On Thu, Mar 19, 2020 at 4:25 PM Peter Maydell <peter.maydell@xxxxxxxxxx> wrote: > On Thu, 19 Mar 2020 at 15:13, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote: > > On Tue, Mar 17, 2020 at 12:58 PM Peter Maydell <peter.maydell@xxxxxxxxxx> wrote: > > > What in particular does this personality setting affect? > > > My copy of the personality(2) manpage just says: > > > > > > PER_LINUX32 (since Linux 2.2) > > > [To be documented.] > > > > > > which isn't very informative. > > > > It's not a POSIX thing (not part of the Single Unix Specification) > > so as with most Linux things it has some fuzzy semantics > > defined by the community... > > > > I usually just go to the source. > > If we're going to decide that this is the way to say > "give me 32-bit semantics" we need to actually document > that and define in at least broad terms what we mean > by it, so that when new things are added that might or > might not check against the setting there is a reference > defining whether they should or not, and so that > userspace knows what it's opting into by setting the flag. > The kernel loves undocumented APIs but userspace > consumers of them are not so enamoured :-) OK I guess we can at least take this opportunity to add some kerneldoc to the include file. > As a concrete example, should "give me 32-bit semantics > via PER_LINUX32" mean "mmap should always return addresses > within 4GB" ? That would seem like it would make sense -- Incidentally that thing in particular has its own personality flag (personalities are additive, it's a bit schizophrenic) so PER_LINUX_32BIT is defined as: PER_LINUX_32BIT = 0x0000 | ADDR_LIMIT_32BIT, and that is specifically for limiting the address space to 32bit. There is also PER_LINUX32_3GB for a 3GB lowmem limit. Since the personality is kind of additive, if we want a flag *specifically* for indicating that we want 32bit hashes from the file system, there are bits left so we can provide that. Is this what we want to do? I just think we shouldn't decide on that lightly as we will be using up personality bug bits, but sometimes you have to use them. PER_LINUX32 as it stands means 32bit personality but very specifically does not include memory range limitations since that has its own flags. Yours, Linus Walleij
