On 13/06/2019 13:28, Szabolcs Nagy wrote: > On 13/06/2019 12:16, Vincenzo Frascino wrote: >> Hi Szabolcs, >> >> thank you for your review. >> >> On 13/06/2019 11:14, Szabolcs Nagy wrote: >>> On 13/06/2019 10:20, Catalin Marinas wrote: >>>> Hi Szabolcs, >>>> >>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote: >>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote: >>>>>> +2. ARM64 Tagged Address ABI >>>>>> +--------------------------- >>>>>> + >>>>>> +From the kernel syscall interface prospective, we define, for the purposes >>>>> ^^^^^^^^^^^ >>>>> perspective >>>>> >>>>>> +of this document, a "valid tagged pointer" as a pointer that either it has >>>>>> +a zero value set in the top byte or it has a non-zero value, it is in memory >>>>>> +ranges privately owned by a userspace process and it is obtained in one of >>>>>> +the following ways: >>>>>> + - mmap() done by the process itself, where either: >>>>>> + * flags = MAP_PRIVATE | MAP_ANONYMOUS >>>>>> + * flags = MAP_PRIVATE and the file descriptor refers to a regular >>>>>> + file or "/dev/zero" >>>>> >>>>> this does not make it clear if MAP_FIXED or other flags are valid >>>>> (there are many map flags i don't know, but at least fixed should work >>>>> and stack/growsdown. i'd expect anything that's not incompatible with >>>>> private|anon to work). >>>> >>>> Just to clarify, this document tries to define the memory ranges from >>>> where tagged addresses can be passed into the kernel in the context >>>> of TBI only (not MTE); that is for hwasan support. FIXED or GROWSDOWN >>>> should not affect this. >>> >>> yes, so either the text should list MAP_* flags that don't affect >>> the pointer tagging semantics or specify private|anon mapping >>> with different wording. >>> >> >> Good point. Could you please propose a wording that would be suitable for this case? > > i don't know all the MAP_ magic, but i think it's enough to change > the "flags =" to > > * flags have MAP_PRIVATE and MAP_ANONYMOUS set or > * flags have MAP_PRIVATE set and the file descriptor refers to... > > Fine by me. I will add it the next iterations. >>>>>> + - a mapping below sbrk(0) done by the process itself >>>>> >>>>> doesn't the mmap rule cover this? >>>> >>>> IIUC it doesn't cover it as that's memory mapped by the kernel >>>> automatically on access vs a pointer returned by mmap(). The statement >>>> above talks about how the address is obtained by the user. >>> >>> ok i read 'mapping below sbrk' as an mmap (possibly MAP_FIXED) >>> that happens to be below the heap area. >>> >>> i think "below sbrk(0)" is not the best term to use: there >>> may be address range below the heap area that can be mmapped >>> and thus below sbrk(0) and sbrk is a posix api not a linux >>> syscall, the libc can implement it with mmap or whatever. >>> >>> i'm not sure what the right term for 'heap area' is >>> (the address range between syscall(__NR_brk,0) at >>> program startup and its current value?) >>> >> >> I used sbrk(0) with the meaning of "end of the process's data segment" not >> implying that this is a syscall, but just as a useful way to identify the mapping. >> I agree that it is a posix function implemented by libc but when it is used with >> 0 finds the current location of the program break, which can be changed by brk() >> and depending on the new address passed to this syscall can have the effect of >> allocating or deallocating memory. >> >> Will changing sbrk(0) with "end of the process's data segment" make it more clear? > > i don't understand what's the relevance of the *end* > of the data segment. > > i'd expect the text to say something about the address > range of the data segment. > > i can do > > mmap((void*)65536, 65536, PROT_READ|PROT_WRITE, MAP_FIXED|MAP_SHARED|MAP_ANON, -1, 0); > > and it will be below the end of the data segment. > As far as I understand the data segment "lives" below the program break, hence it is a way of describing the range from which the user can obtain a valid tagged pointer. Said that, I am not really sure on how do you want me to document this (my aim is for this to be clear to the userspace developers). Could you please propose something? >> >> I will add what you are suggesting about the heap area. >> -- Regards, Vincenzo
