp.module_relocs.vmlinux \
lib/livepatch/test_klp_convert_multi_objs.klp.o >(hexdump)

0000000 0000 0000 0000 0000 0001 0000 0000 0000
0000010 0000 0000 0002 0000
but the symbol tables were merged, sorted and non-unique symbols
removed, leaving a *single* unresolved "state_show" instance:

% eu-readelf --symbols lib/livepatch/test_klp_convert_multi_objs.klp.o | grep '\<state_show\>'
53: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
which means that even though we have separate relocations for each
"state_show" instance:

Relocation section [ 6] '.rela.text.unlikely' for section [ 5] '.text.unlikely' at offset 0x44510 contains 11 entries:
Offset Type Value Addend Name
0x0000000000000003 X86_64_32S 000000000000000000 +0 state_show
...
0x0000000000000034 X86_64_32S 000000000000000000 +0 state_show
...
they share the same rela->sym and there is no way to distinguish which
one correlates to which KLP_SYMPOS structure.


Future Work
-----------
I don't see an easy way to support multiple homonym <object, name>
symbols with unique <position> values in the current livepatch module
Elf format. The only solutions that come to mind right now include
renaming homonym symbols somehow to retain the relocation->symbol
relationship when separate object files are combined. Perhaps an
intermediate linker step could make annotated symbols unique in some way
to achieve this. /thinking out loud

In the meantime, the unique symbol <position> case is already detected
and with a little tweaking we could support multiple common symbol
<position> values.

-- Joe

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