On Thu, Nov 29, 2018 at 02:24:52PM -0600, Josh Poimboeuf wrote: > On Thu, Nov 29, 2018 at 11:27:00AM -0800, Andy Lutomirski wrote: > > On Thu, Nov 29, 2018 at 11:08 AM Linus Torvalds > > <torva...@linux-foundation.org> wrote: > > > > > > On Thu, Nov 29, 2018 at 10:58 AM Linus Torvalds > > > <torva...@linux-foundation.org> wrote: > > > > > > > > In contrast, if the call was wrapped in an inline asm, we'd *know* the > > > > compiler couldn't turn a "call wrapper(%rip)" into anything else. > > > > > > Actually, I think I have a better model - if the caller is done with > > > inline asm. > > > > > > What you can do then is basically add a single-byte prefix to the > > > "call" instruction that does nothing (say, cs override), and then > > > replace *that* with a 'int3' instruction. > > > > > > Boom. Done. > > > > > > Now, the "int3" handler can just update the instruction in-place, but > > > leave the "int3" in place, and then return to the next instruction > > > byte (which is just the normal branch instruction without the prefix > > > byte). > > > > > > The cross-CPU case continues to work, because the 'int3' remains in > > > place until after the IPI. > > > > Hmm, cute. But then the calls are in inline asm, which results in > > giant turds like we have for the pvop vcalls. And, if they start > > being used more generally, we potentially have ABI issues where the > > calling convention isn't quite what the asm expects, and we explode. > > > > I propose a different solution: > > > > As in this patch set, we have a direct and an indirect version. The > > indirect version remains exactly the same as in this patch set. The > > direct version just only does the patching when all seems well: the > > call instruction needs to be 0xe8, and we only do it when the thing > > doesn't cross a cache line. Does that work? In the rare case where > > the compiler generates something other than 0xe8 or crosses a cache > > line, then the thing just remains as a call to the out of line jmp > > trampoline. Does that seem reasonable? It's a very minor change to > > the patch set. > > Maybe that would be ok. If my math is right, we would use the > out-of-line version almost 5% of the time due to cache misalignment of > the address.
BTW, this means that if any of a trampoline's callers crosses cache boundaries then we won't be able to poison the trampoline. Which is kind of sad. -- Josh
