http://sourceware.org/bugzilla/show_bug.cgi?id=12376
--- Comment #5 from dave at hiauly1 dot hia.nrc.ca <dave at hiauly1 dot hia.nrc.ca> 2011-02-14 03:40:21 UTC --- On Mon, 14 Feb 2011, amodra at gmail dot com wrote: > > These in fact point to the same page in physical memory. > > Really? 00010000-00011000 and 00011000-00012000 are not different pages? They map to the same page as far as I can tell (both maps appear in the list iterated using vma_prio_tree_foreach(mpnt, &iter, &mapping->i_mmap, pgoff, pgoff)). This can also be seen by looking at /proc/$PID/maps. When multiple shared writeable mappings exist, I believed they are COWed. So, effectively only one map is writeable. Non equivalent aliases are a problem for architectures such as PA8800/PA8900. They don't support non equivalent aliases in the sense that a write doesn't invalidate non equivalent aliases. The only thing that saves us is the former address range is write protected, and it's rare to try to read using the text map. It seems possible that the text map could be corrupted via the data map. So, this might be a security issue. The V-Class machines are even worse than PA8800/PA8900 because they don't support non equivalent aliases regardless of whether they are read-only or not. These non equivalent aliases occur typically on the boundary page between text and data. The linux dynamic loader mmaps these regions as MAP_FIXED. They are not mapped with MAP_SHARED but it seems the maps are shared for shared libraries. So far, it seems the hppa linux dynamic loader always maps shared pages with equivalent aliases except for the boundary page. I think this is potentially an issue for certain MIPS and ARM cpus but I don't know the details on whether they support non equivalent aliases or not. As far as I can tell, the same occurs for x86, etc, but I don't think the non equivalent aliases matter, at least on linux. On the other hand, it looks like windows starts sections on page boundaries. Probably, it would be best if load segments were aranged in executables to optionally start on a file page boundary. This would avoid the double flush and having two non equivalent address ranges map to the same page. Don't really want to start all sections on a page boundary as this would waste a lot of file space. I have looked a bit at trying to do this, but don't have a solution at the moment. Dave -- Configure bugmail: http://sourceware.org/bugzilla/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are on the CC list for the bug. _______________________________________________ bug-binutils mailing list bug-binutils@gnu.org http://lists.gnu.org/mailman/listinfo/bug-binutils
