libdrgn: x86_64: avoid recursive address translation for swapper_pg_dir

Co-authored-by: Illia Ostapyshyn <[email protected]>
Signed-off-by: Stephen Brennan <[email protected]>

libdrgn/arch_x86_64.c

@@ -18,6 +18,8 @@
 
 #include "arch_x86_64_defs.inc"
 
+#define START_KERNEL_MAP 0xffffffff80000000ULL
+
 static const struct drgn_cfi_row default_dwarf_cfi_row_x86_64 = DRGN_CFI_ROW(
 	// The System V psABI defines the CFA as the value of rsp in the calling
 	// frame.
@@ -616,7 +618,13 @@ linux_kernel_pgtable_iterator_next_x86_64(struct drgn_program *prog,
 	for (;; level--) {
 		uint64_t table;
 		bool table_physical;
-		if (level == levels) {
+		if (level == levels && prog->vmcoreinfo.swapper_pg_dir &&
+		    it->it.pgtable == prog->vmcoreinfo.swapper_pg_dir) {
+			// Avoid recursive address translation on swapper_pg_dir by
+			// directly resolving to a physical address
+			table = it->it.pgtable + prog->vmcoreinfo.phys_base - START_KERNEL_MAP;
+			table_physical = true;
+		} else if (level == levels) {
 			table = it->it.pgtable;
 			table_physical = false;
 		} else {

libdrgn/drgn_program_parse_vmcoreinfo.inc.strswitch

@@ -80,6 +80,14 @@ struct drgn_error *drgn_program_parse_vmcoreinfo(struct drgn_program *prog,
 			prog->vmcoreinfo.pgtable_l5_enabled = tmp;
 			break;
 		}
+		@case "NUMBER(phys_base)"@
+		{
+			err = parse_vmcoreinfo_u64(value, newline, 0,
+						   &prog->vmcoreinfo.phys_base);;
+			if (err)
+				return err;
+			break;
+		}
 		@case "NUMBER(KERNELPACMASK)"@
 			err = parse_vmcoreinfo_u64(value, newline, 16,
 						   &prog->aarch64_insn_pac_mask);
@@ -92,6 +100,7 @@ struct drgn_error *drgn_program_parse_vmcoreinfo(struct drgn_program *prog,
 			if (err)
 				return err;
 			break;
+
 		@endswitch@
 	}
 	if (!prog->vmcoreinfo.osrelease[0]) {

libdrgn/program.h

@@ -169,6 +169,8 @@ struct drgn_program {
 		uint64_t va_bits;
 		/** Whether 5-level paging was enabled on x86-64. */
 		bool pgtable_l5_enabled;
+		/** phys_base on x86_64 */
+		uint64_t phys_base;
 		/** PAGE_SHIFT of the kernel (derived from PAGE_SIZE). */
 		int page_shift;