res_find: Fix fallback logic

The fallback logic was broken if hints were found in multiple environments.
If we found a hint in either the loader environment or the static
environment, fallback would be incremented excessively when we returned to
the environment-selection bits. These checks should have also been guarded
by the fbacklvl checks. As a result, fbacklvl could quickly get to a point
where we skip either the static environment and/or the static hints
depending on which environments contained valid hints.

The impact of this bug is minimal, mostly affecting mips boards that use
static hints and may have hints in either the loader environment or the
static environment.

There may be better ways to express the searchable environments and
describing their characteristics (immutable, already searched, etc.) but
this may be revisited after 12 branches.

Reported by:	Dan Nelson <dnelson_1901@yahoo.com>
Triaged by:	Dan Nelson <dnelson_1901@yahoo.com>
MFC after:	3 days

1 files changed, 30 insertions, 23 deletions

diff --git a/sys/kern/subr_hints.c b/sys/kern/subr_hints.c
index e63fe0cf3564..ba30e23d9f8a 100644
--- a/sys/kern/subr_hints.c
+++ b/sys/kern/subr_hints.c
@@ -174,30 +174,37 @@ fallback:
 			if (dyn_used || fbacklvl >= FBACK_STATIC)
 				return (ENOENT);
 
-			if (fbacklvl <= FBACK_MDENV &&
-			    _res_checkenv(md_envp)) {
-				hintp = md_envp;
-				goto found;
+			switch (fbacklvl) {
+			case FBACK_MDENV:
+				fbacklvl++;
+				if (_res_checkenv(md_envp)) {
+					hintp = md_envp;
+					break;
+				}
+
+				/* FALLTHROUGH */
+			case FBACK_STENV:
+				fbacklvl++;
+				if (!stenv_skip && _res_checkenv(kern_envp)) {
+					hintp = kern_envp;
+					break;
+				} else
+					stenv_skip = true;
+
+				/* FALLTHROUGH */
+			case FBACK_STATIC:
+				fbacklvl++;
+				/* We'll fallback to static_hints if needed/can */
+				if (!sthints_skip &&
+				    _res_checkenv(static_hints))
+					hintp = static_hints;
+				else
+					sthints_skip = true;
+
+				break;
+			default:
+				return (ENOENT);
 			}
-			fbacklvl++;
-
-			if (!stenv_skip && fbacklvl <= FBACK_STENV &&
-			    _res_checkenv(kern_envp)) {
-				hintp = kern_envp;
-				goto found;
-			} else
-				stenv_skip = true;
-
-			fbacklvl++;
-
-			/* We'll fallback to static_hints if needed/can */
-			if (!sthints_skip && fbacklvl <= FBACK_STATIC &&
-			    _res_checkenv(static_hints))
-				hintp = static_hints;
-			else
-				sthints_skip = true;
-found:
-			fbacklvl++;
 		}
 
 		if (hintp == NULL)