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    ftrace/MIPS: Add module support for C version of recordmcount · 412910cd
    Wu Zhangjin 提交于
    Since MIPS modules' address space differs from the core kernel space, to access
    the _mcount in the core kernel, the kernel functions in modules must use long
    call (-mlong-calls): load the _mcount address into one register and jump to the
    address stored by the register:
    
     c:  3c030000        lui     v1,0x0  <-------->  b label
               c: R_MIPS_HI16  _mcount
               c: R_MIPS_NONE  *ABS*
               c: R_MIPS_NONE  *ABS*
    10:  64630000        daddiu  v1,v1,0
              10: R_MIPS_LO16 _mcount
              10: R_MIPS_NONE *ABS*
              10: R_MIPS_NONE *ABS*
    14:	03e0082d 	move	at,ra
    18:	0060f809 	jalr	v1
    label:
    
    In the old Perl version of recordmcount, we only need to record the position of
    the 1st R_MIPS_HI16 type of _mcount, and later, in ftrace_make_nop(), replace
    the instruction in this position by a "b label" and in ftrace_make_call(),
    replace it back.
    
    But, the default C version of recordmcount records all of the _mcount symbols,
    so, we must filter the 2nd _mcount like the Perl version of recordmcount does.
    
    The C version of recordmcount copes with the symbols before they are linked, So
    It doesn't know the type of the symbols and therefore can not filter the
    symbols as the Perl version of recordmcount does. But as we can see above, the
    2nd _mcount symbols of the long call alawys follows the 1st _mcount symbol of
    the same long call, which means the offset from the 1st to the 2nd is fixed, it
    is 0x10-0xc = 4 here, 4 is the length of the 1st load instruciton, for MIPS has
    fixed length of instructions, this offset is always 4.
    
    And as we know, the _mcount is inserted into the entry of every kernel
    function, the offset between the other _mcount's is expected to be always
    bigger than 4. So, to filter the 2ns _mcount symbol of the long call, we can
    simply check the offset between two _mcount symbols, If it is 4, then, filter
    the 2nd _mcount symbol.
    
    To avoid touching too much code, an 'empty' function fn_is_fake_mcount() is
    added for all of the archs, and the specific archs can override it via chaning
    the function pointer: is_fake_mcount in do_file() with the e_machine. e.g. This
    patch adds MIPS_is_fake_mcount() to override the default fn_is_fake_mcount()
    pointed by is_fake_mcount.
    
    This fn_is_fake_mcount() checks if the _mcount symbol is fake, e.g. the 2nd
    _mcount symbol of the long call is fake, for there are 2 _mcount symbols mapped
    to one real mcount call, so, one of them is fake and must be filtered.
    
    This fn_is_fake_mcount() is called in sift_rel_mcount() after finding the
    _mcount symbols and before adding the _mcount symbol into mrelp, so, it can
    prevent the fake mcount symbol going into the last __mcount_loc table.
    Signed-off-by: NWu Zhangjin <wuzhangjin@gmail.com>
    LKML-Reference: <b866f0138224340a132d31861fa3f9300dee30ac.1288176026.git.wuzhangjin@gmail.com>
    Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
    Signed-off-by: NRalf Baechle <ralf@linux-mips.org>
    412910cd
recordmcount.h 13.2 KB