appletlib.c: make it actally follow _BB_SUID_ALWAYS rules

adduser: implement -S and code shrink / fix uid selection
*: sanitize getspnam_r use

   text    data     bss     dec     hex filename
 777042     974    9676  787692   c04ec busybox_old
 776883     974    9676  787533   c044d busybox_unstripped

1 files changed, 10 insertions, 5 deletions

diff --git a/libbb/correct_password.c b/libbb/correct_password.c
index f1793cd17..96bb10e0b 100644
--- a/libbb/correct_password.c
+++ b/libbb/correct_password.c
@@ -40,6 +40,11 @@ int correct_password(const struct passwd *pw)
 {
         char *unencrypted, *encrypted;
         const char *correct;
+#if ENABLE_FEATURE_SHADOWPASSWDS
+        /* Using _r function to avoid pulling in static buffers */
+        struct spwd spw;
+        char buffer[256];
+#endif
 
         /* fake salt. crypt() can choke otherwise. */
         correct = "aa";
@@ -50,11 +55,11 @@ int correct_password(const struct passwd *pw)
         correct = pw->pw_passwd;
 #if ENABLE_FEATURE_SHADOWPASSWDS
         if ((correct[0] == 'x' || correct[0] == '*') && !correct[1]) {
-                /* Using _r function to avoid pulling in static buffers */
-                struct spwd spw;
-                struct spwd *result;
-                char buffer[256];
-                correct = (getspnam_r(pw->pw_name, &spw, buffer, sizeof(buffer), &result)) ? "aa" : spw.sp_pwdp;
+                /* getspnam_r may return 0 yet set result to NULL.
+                 * At least glibc 2.4 does this. Be extra paranoid here. */
+                struct spwd *result = NULL;
+                int r = getspnam_r(pw->pw_name, &spw, buffer, sizeof(buffer), &result);
+                correct = (r || !result) ? "aa" : result->sp_pwdp;
         }
 #endif