Messages in this thread | | | Date | Tue, 15 Feb 2022 21:53:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups |
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On Tue, Feb 15, 2022 at 08:56:03AM -0800, Sami Tolvanen wrote: > On Mon, Feb 14, 2022 at 2:25 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Feb 14, 2022 at 01:38:18PM -0800, Sami Tolvanen wrote: > > > I'm fine with adding a trap mode that's used by default, but having > > > more helpful diagnostics when something fails is useful even in > > > production systems in my experience. This change results in a vmlinux > > > that's another 0.92% smaller. > > > > You can easily have the exception generate a nice warning, you can even > > have it continue. You really don't need a call for that. > > Sure, but wouldn't that require us to generate something like > __bug_table, so we know where the CFI specific traps are?
Yes, but since you're going to emit a retpoline, and objtool already generates a list of retpoline sites, we can abuse that. If the instruction after the trap is a retpoline site, we a CFI fail.
> > > In this case the function has two indirect calls and Clang seems to > > > prefer to emit just one ud2. > > > > That will not allow you to recover from the exception. UD2 is not an > > unconditional fail. It should have an out-going edge in this case too. > > Yes, CFI failures are not recoverable in that code. In fact, LLVM > assumes that the llvm.trap intrinsic (i.e. ud2) never returns, but I > suppose we could just use an int3 instead. I assume that's sufficient > to stop speculation?
It is. int3 is also smaller by one byte, but the exception is already fairly complicated, but I can see if we can make it work.
> > Also, you really should add a CS prefix to the retpoline thunk call if > > you insist on using r11 (or any of the higher regs). > > I actually didn't touch the retpoline thunk call, that's exactly the > code Clang normally generates.
Yeah, and it needs help, also see:
https://lore.kernel.org/lkml/20211119165630.276205624@infradead.org/
Specifically, clang needs to:
- CS prefix stuff the retpoline thunk call/jmp; - stop doing conditional indirect tail-calls.
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