On Thu, Jan 12, 2017 at 02:15:11PM -0600, Josh Poimboeuf wrote:
> On Thu, Jan 12, 2017 at 12:08:07PM -0800, Andy Lutomirski wrote:
> > On Thu, Jan 12, 2017 at 11:51 AM, Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > >>
> > >> Just to clarify, I think you're asking if, for versions of gcc which
> > >> don't support -mpreferred-stack-boundary=3, objtool can analyze all C
> > >> functions to ensure their stacks are 16-byte aligned.
> > >>
> > >> It's certainly possible, but I don't see how that solves the problem.
> > >> The stack will still be misaligned by entry code. Or am I missing
> > >> something?
> > >
> > > I think the argument is that we *could* try to align things, if we
> > > just had some tool that actually then verified that we aren't missing
> > > anything.
> > >
> > > I'm not entirely happy with checking the generated code, though,
> > > because as Ingo says, you have a 50:50 chance of just getting it right
> > > by mistake. So I'd much rather have some static tool that checks
> > > things at a code level (ie coccinelle or sparse).
> >
> > What I meant was checking the entry code to see if it aligns stack
> > frames, and good luck getting sparse to do that. Hmm, getting 16-byte
> > alignment for real may actually be entirely a lost cause. After all,
> > I think we have some inline functions that do asm volatile ("call
> > ..."), and I don't see any credible way of forcing alignment short of
> > generating an entirely new stack frame and aligning that.
>
> Actually we already found all such cases and fixed them by forcing a new
> stack frame, thanks to objtool. For example, see 55a76b59b5fe.
>
> > Ick. This
> > whole situation stinks, and I wish that the gcc developers had been
> > less daft here in the first place or that we'd noticed and gotten it
> > fixed much longer ago.
> >
> > Can we come up with a macro like STACK_ALIGN_16 that turns into
> > __aligned__(32) on bad gcc versions and combine that with your sparse
> > patch?
This could work. Only concerns I'd have are:
- Are there (or will there be in the future) any asm functions which
assume a 16-byte aligned stack? (Seems unlikely. Stack alignment is
common in the crypto code but they do the alignment manually.)
- Who's going to run sparse all the time to catch unauthorized users of
__aligned__(16)?
--
Josh
--
To unsubscribe from this list: send the line "unsubscribe linux-crypto" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
