[Crash-utility] [PATCH 2/2] arm64: fix the "pud page" crash for live system

[Huang Shijie]

1.) When I tested live system with "crash vmlinux /proc/kcore" in kernel v5.7,
    I met the following crash issue:
       ........................................
       crash: seek error: kernel virtual address: ffff75e9fffff000  type: "pud page"
       ........................................

2.) The root cause is the PTOV does not work correctly for some kernel,
    and then arm64_vtop_4level_4k() does not work correctly too.

    Why PTOV does not work?

    The PHYS_OFFSET is just wrapper of memstart_addr.
     ...............................
     #define PHYS_OFFSET  ({ VM_BUG_ON(memstart_addr & 1); memstart_addr; })
     ...............................

    Because memstart_addr is changed after physvirt_offset is initialized.
    so the NUMBER(PHYS_OFFSET) does not return the correct value.

3.) How many kernel versions have this bug?
      1) In kernel v5.4, the patch:
          "5383cc6efed137 arm64: mm: Introduce vabits_actual"

          makes the NUMBER(PHYS_OFFSET) do not work correctly.

      2) In kernel v5.10, the patch:
          "7bc1a0f9e17658 arm64: mm: use single quantity
                                 to represent the PA to VA translation"

          makes the NUMBER(PHYS_OFFSET) work again.

4.) What does this patch do?
    This patch uses the same method as makedumpfile does:
      Use the PT_LOAD segments to get the phys_offset.

Signed-off-by: Huang Shijie <shijie at os.amperecomputing.com>
---
 arm64.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/arm64.c b/arm64.c
index e3fa316..d81c2b9 100644
--- a/arm64.c
+++ b/arm64.c
@@ -1426,6 +1426,51 @@ arm64_calc_physvirt_offset(void)
 
 }
 
+/*
+ * Check if an virtual address is a linear address.
+ */
+#define PAGE_END 	(_PAGE_END(ms->VA_BITS_ACTUAL))
+static int arm64_is_linear_addr(struct machine_specific *ms,
+				unsigned long va)
+{
+	return (va - PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET);
+}
+
+/*
+ * This function only works for kernel range: [5.4, 5.10).
+ *
+ *      1) In kernel v5.4, the patch:
+ *          "5383cc6efed137 arm64: mm: Introduce vabits_actual"
+ *
+ *          makes the NUMBER(PHYS_OFFSET) do not work correctly.
+ *
+ *      2) In kernel v5.10, the patch:
+ *          "7bc1a0f9e17658 arm64: mm: use single quantity
+ *                                 to represent the PA to VA translation"
+ *
+ *          makes the NUMBER(PHYS_OFFSET) work again.
+ *
+ * This function tries to get the phys_offset from PT_LOAD segments.
+ * This method was originally used by the makedumpfile tool.
+ */
+static void arm64_get_phys_offset_by_pt_load(struct machine_specific *ms)
+{
+	int i;
+	Elf64_Phdr *h;
+	struct proc_kcore_data *pkd = arm64_get_pkd();
+
+	for (i = 0; i < pkd->segments; i++) {
+		h = &pkd->load64[i];
+
+		if (arm64_is_linear_addr(ms, h->p_vaddr)) {
+			ms->phys_offset = h->p_paddr - (h->p_vaddr & ~PAGE_OFFSET);
+			return;
+		}
+	}
+
+	error(FATAL, "We cannot get the correct phys_offset!\n");
+}
+
 static void
 arm64_calc_phys_offset(void)
 {
@@ -1454,6 +1499,14 @@ arm64_calc_phys_offset(void)
 		if ((machdep->flags & NEW_VMEMMAP) &&
 		    ms->kimage_voffset && (sp = kernel_symbol_search("memstart_addr"))) {
 			if (pc->flags & PROC_KCORE) {
+				unsigned long v = arm64_get_kernel_version();
+
+				/* Do special operation for kernel [5.4, 5.10) */
+				if (LINUX(5, 4, 0) <= v && v < LINUX(5, 10, 0)) {
+					arm64_get_phys_offset_by_pt_load(ms);
+					return;
+				}
+
 				if ((string = pc->read_vmcoreinfo("NUMBER(PHYS_OFFSET)"))) {
 					ms->phys_offset = htol(string, QUIET, NULL);
 					free(string);
-- 
2.30.2