objtool/klp: Allow new references to module exports

klp_reloc_needed() returns true for module exports to support
late-module patching.  However, clone_reloc_klp() unconditionally
rejects symbols without a twin (i.e., new references added by the
patch), even when the symbol is a known export from Module.symvers.

Relax the check: allow new references to exported symbols by only
erroring on !twin when there is no export.  The export metadata from
Module.symvers provides sufficient context to emit the klp-relocation
without a twin.

For a module export that isn't sufficient on its own though, as the
resulting klp relocation will only be resolved at patch-enable time if
the exporting module is loaded.

If the original (unpatched) module already depends on the exporting
module, the dependency is safe: the module loader ensures the dependency
is satisfied before the patched module can be loaded, so the
klp relocation target will exist.

However, if the patch introduces a reference to a module that the
original doesn't depend on, there is no such guarantee.  The exporting
module could be absent or could be unloaded at any time, leading to a
relocation failure or use-after-free.

So also add a build-time check: when a new symbol reference (no twin)
targets a module export, verify that the original module already has at
least one UNDEF symbol resolving to that same exporting module.  If not,
error out with a diagnostic message.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://patch.msgid.link/e0d725acb4774747f0e271308b4ca33daae2d5db.1786138493.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

0 files changed, 0 insertions, 0 deletions