Messages in this thread | | | Subject | Re: [PATCH v5 4/4] sparc64: Add support for ADI (Application Data Integrity) | From | Khalid Aziz <> | Date | Wed, 25 Jan 2017 15:57:36 -0700 |
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On 01/25/2017 03:50 PM, Rob Gardner wrote: > On 01/25/2017 03:20 PM, Khalid Aziz wrote: >> On 01/25/2017 03:00 PM, Rob Gardner wrote: >>> On 01/25/2017 12:57 PM, Khalid Aziz wrote: >>>> >>>> @@ -157,6 +158,24 @@ int __get_user_pages_fast(unsigned long start, >>>> int nr_pages, int write, >>>> pgd_t *pgdp; >>>> int nr = 0; >>>> +#ifdef CONFIG_SPARC64 >>>> + if (adi_capable()) { >>>> + long addr = start; >>>> + >>>> + /* If userspace has passed a versioned address, kernel >>>> + * will not find it in the VMAs since it does not store >>>> + * the version tags in the list of VMAs. Storing version >>>> + * tags in list of VMAs is impractical since they can be >>>> + * changed any time from userspace without dropping into >>>> + * kernel. Any address search in VMAs will be done with >>>> + * non-versioned addresses. Ensure the ADI version bits >>>> + * are dropped here by sign extending the last bit before >>>> + * ADI bits. IOMMU does not implement version tags. >>>> + */ >>>> + addr = (addr << (long)adi_nbits()) >> (long)adi_nbits(); >>> >>> >>> So you are depending on the sign extension to clear the ADI bits... but >>> this only happens if there is a zero in that "last bit before ADI bits". >>> If the last bit is a 1, then the ADI bits will be set instead of >>> cleared. That seems like an unintended consequence given the comment. I >>> am aware of the value of adi_nbits() and of the number of valid bits in >>> a virtual address on the M7 processor, but wouldn't using 'unsigned >>> long' for everything here guarantee the ADI bits get cleared regardless >>> of the state of the last non-adi bit? >> >> Sign extension is the right thing to do. MMU considers values of 0 and >> 15 for bits 63-60 to be untagged addresses and expects bit 59 to be >> sign-extended for untagged virtual addresses. The code I added is >> explicitly meant to sign-extend, not zero out the top 4 bits. > > OK, that wasn't perfectly clear from the comment, which said "version > bits are dropped". > > So sign extending will produce an address that the MMU can use, but will > it produce an address that will allow a successful search in the page > tables? ie, was this same sign extending done when first handing out > that virtual address to the user? >
Yes to both your questions. When virtual addresses are handed out, the last implemented virtual address bit is sign-extended. Sign-extending when dropping version bits preserves that original sign-extension. This is why MMU considers tag values of 0 as well as 15 to be invalid because they both represent sign-extension of the last implemented virtual address.
-- Khalid
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