Messages in this thread | | | Date | Tue, 28 Jan 2014 13:39:44 +0800 | From | Ren Qiaowei <> | Subject | Re: [PATCH v3 2/4] x86, mpx: hook #BR exception handler to allocate bound tables |
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On 01/28/2014 01:21 PM, Andy Lutomirski wrote: > On Mon, Jan 27, 2014 at 7:35 PM, Ren Qiaowei <qiaowei.ren@intel.com> wrote: >> On 01/28/2014 04:36 AM, Andy Lutomirski wrote: >>>> >>>> + bd_entry = status & MPX_BNDSTA_ADDR_MASK; >>>> + if ((bd_entry >= bd_base) && (bd_entry < bd_base + bd_size)) >>>> + allocate_bt(bd_entry); >>> >>> >>> What happens if this fails? Retrying forever isn't very nice. >>> >> If allocation of the bound table fail, the related entry in the bound >> directory is still invalid. The following access to this entry still produce >> #BR fault. >> > > By the "following access" I think you mean the same instruction that > just trapped -- it will trap again because the exception hasn't been > fixed up. Then mmap will fail again, and you'll retry again, leading > to an infinite loop. > I don't mean the same instruction that just trapped.
> I think that failure to fix up the exception should either let the > normal bounds error through or should raise SIGBUS. > Maybe we need HPA help answer this question. Peter, what do you think about it? If allocation of the bound table fail, what should we do?
>> >>>> + if (!user_mode(regs)) { >>>> + if (!fixup_exception(regs)) { >>>> + tsk->thread.error_code = error_code; >>>> + tsk->thread.trap_nr = X86_TRAP_BR; >>>> + die("bounds", regs, error_code); >>>> + } >>> >>> >>> Why the fixup? Unless I'm missing something, the kernel has no business >>> getting #BR on access to a user address. >>> >>> Or are you adding code to allow the kernel to use MPX itself? If so, >>> shouldn't this use an MPX-specific fixup to allow normal C code to use >>> this stuff? >>> >> It checks whether #BR come from user-space. You can see do_trap_no_signal(). > > Wasn't #BR using do_trap before? do_trap doesn't call > fixup_exception. I don't see why it should do it now. (I also don't > think it should come from kernel space until someone adds kernel-mode > MPX support.) > do_trap() -> do_trap_no_signal() call similar code to check if the fault occurred in userspace or kernel space. You can see previous discussion for the first version of this patchset.
Thanks, Qiaowei
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