On Tue, 2014-08-05 at 15:28 +0800, Wang Nan wrote:
> This patch introduce kprobeopt for ARM 32.
>
> Limitations:
> - Currently only kernel compiled with ARM ISA is supported.
>
> - offset between probe point and kprobe pre_handler must not larger
> than 64MiB. Masami Hiramatsu suggests replacing 2 words, it will
> make things complex. Futher patch can make such optimization.
>
> I have tested this patch on qemu vexpress a9 platform.
>
> Kprobe opt on ARM is relatively simpler than kprobe opt on x86 because
> ARM instruction is always 4 bytes aligned. This patch replace probed
> instruction by a 'b', branch to trampoline code and then calls
> optimized_callback(). optimized_callback() calls kprobe_handler() to
> execute kprobe handler. It also emulate/simulate replaced instruction.
>
> When unregistering kprobe, the deferred manner of unoptimizer may leave
> branch instruction before optimizer is called. Different from x86_64,
> which only copy the probed insn after optprobe_template_end and
> reexecute them, this patch call singlestep to emulate/simulate the insn
> directly. Futher patch can optimize this behavior.
>
> Signed-off-by: Wang Nan <wangn...@huawei.com>
> Cc: Masami Hiramatsu <masami.hiramatsu...@hitachi.com>
I've not reviewed this patch properly as I don't have time at the moment
to study the details of how optprobes work, but I did give it quick look
over to check that it was using the helper functions like patch_text()
which it does :-). I also spotted one duplication of an existing
function, see inline comment below...
[...]
> +static inline int
> +arm_branch_to_addr(unsigned int *pinst, void *src, void *dest)
> +{
> + unsigned int inst = 0xea000000;
> + long offset = (unsigned long)(dest) -
> + ((unsigned long)(src) + 8);
> + if ((offset > 0x3fffffc) || (offset < -0x3fffffc)) {
> + printk(KERN_WARNING "Failed to instrument %pS to %pS\n", src,
> dest);
> + return -EINVAL;
> + }
> +
> + inst |= (((unsigned long)offset) >> 2) & (0x00ffffffUL);
> + *pinst = inst;
> + return 0;
> +}
> +
This looks remarkably similar to the code in arch/arm/kernel/insn.c so I
think you can probably just use the existing arm_gen_branch() function.
[...]
--
Tixy
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