On Thu, Jan 12, 2017 at 7:11 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > 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. Just to check: did you do your experiment with -mpreferred-stack-boundary=4? --Andy -- 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