Re: [tip: x86/core] objtool: Find unused ENDBR instructions

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Mar 17, 2022 at 03:22:54PM -0700, Josh Poimboeuf wrote:
> On Tue, Mar 15, 2022 at 03:39:52PM +0000, David Laight wrote:
> > From: Peter Zijlstra
> > > 
> > > objtool: Find unused ENDBR instructions
> > > 
> > > Find all ENDBR instructions which are never referenced and stick them
> > > in a section such that the kernel can poison them, sealing the
> > > functions from ever being an indirect call target.
> > 
> > Thought, what happens if the only indirect call is from
> > code in a module?
> 
> Then <boom>, I guess.  Is it safe to assume in-tree modules don't need
> to do indirect calls to exported functions?  I guess we'll find out :-)

So exported functions will keep their ENDBR. Specifically, their address
is taken by the EXPORT_SYMBOL thing.

Sealing them might work, but let's not do that just now ;-)

Any unexported function discovered through kallsyms OTOH... those
deservedly will go *boom*.