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    SubjectRe: [PATCH 0/3][RFC] ftrace: track dynamic ftrace update failures

    On Tue, 3 Jun 2008, Abhishek Sagar wrote:
    > > Have you also taken into account adding and removing of modules? This can
    > > cause some funny behaviours with ftrace that can be rather dangerous.
    > I've tried to accommodate them (fixed the wrong use of
    > kernel_text_address). This makes
    > init and module function records eligible to be freed. Only core
    > kernel function update failures now get tracked/remembered.
    > There might still be a need to find a way to track external modifiers
    > of mcount call-sites such as (but not limited to) kprobes. Consider
    > this (rather long) contrived scenario:
    > 1. Function foo gets recorded by ftrace.
    > 2. Mcount call site of foo is patched with a NOP by ftraced.
    > 3. foo gets 'enabled' by some ftrace filter rule. NOP changes to a
    > call to the function tracer.
    > 4. A kprobe is registered on the mcount call-site. It places a trap
    > generating instruction at the
    > mcount call site and stores the call-to-function-tracer instruction
    > elsewhere internally.
    > 5. Tracing is disabled by end user.
    > 6. Disabling of foo fails because Kprobe had placed a trap at the
    > mcount call site.
    > 7. foo still keeps getting traced because Kprobes will keep
    > single-stepping the call-to-function-tracer instruction each time the
    > mcount call site is executed.
    > Not sure what to do with such cases...

    Can we add an API to kprobes or perhaps a way to register code
    modification in ftrace, that ftrace can ignore? Let the two know about
    each other, but in a way that they dont need to know the internals of one

    -- Steve

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