Re: [PATCH i386] Allow sibcalls in no-PLT PIC

[Rich Felker <dalias at libc dot org>]

On Tue, May 19, 2015 at 04:43:53PM +0200, Michael Matz wrote:
> Hi,
> 
> On Fri, 15 May 2015, Rich Felker wrote:
> 
> > Forget lazy binding. It's dead anyway because serious distros want
> > PIE+relro+bindnow+...
> 
> You keep saying this, but I can't help the feeling it's mostly because 
> musl doesn't support it ;-)

Well the reasons musl doesn't support it are partly the above, and
partly that it's been a continuous source of subtle bugs in glibc --
things like clobbering new vector registers, missing synchronization,
failures to be async-signal-safe, etc. So it's not that I think lazy
binding is bad because musl doesn't support it, but rather that musl
doesn't support lazy binding because I think it's bad. :-)

> No, you don't have to use bindnow to get the effects of relro.  Sure 
> there's more parts of the GOT protected with it, but if that's really that 
> much more hardened is up for debate.

Normally it's function addresses that you care about protecting --
they're the easy vector for arbitrary code execution -- and they're
unprotected without bindnow. Addresses of global data could also be an
attack vector, but a more difficult one to exploit.

> > If people really want lazy binding, they can use options which support 
> > it, but I don't want to keep suffering the codegen cost of lazy binding 
> > despite never using it.
> 
> > There should be an option to generate optimal code equivalent to what 
> > you get with Alexander Monakov's patches for those of us who aren't 
> > trying to support this legacy feature that precludes good performance 
> > and precludes hardening.
> 
> H.J.'s branch is for _improving_ code on top of the no-plt code, it's not 
> replacing it or an alternative for it.

Thanks for the clarification -- this was the part I was failing to
understand. I'm still mildly worried that concerns for supporting
relaxation might lead to decisions not to optimize code in ways that
would be difficult to relax (e.g. certain types of address load
reordering or hoisting) but I don't understand GCC internals
sufficiently to know if this concern is warranted or not. As long as
his work isn't interfering with the ability of -fno-plt to generate
optimal code, I agree it's both inappropriate and counter-productive
for me to be objecting to part or all of it.

I would still like to see the @GOTPCREL stuff added and used instead
of @GOT, as I mentioned earlier in the thread, but I agree that's
independent of relaxation support and shouldn't block it.

Rich