Re: [PATCH v2 7/7] arm64: uprobes - ARM32 instruction probing

[Maciej Slodczyk]

Hi Robin,

Thank you for having a look at my patchset.

On 27.09.2018 19:01, Robin Murphy wrote:
> On 26/09/18 13:12, Maciej Slodczyk wrote:
> [...]
>> @@ -38,16 +78,44 @@ int arch_uprobe_analyze_insn(struct arch_uprobe 
>> *auprobe, struct mm_struct *mm,
>> ÂÂÂÂÂÂÂÂÂ unsigned long addr)
>> Â {
>> ÂÂÂÂÂ probes_opcode_t insn;
>> +ÂÂÂ enum probes_insn retval;
>> +ÂÂÂ unsigned int bpinsn;
>> -ÂÂÂ /* TODO: Currently we do not support AARCH32 instruction probing */
>> -ÂÂÂ if (mm->context.flags & MMCF_AARCH32)
>> -ÂÂÂÂÂÂÂ return -ENOTSUPP;
>> -ÂÂÂ else if (!IS_ALIGNED(addr, AARCH64_INSN_SIZE))
>> +ÂÂÂ insn = *(probes_opcode_t *)(&auprobe->insn[0]);
>> +
>> +ÂÂÂ if (!IS_ALIGNED(addr, AARCH64_INSN_SIZE))
>> ÂÂÂÂÂÂÂÂÂ return -EINVAL;
>> -ÂÂÂ insn = *(probes_opcode_t *)(&auprobe->insn[0]);
>> +ÂÂÂ /* check if AARCH32 */
>> +ÂÂÂ if (is_compat_task()) {
>> +
>> +ÂÂÂÂÂÂÂ /* Thumb is not supported yet */
>> +ÂÂÂÂÂÂÂ if (addr & 0x3)
> 
> I'm only skimming, so forgive me if I'm missing something which should 
> be obvious, but this has a big red flag all over it. If "addr" is the 
> actual instruction address (or even a branch target, for a 
> non-interworking branch), plenty of Thumb instructions will just happen 
> to lie at 4-byte-aligned addresses anyway.
> 
That's the same way Thumb instructions are filtered out in arch/arm 
uprobes and kprobes code. I believe that at this point all Thumb 
instruction have bit 0 set. Please correct me if I'm wrong.

> Furthermore, how would this check ever catch anything anyway given 
> !IS_ALIGNED(addr, AARCH64_INSN_SIZE) above?

You're right, there's no point in checking it here. I'll fix it in v3.

Thank you,
Maciej