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    SubjectRe: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
    On Wed, Nov 28, 2018 at 10:05:54PM -0800, Andy Lutomirski wrote:

    > >> +static void static_call_bp_handler(struct pt_regs *regs, void *_data)
    > >> +{
    > >> + struct static_call_bp_data *data = _data;
    > >> +
    > >> + /*
    > >> + * For inline static calls, push the return address on the stack so the
    > >> + * "called" function will return to the location immediately after the
    > >> + * call site.
    > >> + *
    > >> + * NOTE: This code will need to be revisited when kernel CET gets
    > >> + * implemented.
    > >> + */
    > >> + if (data->ret) {
    > >> + regs->sp -= sizeof(long);
    > >> + *(unsigned long *)regs->sp = data->ret;
    > >> + }
    >
    > You can’t do this. Depending on the alignment of the old RSP, which
    > is not guaranteed, this overwrites regs->cs. IRET goes boom.

    I don't get it; can you spell that out?

    The way I understand it is that we're at a location where a "E8 - Near
    CALL" instruction should be, and thus RSP should be the regular kernel
    stack, and the above simply does "PUSH ret", which is what that CALL
    would've done too.


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