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objtool/klp: Allow new references to module exports

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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>
This commit is contained in:
Joe Lawrence
2026-08-07 14:37:52 -07:00
committed by Josh Poimboeuf
parent 86a697572c
commit 72d76d0c18
1 changed files with 33 additions and 2 deletions
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35
tools/objtool/klp-diff.c
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@@ -1295,6 +1295,28 @@ static int convert_reloc_sym(struct elf *elf, struct reloc *reloc)
return convert_reloc_secsym_to_sym(elf, reloc);
}
/*
* Check if the original module already has a dependency on dep_mod, i.e. it
* already references at least one export from that module.
*/
static bool has_module_dep(struct elfs *e, const char *dep_mod)
{
struct symbol *sym;
for_each_sym(e->orig, sym) {
struct export *exp;
if (!is_undef_sym(sym) || is_weak_sym(sym))
continue;
exp = find_export(sym);
if (exp && !strcmp(exp->mod, dep_mod))
return true;
}
return false;
}
/*
* Convert a regular relocation to a klp relocation (sort of).
*/
@@ -1314,8 +1336,17 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
unsigned long sympos;
if (!patched_sym->twin) {
ERROR("unexpected klp reloc for new symbol %s", patched_sym->name);
return -1;
if (!export) {
ERROR("unexpected klp reloc for new symbol %s", patched_sym->name);
return -1;
}
if (strcmp(export->mod, "vmlinux") &&
!has_module_dep(e, export->mod)) {
ERROR("%s: new reference to %s (exported by %s) would create an undeclared module dependency",
patched_sym->name, export->sym, export->mod);
return -1;
}
}
/*