On Thu, Jan 12, 2017 at 05:46:55PM -0800, Andy Lutomirski wrote: > On Thu, Jan 12, 2017 at 12:15 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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. > > What I mean is: what guarantees that the stack is properly aligned for > the subroutine call? gcc promises to set up a stack frame, but does > it promise that rsp will be properly aligned to call a C function? Yes, I did an experiment and you're right. I had naively assumed that all stack frames would be aligned. -- 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