Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

Sami Tolvanen
2022-02-15T16:56:18Z

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?

> > 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?

> 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.

> > How would you like to deal with the 4-byte hashes in objtool? We
> > either need to annotate all function symbols in the kernel, or we need
> > a way to distinguish the hashes from random instructions, so we can
> > also have functions that don't have a type hash.
>
> Easiest would be to create a special section with all the hash offsets
> in I suppose. A bit like -mfentry-section=name.

OK, I'll take a look. With 64-bit hashes I was planning to use a known
prefix, but that's not really an option with a 32-bit hash.

Sami