lkml.org 
[lkml]   [2019]   [May]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
    On Mon, May 6, 2019 at 11:57 AM Steven Rostedt <rostedt@goodmis.org> wrote:
    >
    > You should have waited another week to open that merge window ;-)

    Hmm. I'm looking at it while the test builds happen, and since I don't
    see what's wrong in the low-level entry code, I'm looking at the
    ftrace code instead.

    What's going on here?

    *pregs = int3_emulate_call(regs, (unsigned
    long)ftrace_regs_caller);

    that line makes no sense. Why would we emulate a call to
    ftrace_regs_caller()? That function sets up a pt_regs, and then calls
    ftrace_stub().

    But we *have* pt_regs here already with the right values. Why isn't
    this just a direct call to ftrace_stub() from within the int3 handler?

    And the thing is, calling ftrace_regs_caller() looks wrong, because
    that's not what happens for *real* mcount handling, which uses that
    "create_trampoline()" to create the thing we're supposed to really
    use?

    Anyway, I simply don't understand the code, so I'm confused. But why
    is the int3 emulation creating a call that doesn't seem to match what
    the instruction that we're actually rewriting is supposed to do?

    IOW, it looks to me like ftrace_int3_handler() is actually emulating
    something different than what ftrace_modify_code() is actually
    modifying the code to do!

    Since the only caller of ftrace_modify_code() is update_ftrace_func(),
    why is that function not just saving the target and we'd emulate the
    call to that? Using anything else looks crazy?

    But as mentioned, I just don't understand the ftrace logic. It looks
    insane to me, and much more likely to be buggy than the very simple
    entry code.

    Linus

    \
     
     \ /
      Last update: 2019-05-06 21:54    [W:4.040 / U:0.080 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site