Re: [RFC PATCH 00/11] Kernel FineIBT Support

Peter Zijlstra
2022-04-20T07:41:14Z

On Tue, Apr 19, 2022 at 05:42:30PM -0700, joao@overdrivepizza.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