• Michael Ellerman's avatar
    powerpc/livepatch: Add live patching support on ppc64le · 85baa095
    Michael Ellerman authored
    Add the kconfig logic & assembly support for handling live patched
    functions. This depends on DYNAMIC_FTRACE_WITH_REGS, which in turn
    depends on the new -mprofile-kernel ftrace ABI, which is only supported
    currently on ppc64le.
    
    Live patching is handled by a special ftrace handler. This means it runs
    from ftrace_caller(). The live patch handler modifies the NIP so as to
    redirect the return from ftrace_caller() to the new patched function.
    
    However there is one particularly tricky case we need to handle.
    
    If a function A calls another function B, and it is known at link time
    that they share the same TOC, then A will not save or restore its TOC,
    and will call the local entry point of B.
    
    When we live patch B, we replace it with a new function C, which may
    not have the same TOC as A. At live patch time it's too late to modify A
    to do the TOC save/restore, so the live patching code must interpose
    itself between A and C, and do the TOC save/restore that A omitted.
    
    An additionaly complication is that the livepatch code can not create a
    stack frame in order to save the TOC. That is because if C takes > 8
    arguments, or is varargs, A will have written the arguments for C in
    A's stack frame.
    
    To solve this, we introduce a "livepatch stack" which grows upward from
    the base of the regular stack, and is used to store the TOC & LR when
    calling a live patched function.
    
    When the patched function returns, we retrieve the real LR & TOC from
    the livepatch stack, restore them, and pop the livepatch "stack frame".
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Reviewed-by: default avatarTorsten Duwe <duwe@suse.de>
    Reviewed-by: default avatarBalbir Singh <bsingharora@gmail.com>
    85baa095
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