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    SubjectRe: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)
    On Mon, Aug 22, 2011 at 6:13 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
    >
    > *UGH*.  OK,
    >        1) I'm an idiot; int_ret_from_sys_call does *not* usually step on
    > rbp (it's callee-saved).  So normally ebp is left as is on the way out,
    > which is why we don't see stuff getting buggered left, right and center.

    Check.

    And the system call restart should actually work fine too, because at
    syscall entry we save %ebp *both* in the slot for ebp and ecx when we
    enter the first time. So the second time, we'll re-load the third
    argument from ebp again, but that's fine - it's still going to be the
    right value. Yes? No?

    However, I note that the cstar entrypont has a comment about not saving ebp:

    * %ebp Arg2 [note: not saved in the stack frame, should not be touched]

    which sounds odd. Why don't we save it? If we take a signal handler
    there, don't we want %ebp on the kernel stack in pt_regs, in order to
    do everything right?

    Now I'm *really* confused.

    Linus
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