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    SubjectRe: [PATCH 08/13] x86/dumpstack: When OOPSing, rewind the stack before do_exit
    On Wed, Jun 15, 2016 at 05:28:30PM -0700, Andy Lutomirski wrote:
    > If we call do_exit with a clean stack, we greatly reduce the risk of
    > recursive oopses due to stack overflow in do_exit, and we allow
    > do_exit to work even if we OOPS from an IST stack. The latter gives
    > us a much better chance of surviving long enough after we detect a
    > stack overflow to write out our logs.
    >
    > I intentionally separated this from the preceding patch that
    > disables do_exit-on-OOPS on IST stacks. This way, if we need to
    > revert this patch, we still end up in an acceptable state wrt stack
    > overflow handling.
    >
    > Signed-off-by: Andy Lutomirski <luto@kernel.org>
    > ---
    > arch/x86/entry/entry_32.S | 11 +++++++++++
    > arch/x86/entry/entry_64.S | 11 +++++++++++
    > arch/x86/kernel/dumpstack.c | 13 +++++++++----
    > 3 files changed, 31 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
    > index 983e5d3a0d27..1499db695a88 100644
    > --- a/arch/x86/entry/entry_32.S
    > +++ b/arch/x86/entry/entry_32.S
    > @@ -1153,3 +1153,14 @@ ENTRY(async_page_fault)
    > jmp error_code
    > END(async_page_fault)
    > #endif
    > +
    > +ENTRY(rewind_stack_do_exit)
    > + /* Prevent any naive code from trying to unwind to our caller. */
    > + xorl %ebp, %ebp
    > +
    > + movl PER_CPU_VAR(cpu_current_top_of_stack), %esi
    > + leal -TOP_OF_KERNEL_STACK_PADDING-PT_OLDSS(%esi), %esp
    > +
    > + call do_exit
    > +1: jmp 1b
    > +END(rewind_stack_do_exit)
    > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
    > index 9ee0da1807ed..394cad73e890 100644
    > --- a/arch/x86/entry/entry_64.S
    > +++ b/arch/x86/entry/entry_64.S
    > @@ -1423,3 +1423,14 @@ ENTRY(ignore_sysret)
    > mov $-ENOSYS, %eax
    > sysret
    > END(ignore_sysret)
    > +
    > +ENTRY(rewind_stack_do_exit)
    > + /* Prevent any naive code from trying to unwind to our caller. */
    > + xorl %ebp, %ebp
    > +
    > + movq PER_CPU_VAR(cpu_current_top_of_stack), %rax
    > + leaq -TOP_OF_KERNEL_STACK_PADDING-SS(%rax), %rsp

    I think this should be:

    leaq -TOP_OF_KERNEL_STACK_PADDING-SIZEOF_PTREGS, %rsp

    That way when it calls do_exit(), the stack frame will be placed at the
    conventional spot where a smart unwinder would expect to find it.

    --
    Josh

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