Re: [PATCH] Alternate futex non-page-pinning and COW fix

[Jamie Lokier]

Linus Torvalds wrote:
> Actually: the VM_SHARED flag will never change, so testing VM_SHARED is 
> actually the _right_ thing from a mm perspective.

Yes it can.  See sys_mprotect().  If that's not intended, it's a bug
in mprotect().  What does PROT_SEM mean for Linux, btw?

See:
	if (prot & ~(PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM))
		return -EINVAL;

and:
	newflags = prot | (vma->vm_flags & ~(PROT_READ | PROT_WRITE | PROT_EXEC));
	if ((newflags & ~(newflags >> 4)) & 0xf) {
		error = -EACCES;
		goto out;
	}

newflags is than used to index protection_map[], like this:
	newprot = protection_map[newflags & 0xf];

and that is stored in the page tables.

-- Jamie
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