• Michael Ellerman's avatar
    powerpc/module: Mark module stubs with a magic value · f17c4e01
    Michael Ellerman authored
    When a module is loaded, calls out to the kernel go via a stub which is
    generated at runtime. One of these stubs is used to call _mcount(),
    which is the default target of tracing calls generated by the compiler
    with -pg.
    
    If dynamic ftrace is enabled (which it typically is), another stub is
    used to call ftrace_caller(), which is the target of tracing calls when
    ftrace is actually active.
    
    ftrace then wants to disable the calls to _mcount() at module startup,
    and enable/disable the calls to ftrace_caller() when enabling/disabling
    tracing - all of these it does by patching the code.
    
    As part of that code patching, the ftrace code wants to confirm that the
    branch it is about to modify, is in fact a call to a module stub which
    calls _mcount() or ftrace_caller().
    
    Currently it does that by inspecting the instructions and confirming
    they are what it expects. Although that works, the code to do it is
    pretty intricate because it requires lots of knowledge about the exact
    format of the stub.
    
    We can make that process easier by marking the generated stubs with a
    magic value, and then looking for that magic value. Altough this is not
    as rigorous as the current method, I believe it is sufficient in
    practice.
    Reviewed-by: default avatarBalbir Singh <bsingharora@gmail.com>
    Reviewed-by: default avatarTorsten Duwe <duwe@suse.de>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    f17c4e01
ftrace.c 12.5 KB