On Thu, Nov 05, 2015 at 08:20:42AM -0800, Laura Abbott wrote: > On 11/05/2015 01:46 AM, Russell King - ARM Linux wrote: > >On Wed, Nov 04, 2015 at 05:00:39PM -0800, Laura Abbott wrote: > >>Currently, read only permissions are not being applied even > >>when CONFIG_DEBUG_RODATA is set. This is because section_update > >>uses current->mm for adjusting the page tables. current->mm > >>need not be equivalent to the kernel version. Use pgd_offset_k > >>to get the proper page directory for updating. > > > >What are you trying to achieve here? You can't use these functions > >at run time (after the first thread has been spawned) to change > >permissions, because there will be multiple copies of the kernel > >section mappings, and those copies will not get updated. > > > >In any case, this change will probably break kexec and ftrace, as > >the running thread will no longer see the updated page tables. > > > > I think I was hitting that exact problem with multiple copies > not getting updated. The section_update code was being called > and I was seeing the tables get updated but nothing was being > applied when I tried to write to text or check the debugfs > page table. The current flow is: > > rest_init -> kernel_thread(kernel_init) and from that thread > mark_rodata_ro. So mark_rodata_ro is always going to happen > in a thread. > > Do we need to update for both init_mm and the first running > thread? The "first running thread" is merely coincidental for things like kexec. Hmm. Actually, I think the existing code _should_ be fine. At the point where mark_rodata_ro() is, we should still be using init_mm, so updating the current threads page tables should actually be updating the swapper_pg_dir. The other cases (kexec and ftrace) I think are also fine as they stand - we want to be changing the currently active page tables there. So, I really think we do not want to be using pgd_offset_k() here at all - but we need to find out what's changed to cause (presumably) mark_rodata_ro() not to be hitting the swapper page tables. Maybe some debug in mark_rodata_ro() to find out what current->mm is? -- FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>