On Fri, 10 Nov 2017 12:08:35 +0000 David Laight <David.Laight@xxxxxxxxxx> wrote: > From: Matthew Wilcox > > Sent: 09 November 2017 19:44 > > > > On Fri, Nov 10, 2017 at 04:15:26AM +1100, Nicholas Piggin wrote: > > > So these semantics are what we're going with? Anything that does mmap() is > > > guaranteed of getting a 47-bit pointer and it can use the top 17 bits for > > > itself? Is intended to be cross-platform or just x86 and power specific? > > > > It is x86 and powerpc specific. The arm64 people have apparently stumbled > > across apps that expect to be able to use bit 48 for their own purposes. > > And their address space is 48 bit by default. Oops. > > (Do you mean 49bit?) I think he meant bit 47, which makes sense because they were probably ported from x86-64 with 47 bit address. That seems to be why x86-64 5-level and powerpc decided to limit to a 47 bit address space by default. > > Aren't such apps just doomed to be broken? Well they're not portable but they are not broken if virtual address is limited. > > ISTR there is something on (IIRC) sparc64 that does a 'match' > on the high address bits to make it much harder to overrun > one area into another. I'm not sure about that but I think the problem would be the app masking out bits from the pointer for its own use before ever dereferencing it. Thanks, Nick
