Hi Ken, On Dec 17 10:34, Ken Brown via Cygwin wrote: > On 12/16/2024 2:58 PM, Ken Brown via Cygwin wrote: > > On 12/16/2024 8:32 AM, Corinna Vinschen via Cygwin wrote: > > > Right now, mmaping with PROT_NONE and then re-mmaping with PROT_WRITE > > > doesn't work. Cygwin implements PROT_NONE not as MAP_RESERVE, but as > > > MEM_COMMIT with PAGE_NOACCESS. mmap() doesn't check if the requested > > > pages are already allocated with PAGE_NOACCESS and then succeeds while > > > in fact just changing the page protection. This is basically what > > > you want. Right now, you'd have to call mprotect() instead. > > > > > > With anonymous mappings only, this is all just adding a bit of code to > > > mmap() to do what mprotect() does in this case. > > > > [...] > > > > > So only anonymous mappings would be possible, assuming we tweak mmap() > > > to check if the old mapping was anonymous either and then allow to > > > just change the page protection, as if mprotect has been called. > > > > > > And, funny enough, something pretty similar already exists in mmap(). > > > See mmap.cc, line 1051 and the mmap_list::try_map() method. Right > > > now it only checks if an anonymous mapping has been partially unmapped > > > and can be recycled. But it could be improved by allowing to recycle > > > the anonymous mapping either way, as long as the new mapping is also > > > anonymous and the SHARED/PRIVATE flags match. > > Thanks! This looks doable. I hope to get back to you with a patch in > > the not-too-distant future. > While looking at this, I came across a bug in the use of > mmap_list::try_map(): On success, the recycled pages are given the same > protection they had as part of the original mmap_record instead of the > protection requested in the mmap call. This is done in > mmap_record::map_pages.
Thanks for catching! > Would you like a separate patch to fix this, or should I just fix it as part > of the bigger project? Ideally a separate patch. If you see a chance to send it to cygwin-patches this week, we can even merge it into 3.5.5, which I'm planning to release end of this week (after that --> vacation :))) Thanks, Corinna -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
