On 13/06/2019 16:32, Szabolcs Nagy wrote: > On 13/06/2019 15:03, Vincenzo Frascino wrote: >> On 13/06/2019 13:28, Szabolcs Nagy wrote: >>> On 13/06/2019 12:16, Vincenzo Frascino wrote: >>>> On 13/06/2019 11:14, Szabolcs Nagy wrote: >>>>> On 13/06/2019 10:20, Catalin Marinas wrote: >>>>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote: >>>>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote: >>>>>>>> + - 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? > > [...], it is in the memory ranges privately owned by a > userspace process and it is obtained in one of the > following ways: > > - mmap done by the process itself, [...] > > - brk syscall done by the process itself. > (i.e. the heap area between the initial location > of the program break at process creation and its > current location.) > > - any memory mapped by the kernel [...] > > the data segment that's part of the process image is > already covered by the last point. > Thanks Szabolcs, I will update the document accordingly. -- Regards, Vincenzo
