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    SubjectRe: [patch 00/38] x86/retbleed: Call depth tracking mitigation
    On Mon, Jul 18, 2022 at 05:11:27PM -0700, Linus Torvalds wrote:
    > On Mon, Jul 18, 2022 at 5:03 PM Linus Torvalds
    > <torvalds@linux-foundation.org> wrote:
    > >
    > > So it already only adds the pattern to things that have their address
    > > taken, not all functions?

    The preamble is added to address-taken static functions and all global
    functions, because those might be indirectly called from other
    translation units. With LTO, we could prune unnecessary preambles from
    non-address-taken globals too.

    > > If so, that's simple enough to sort out: don't do any RSB stack
    > > adjustment for those thunks AT ALL.
    > >
    > > Because they should just then end up with a jump to the "real" target,
    > > and that real target will do the RSB stack thing.
    >
    > Put another way, let's say that you have a function that looks like this:
    >
    > int silly(void)
    > {
    > return 0;
    > }
    >
    > and now you have two cases:
    >
    > - the "direct callable version" of that function looks exactly the
    > way it always has looked, and gets the 16 bytes of padding for it, and
    > the RSB counting can happen in that padding
    >
    > - the "somebody took the address of this function" creates code that
    > has the hash marker before it, and has the hash check, and then does a
    > "jmp silly" to actually jump to the real code.

    Clang's current CFI implementation is somewhat similar to this. It
    creates separate thunks for address-taken functions and changes
    function addresses in C code to point to the thunks instead.

    While this works, it creates painful situations when interacting with
    assembly (e.g. a function address taken in assembly cannot be used
    for indirect calls in C as it doesn't point to the thunk) and needs
    unpleasant hacks when we want take the actual function address in C
    (i.e. scattering the code with function_nocfi() calls).

    I have to agree with Peter on this, I would rather avoid messing with
    function pointers in KCFI to avoid these issues.

    Sami

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