Messages in this thread | | | Date | Wed, 11 Oct 2023 09:41:42 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/3] Revert "x86/retpoline: Remove .text..__x86.return_thunk section" |
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On Tue, Oct 10, 2023 at 02:22:54PM -0700, Josh Poimboeuf wrote:
> From: Josh Poimboeuf <jpoimboe@kernel.org> > Subject: [PATCH] objtool: Fix return thunk patching in retpolines > > 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. > > 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> > --- > tools/objtool/check.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index e308d1ba664e..556469db4239 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -1610,6 +1610,15 @@ static int add_jump_destinations(struct objtool_file *file) > return -1; > } > > + /* > + * Since retpolines are in the same section as the return > + * thunk, they might not use a relocation when branching to it. > + */ > + if (jump_dest->sym && jump_dest->sym->return_thunk) { > + add_return_call(file, insn, true); > + continue; > + }
*urgh*... I mean, yes, that obviously works, but should we not also have the retpoline thingy for consistency? That case makes less sense though :/
Perhaps warn about this instead of fixing it? Forcing people to play the section game?
I dunno.. no real strong opinions.
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