libdrgn: x86_64: allow phys_base to be 0

Mark reported recursive address translation errors when debugging a QEMU
guest memory dump. We're supposed to be handling this in
linux_kernel_pgtable_iterator_next_x86_64() by translating
swapper_pg_dir to a physical address. However, it decides whether
phys_base was present in the VMCOREINFO by comparing phys_base to 0, and
this dump legitimately had a phys_base of 0. Add a separate flag so we
can distinguish missing vs 0.

Reported-and-tested-by: Mark Harmstone <maharmstone@fb.com>
Fixes: 5b39bfb ("libdrgn: x86_64: avoid recursive address translation for swapper_pg_dir")
Signed-off-by: Omar Sandoval <osandov@osandov.com>

libdrgn/arch_x86_64.c

@@ -607,15 +607,13 @@ linux_kernel_pgtable_iterator_next_x86_64(struct drgn_program *prog,
 	for (;; level--) {
 		uint64_t table;
 		bool table_physical;
-		if (level == levels && prog->vmcoreinfo.phys_base &&
+		if (level == levels && prog->vmcoreinfo.have_phys_base &&
 		    it->it.pgtable == prog->vmcoreinfo.swapper_pg_dir) {
-			// Avoid recursive address translation on swapper_pg_dir by
-			// directly resolving to a physical address. Don't do
-			// this if phys_base is 0, since that likely means it
-			// was not present in the vmcoreinfo. It has been
-			// present since Linux kernel commit 401721ecd1dc
-			// ("kexec: export the value of phys_base instead of
-			// symbol address") (in v4.10).
+			// Avoid recursive address translation on swapper_pg_dir
+			// by directly resolving to a physical address.
+			// phys_base has been present since Linux kernel commit
+			// 401721ecd1dc ("kexec: export the value of phys_base
+			// instead of symbol address") (in v4.10).
 			table = it->it.pgtable + prog->vmcoreinfo.phys_base - START_KERNEL_MAP;
 			table_physical = true;
 		} else if (level == levels) {

libdrgn/drgn_program_parse_vmcoreinfo.inc.strswitch

@@ -89,6 +89,7 @@ struct drgn_error *drgn_program_parse_vmcoreinfo(struct drgn_program *prog,
 						   &prog->vmcoreinfo.phys_base);
 			if (err)
 				return err;
+			prog->vmcoreinfo.have_phys_base = true;
 			break;
 		}
 		@case "NUMBER(KERNELPACMASK)"@

libdrgn/program.h

@@ -191,6 +191,8 @@ struct drgn_program {
 				bool arm_lpae;
 				/** Whether `CRASHTIME` was in the VMCOREINFO. */
 				bool have_crashtime;
+				/** Whether `phys_base` was in the VMCOREINFO. */
+				bool have_phys_base;
 				/**
 				 * `PAGE_SHIFT` of the kernel (derived from
 				 * `PAGE_SIZE`).