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... >>>>> + - 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. > > I will add what you are suggesting about the heap area. >
