On 08/30/17 13:14, Josh Poimboeuf wrote: > On Wed, Aug 30, 2017 at 12:23:24PM -0700, H. Peter Anvin wrote: >> On 08/30/17 02:43, tip-bot for Josh Poimboeuf wrote: >>> >>> Those warnings are caused by an unusual GCC non-optimization where it >>> uses an intermediate register to adjust the stack pointer. It does: >>> >>> lea 0x8(%rsp), %rcx >>> ... >>> mov %rcx, %rsp >>> >>> Instead of the obvious: >>> >>> add $0x8, %rsp >>> >>> It makes no sense to use an intermediate register, so I opened a GCC bug >>> to track it: >>> >>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81813 >>> >>> But it's not exactly a high-priority bug and it looks like we'll be >>> stuck with this issue for a while. So for now we have to track register >>> values when they're loaded with stack pointer offsets. >>> >> >> This seems like a good reason to try to extract this information from >> the DWARF data *if available*? > > Well, I haven't ruled that out for the future, but in this case, > integrating DWARF would be a lot more work than this relatively simple > patch. > > If we did go that route, it could be tricky deciding when to trust > DWARF vs. when to trust objtool's reverse engineering. > > Another (vague) idea I'm thinking about is to write a GCC plugin which > annotates the object files in a way that would help objtool become more > GCC-ignorant. If it worked, this approach would be more powerful and > less error-prone than relying on DWARF. > > Depending on how much work we can offload to the plugin, it might also > help make it easier to port objtool to other arches and compilers (e.g., > clang). > > I'm not 100% sold on that idea either, because it still requires objtool > to trust the compiler to some extent. But I think it would be worth it > because it would make the objtool code simpler, more portable, more > robust, and easier to maintain (so I don't always have to stay on top of > all of GCC's latest optimizations). > > In the meantime, objtool's current design is working fine (for now). I > haven't found any issues it can't handle (yet). > Reverse engineering this way is at least NP-complete, and quite possibly undecidable. A gcc plugin would tie the kernel *way* harder to gcc than it is now, and it seems incredibly unlikely that you would come up with something simpler and more reliable than a DWARF parser. What you *can* do, of course, is cross-correlate the two, and *way* more importantly, you cover assembly. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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