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Date: Wed, 20 Apr 2022 09:40:44 +0200 From: Peter Zijlstra <peterz@...radead.org> To: joao@...rdrivepizza.com Cc: linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com, keescook@...omium.org, samitolvanen@...gle.com, mark.rutland@....com, hjl.tools@...il.com, alyssa.milburn@...ux.intel.com, ndesaulniers@...gle.com, gabriel.gomes@...ux.intel.com, rick.p.edgecombe@...el.com Subject: Re: [RFC PATCH 00/11] Kernel FineIBT Support On Tue, Apr 19, 2022 at 05:42:30PM -0700, joao@...rdrivepizza.com wrote: > @PeterZ @JoshP > > I'm a bit unaware of the details on why the objtool approach to bypass ENDBRs > was removed from the IBT series. Is this approach now sensible considering that > it is a requirement for a new/enhanced feature? If not, how extending the Linker > to emit already fixed offsets sounds like? Josh hates objtool modifying actualy code. He much prefers objtool only emits out of band data. Now, I did sneak in that jump_label nop'ing, and necessity (broken compilers) had us do the KCOV nop'ing in noinstr, but if you look at the recent objtool series here: https://lkml.kernel.org/r/cover.1650300597.git.jpoimboe@redhat.com you'll see his thoughs on that :-) Now, I obviously don't mind, it's easy enough to figure out what objtool actually does with something like: $ OBJTOOL_ARGS="--backup" make O=ibt-build/ -j$lots vmlinux $ objdiff.sh ibt-build/vmlinux.o Where objdiff.sh is the below crummy script. Now, one compromise that I did get out of Josh was that he objected less to rewriting relocations than to rewriting the immediates. From my testing the relocations got us the vast majority of direct call sites, very few are immediates. Josh, any way you might reconsider all that? :-) --- #!/bin/bash name=$1 pre=${name}.orig post=${name} function to_text { obj=$1 ( objdump -wdr $obj; readelf -W --relocs --symbols $obj | awk '/^Relocation section/ { $6=0 } { print $0 }' ) > ${obj}.tmp } to_text $pre to_text $post diff -u ${pre}.tmp ${post}.tmp rm ${pre}.tmp ${post}.tmp
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