On 26/03/2015 10:48, Ingo Molnar wrote:

* Benjamin Herrenschmidt [off-list ref] wrote:

quoted

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.

The same is done on PowerPC. So calling mremap() to extend the vDSO is
failing but splitting it or unmapping a part of it is allowed but lead
to an unusable vDSO.

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

On PowerPC, you'll get the same result.

Should we prevent the user to break its vDSO ?

Thanks,
Laurent.