Trying to answer both questions to this patch on this one. On Mon, 2023-01-23 at 10:28 +0100, David Hildenbrand wrote: > > +/* > > + * Normally COW memory can result in Dirty=1,Write=0 PTEs. But in > > the case > > + * of X86_FEATURE_USER_SHSTK, the software COW bit is used, since > > the > > + * Dirty=1,Write=0 will result in the memory being treated as > > shadow stack > > + * by the HW. So when creating COW memory, a software bit is used > > + * _PAGE_BIT_COW. The following functions pte_mkcow() and > > pte_clear_cow() > > + * take a PTE marked conventionally COW (Dirty=1) and transition > > it to the > > + * shadow stack compatible version of COW (Cow=1). > > + */ > > TBH, I find that all highly confusing. > > Dirty=1,Write=0 does not indicate a COW page reliably. You could > have > both, false negatives and false positives. > > False negative: fork() on a clean anon page. > > False positives: wrpotect() of a dirty anon page. > > > I wonder if it really has to be that complicated: what you really > want > to achieve is to disallow "Dirty=1,Write=0" if it's not a shadow > stack > page, correct? The other thing is to save that the PTE is/was Dirty=1 somewhere (for non-shadow stack memory). A slightly different but related thing. But losing that information would would introduce differences for pte_dirty() between when shadow stack was enabled or not. GUP/COW doesn't need this anymore but there are lots of other places it gets checked. Perhaps following your GUP changes, _PAGE_COW is just now the wrong name for it. _PAGE_SAVED_DIRTY maybe?
