Messages in this thread | | | Date | Thu, 12 Oct 2023 10:16:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] objtool: Fix return thunk patching in retpolines |
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On Wed, Oct 11, 2023 at 07:47:37PM -0700, Josh Poimboeuf wrote: > With CONFIG_RETHUNK enabled, the compiler replaces every RET with a tail > call to a return thunk ('JMP __x86_return_thunk'). Objtool annotates > all such return sites so they can be patched during boot by > apply_returns(). > > The implementation of __x86_return_thunk() is just a bare RET. It's > only meant to be used temporarily until apply_returns() patches all > return sites with either a JMP to another return thunk or an actual RET. > > The following commit > > e92626af3234 ("x86/retpoline: Remove .text..__x86.return_thunk section") retpolines > > broke objtool's detection of return sites in retpolines. Since > retpolines and return thunks are now in the same section, the compiler > no longer uses relocations for the intra-section jumps between the > retpolines and the return thunk, causing objtool to overlook them. > > As a result, none of the retpolines' return sites get patched. Each one > stays at 'JMP __x86_return_thunk', effectively a bare RET. > > Fix it by teaching objtool to detect when a non-relocated jump target is > a return thunk (or retpoline). > > Fixes: e92626af3234 ("x86/retpoline: Remove .text..__x86.return_thunk section") > Reported-by: David Kaplan <david.kaplan@amd.com> > Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> --- > tools/objtool/check.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index e308d1ba664e..e94756e09ca9 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -1610,6 +1610,22 @@ static int add_jump_destinations(struct objtool_file *file) > return -1; > } > > + /* > + * An intra-TU jump in retpoline.o might not have a relocation > + * for its jump dest, in which case the above > + * add_{retpoline,return}_call() didn't happen. > + */ > + if (jump_dest->sym && jump_dest->offset == jump_dest->sym->offset) { > + if (jump_dest->sym->retpoline_thunk) { > + add_retpoline_call(file, insn); > + continue; > + } > + if (jump_dest->sym->return_thunk) { > + add_return_call(file, insn, true); > + continue; > + } > + } > + > /* > * Cross-function jump. > */ > -- > 2.41.0 >
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