* Benjamin Herrenschmidt [off-list ref] wrote:

quoted

quoted

+#define __HAVE_ARCH_REMAP
+static inline void arch_remap(struct mm_struct *mm,
+			      unsigned long old_start, unsigned long old_end,
+			      unsigned long new_start, unsigned long new_end)
+{
+	/*
+	 * mremap() doesn't allow moving multiple vmas so we can limit the
+	 * check to old_start == vdso_base.
+	 */
+	if (old_start == mm->context.vdso_base)
+		mm->context.vdso_base = new_start;
+}

mremap() doesn't allow moving multiple vmas, but it allows the 
movement of multi-page vmas and it also allows partial mremap()s, 
where it will split up a vma.

In particular, what happens if an mremap() is done with 
old_start == vdso_base, but a shorter end than the end of the vDSO? 
(i.e. a partial mremap() with fewer pages than the vDSO size)

Is there a way to forbid splitting ? Does x86 deal with that case at 
all or it doesn't have to for some other reason ?

So we use _install_special_mapping() - maybe PowerPC does that too? 
That adds VM_DONTEXPAND which ought to prevent some - but not all - of 
the VM API weirdnesses.

On x86 we'll just dump core if someone unmaps the vdso.

Thanks,

	Ingo