Re: [PATCH v2 2/6] mips dynamic function tracer support

[wu zhangjin <wuzhangjin@gmail.com>]

hi,

sorry, I'm so late to reply your E-mail, a little busy these days.

On Sun, May 31, 2009 at 2:47 PM, Wang Liming <liming.wang@windriver.com> wrote:
> hi Wu,
>
> I have one question about dynamic ftrace support for modules on mips arch.
> I have also ported a dynamic ftrace of mips before, but when I using modules
> with dynamic ftrace enabled in mips, I have following error:
>
> # insmod ./usbcore
> module usbcore: relocation overflow
> insmod: cannot insert './usbcore.ko': invalid module format
> #
>
> I have no time to test your patch, but I think your patch also has this
> problem.
>
> Below I explain possibility of this problem in details:
>
> We know that on mips, kernel symbol will locate in 0x80000000~0x90000000,
> and modules' symbol will locate in >0xc0000000.
>
> On mips, gcc will insert following assembler code before every functions:
>
>    move    at,ra
>    jal     0 <usb_ifnum_to_if>
>    10: R_MIPS_26   _mcount
>
>
> Here "_mcount" is a kernel symbol in 0x80000000~0x90000000, for
> example at 0x8027a740.
>
> When loading a module, kernel should relocate "jal 0" instruction to jump to
> the kernel's "_mcount" address, 0x8027a740. But "jal 0" is a 26 bits offset
> jump
> instruction, it only can be relocated to jump to modules' symbol address
> space(>0xc0000000) and it can't be relocated to jump to kernel symbol
> address
> space(such as 0x8027a740), because offset is 32 bits(is 0xc0000000 -
> 0x80000000). We can't modify this instruction to jump to a 32 offset address
> except that we modify gcc compiler to insert other assembler codes.
>
> So, when installing modules, the kernel will try to relocate "jal 0"
> instruction
>  to instruction of jumping to >0xc0000000, but it finds that _mcount's
> address
> is 0x8027a740 and then print error indicated that it can't be reloacted.
>
> The following source code is the checking place:
>
>
> ----------arch/mips/kernel/module.c:apply_r_mips_26_rel()-------------------
>
> static int apply_r_mips_26_rel(struct module *me, u32 *location, Elf_Addr v)
> {
>    ...
>    if ((v & 0xf0000000) != (((unsigned long)location + 4) & 0xf0000000)) {
>                printk(KERN_ERR
>                       "module %s: relocation overflow, v:%x, location:%x\n",
>                       me->name, v, (unsigned long)location);
>                return -ENOEXEC;
>        }
>
>    *location = (*location & ~0x03ffffff) |
>                    ((*location + (v >> 2)) & 0x03ffffff);
>
> }
> ----------arch/mips/kernel/module.c:apply_r_mips_26_rel()-------------------
>
> v is kernel _mcount's address, location is the address of the instrution
> that should be relocated;
>
> To resolve this problem, we may need to do more work, either on gcc or on
> the kernel. So I want to hear your test result and if you have solution,
> please let me know.
>

yes, current version of mips-specific dynamic ftrace not support modules yet.

there is similar solution implemented in PowerPC(something named trampoline),
although I did not look into it, but I'm sure we can implement the
mips-specific one
via imitating it.

and currently, I'm planning to fix it in the -v3 of mips-specific
ftrace support.

best wishes,
Wu Zhangjin