1 files changed, 92 insertions, 0 deletions
diff --git a/mm/vma_exec.c b/mm/vma_exec.c
new file mode 100644
index 000000000000..6736ae37f748
--- /dev/null
+++ b/mm/vma_exec.c
@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * Functions explicitly implemented for exec functionality which however are
+ * explicitly VMA-only logic.
+ */
+
+#include "vma_internal.h"
+#include "vma.h"
+
+/*
+ * Relocate a VMA downwards by shift bytes. There cannot be any VMAs between
+ * this VMA and its relocated range, which will now reside at [vma->vm_start -
+ * shift, vma->vm_end - shift).
+ *
+ * This function is almost certainly NOT what you want for anything other than
+ * early executable temporary stack relocation.
+ */
+int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift)
+{
+	/*
+	 * The process proceeds as follows:
+	 *
+	 * 1) Use shift to calculate the new vma endpoints.
+	 * 2) Extend vma to cover both the old and new ranges.  This ensures the
+	 *    arguments passed to subsequent functions are consistent.
+	 * 3) Move vma's page tables to the new range.
+	 * 4) Free up any cleared pgd range.
+	 * 5) Shrink the vma to cover only the new range.
+	 */
+
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long old_start = vma->vm_start;
+	unsigned long old_end = vma->vm_end;
+	unsigned long length = old_end - old_start;
+	unsigned long new_start = old_start - shift;
+	unsigned long new_end = old_end - shift;
+	VMA_ITERATOR(vmi, mm, new_start);
+	VMG_STATE(vmg, mm, &vmi, new_start, old_end, 0, vma->vm_pgoff);
+	struct vm_area_struct *next;
+	struct mmu_gather tlb;
+	PAGETABLE_MOVE(pmc, vma, vma, old_start, new_start, length);
+
+	BUG_ON(new_start > new_end);
+
+	/*
+	 * ensure there are no vmas between where we want to go
+	 * and where we are
+	 */
+	if (vma != vma_next(&vmi))
+		return -EFAULT;
+
+	vma_iter_prev_range(&vmi);
+	/*
+	 * cover the whole range: [new_start, old_end)
+	 */
+	vmg.middle = vma;
+	if (vma_expand(&vmg))
+		return -ENOMEM;
+
+	/*
+	 * move the page tables downwards, on failure we rely on
+	 * process cleanup to remove whatever mess we made.
+	 */
+	pmc.for_stack = true;
+	if (length != move_page_tables(&pmc))
+		return -ENOMEM;
+
+	tlb_gather_mmu(&tlb, mm);
+	next = vma_next(&vmi);
+	if (new_end > old_start) {
+		/*
+		 * when the old and new regions overlap clear from new_end.
+		 */
+		free_pgd_range(&tlb, new_end, old_end, new_end,
+			next ? next->vm_start : USER_PGTABLES_CEILING);
+	} else {
+		/*
+		 * otherwise, clean from old_start; this is done to not touch
+		 * the address space in [new_end, old_start) some architectures
+		 * have constraints on va-space that make this illegal (IA64) -
+		 * for the others its just a little faster.
+		 */
+		free_pgd_range(&tlb, old_start, old_end, new_end,
+			next ? next->vm_start : USER_PGTABLES_CEILING);
+	}
+	tlb_finish_mmu(&tlb);
+
+	vma_prev(&vmi);
+	/* Shrink the vma to just the new range */
+	return vma_shrink(&vmi, vma, new_start, new_end, vma->vm_pgoff);
+}