Hi Masami and everyone,
When checking my code I found a problem: if we replace a stack operatinon
instruction,
it is possible that the emulate execution of such instruction destroy the stack
used
by kprobeopt:
> +
> +asm (
> + ".global optprobe_template_entry\n"
> + "optprobe_template_entry:\n"
> + " sub sp, sp, #80\n"
> + " stmia sp, {r0 - r14} \n"
Here, trampoline code sub sp with 80 (0x50, I choose this number without much
thinking), and then
use stmia to push r0 - r14 (registers except pc) onto the stack. Assume the
original sp is
0xd0000050, the stack becomes:
0xd0000000: r0
0xd0000004: r1
0xd0000008: r2
...
0xd0000038: r14
0xd000003c: r15 (place holder)
0xd0000040: cpsr (place holder)
0xd0000044: ?
0xd0000048: ?
0xd000004c: ?
0xd0000050: original stack
If the replaced code operates stack, for example, push {r0 - r10}, it will
overwrite our register.
For that reason, sub sp, #80 is not enough, we need at least 64 bytes stack
space, so the first instruction
here should be sub sp, #128.
However, it increase stack requirement. Moreover, although rare, there may be
sp relative addressing,
such as: str r1, [sp, #-132].
To make every situations safe, do you think we need to alloc a pre-cpu optprobe
private stack?
For example:
str sp, [pc, #??] (store original sp first)
ldr sp, [pc, #??] (load pre-cpu stack)
sub sp, #68
stmia sp, {r0 - r12}
... (fix sp and pc in stack)
ldmia sp, {r0 - r15}
optprobe_template_sp:
1: .long 0 (placeholder for saved sp)
optprobe_template_private_stack:
2: .long 0 (placeholder for per-cpu private stack)
optprobe_template_pc:
3: .long 0 (placeholder for pc)
> + " add r3, sp, #80\n"
> + " str r3, [sp, #52]\n"
> + " mrs r4, cpsr\n"
> + " str r4, [sp, #64]\n"
> + " mov r1, sp\n"
> + " ldr r0, 1f\n"
> + " ldr r2, 2f\n"
> + " blx r2\n"
> + " ldr r1, [sp, #64]\n"
> + " msr cpsr_fs, r1\n"
> + " ldmia sp, {r0 - r15}\n"
> + ".global optprobe_template_val\n"
> + "optprobe_template_val:\n"
> + "1: .long 0\n"
> + ".global optprobe_template_call\n"
> + "optprobe_template_call:\n"
> + "2: .long 0\n"
> + ".global optprobe_template_end\n"
> + "optprobe_template_end:\n");
> +
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