Re: ftrace for MIPS

From: Steven Rostedt
Date: Tue Oct 20 2009 - 12:22:02 EST


On Tue, 2009-10-20 at 23:31 +0800, Wu Zhangjin wrote:

> Just added tracing_stop() and tracing_start() around

That seems a bit heavy handed. I still think writing it in "asm" the way
x86 and powerpc do is the best.

> probe_kernel_read(), it works(not hang again), and i can get the stack
> address of the ra register(return address) now, but failed when trying
> to hijack the return address via writing &return_to_handler in the stack
> address:
>
> I can write hijack some of the addresses, but failed with this error at
> last:
>
> Unable to handle kernel paging request at 0000000000000000, epc =
> 0000000000000000, ra = 000000000000.

hmm, looks like you jumped to "0"

>
> Need to check which registers is missing when saving/restoring for
> _mcount:
>
>
> NESTED(ftrace_graph_caller, PT_SIZE, ra)
> MCOUNT_SAVE_REGS
> PTR_S v0, PT_R2(sp)
>
> MCOUNT_SET_ARGS
> jal prepare_ftrace_return
> nop
>
> /* overwrite the parent as &return_to_handler: v0 -> $1(at) */
> move $1, v0

I'm confused here? I'm not exactly sure what the above is doing. Is $1 a
register (AT)? And how is this register used before calling mcount?

>
> PTR_L v0, PT_R2(sp)
> MCOUNT_RESTORE_REGS
> RETURN_BACK
> END(ftrace_graph_caller)
>
> .align 2
> .globl return_to_handler
> return_to_handler:
> PTR_SUBU sp, PT_SIZE
> PTR_S v0, PT_R2(sp)

BTW, is v0 the only return register? I know x86 can return two different
registers depending on what it returns. What happens if a function
returns a 64 bit value on a 32bit box? Does it use two registers for
that?

-- Steve

>
> jal ftrace_return_to_handler
> nop
>
> /* restore the real parent address: v0 -> ra */
> move ra, v0
>
> PTR_L v0, PT_R2(sp)
> PTR_ADDIU sp, PT_SIZE
>
> jr ra
>
> ...
>
> .macro MCOUNT_SAVE_REGS
> PTR_SUBU sp, PT_SIZE
> PTR_S ra, PT_R31(sp)
> PTR_S AT, PT_R1(sp)
> PTR_S a0, PT_R4(sp)
> PTR_S a1, PT_R5(sp)
> PTR_S a2, PT_R6(sp)
> PTR_S a3, PT_R7(sp)
> #ifdef CONFIG_64BIT
> PTR_S a4, PT_R8(sp)
> PTR_S a5, PT_R9(sp)
> PTR_S a6, PT_R10(sp)
> PTR_S a7, PT_R11(sp)
> #endif
> .endm
>
> .macro MCOUNT_RESTORE_REGS
> PTR_L ra, PT_R31(sp)
> PTR_L AT, PT_R1(sp)
> PTR_L a0, PT_R4(sp)
> PTR_L a1, PT_R5(sp)
> PTR_L a2, PT_R6(sp)
> PTR_L a3, PT_R7(sp)
> #ifdef CONFIG_64BIT
> PTR_L a4, PT_R8(sp)
> PTR_L a5, PT_R9(sp)
> PTR_L a6, PT_R10(sp)
> PTR_L a7, PT_R11(sp)
> #endif
> PTR_ADDIU sp, PT_SIZE
>
> Regards,
> Wu Zhangjin
>

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