On Thu, Mar 19, 2020 at 4:25 PM Peter Maydell peter.maydell@linaro.org wrote:
On Thu, 19 Mar 2020 at 15:13, Linus Walleij linus.walleij@linaro.org wrote:
On Tue, Mar 17, 2020 at 12:58 PM Peter Maydell peter.maydell@linaro.org 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