x86: don't destroy %rbp on kernel-mode faults

From the code:

    "B stepping K8s sometimes report an truncated RIP for IRET exceptions
    returning to compat mode. Check for these here too."

The code then proceeds to truncate the upper 32 bits of %rbp. This means
that when do_page_fault() is finally called, its prologue,

    do_page_fault:
        push %rbp
        movl %rsp, %rbp

will put the truncated base pointer on the stack. This means that the
stack tracer will not be able to follow the base-pointer changes and
will see all subsequent stack frames as unreliable.

This patch changes the code to use a different register (%rcx) for the
checking and leaves %rbp untouched.

Signed-off-by: Vegard Nossum <[email protected]>
Signed-off-by: Pekka Enberg <[email protected]>
Acked-by: Arjan van de Ven <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Pekka Enberg <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions