Messages in this thread | | | Subject | mprotect pgprot handling weirdness | From | Benjamin Herrenschmidt <> | Date | Tue, 06 Apr 2010 15:09:26 +1000 |
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Hi folks !
While looking at untangling a bit some of the mess with vm_flags and pgprot (*), I notices a few things I can't quite explain... they may .. or may not be bugs, but I though it was worth mentioning:
- In mprotect_fixup() :
/* * vm_flags and vm_page_prot are protected by the mmap_sem * held in write mode. */ vma->vm_flags = newflags; vma->vm_page_prot = pgprot_modify(vma->vm_page_prot, vm_get_page_prot(newflags));
if (vma_wants_writenotify(vma)) { vma->vm_page_prot = vm_get_page_prot(newflags & ~VM_SHARED); dirty_accountable = 1; }
So as you can see above, we take great care (using pgprot_modify) to avoid blasting away some PAT related flags on x86 (no other arch implements pgprot_modify() today).... but if we hit vma_wants_writenotify(), then we unconditionally override the entire vma->vm_page_prot field with some new prot bits born of the new vm_flags. That sounds odd...
- in sys_mprotect:
newflags = vm_flags | (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
Do I read correctly that this means we cannot -remove- any flag than VM_READ, VM_WRITE or VM_EXEC ? That means that we cannot remove PROT_SAO which gets turned into VM_SAO on powerpc ... Yet another reason to take those arch specific mapping attributes out of the vm_flags.
(*) Right now it's near impossible to add arch specific PROT_* bits to mmap/mprotect for fancy things like cachability attributes, or other nifty things like reverse-endian mappings that we have on some embedded platforms, I'm investigating ways to better separate vm_page_prot from vm_flags so some PROT_* bits can go straight to the former without having to be mirrored in some way in the later.
Cheers, Ben.
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