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

    It also means the trap handler needs to do a bunch of instruction
    decoding to find the address that was going to be jumped to, etc.

    > > > 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?

    Peter, is there a reason you want things in the specific order of:

    cmp, je-to-call, trap, call

    Isn't it more run-time efficient to have an out-of-line failure of
    the form:

    cmp, jne-to-trap, call, ...code..., trap, jmp-to-call

    I thought the static label stuff allowed the "default out of line"
    option, as far as pessimizing certain states, etc? The former is certainly
    code-size smaller, though, yes, but doesn't it waste space in the cache
    line for the unlikely case, etc?

    > > 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.

    32-bit hashes would have both code size and runtime benefits: fewer
    instructions for the compare therefore a smaller set of instructions
    added.

    --
    Kees Cook

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