From cb65f0d0e966464612b6fa306260c4135efad60f Mon Sep 17 00:00:00 2001
From: Grisha Levit <grishalevit@gmail.com>
Date: Fri, 30 Jun 2023 03:05:21 -0400
Subject: [PATCH] avoid calling setgid when ids already match

On some Android versions attemptig to set the current egid causes
a SIGSYS to be sent even when ids already match. Avoid calling the
sete[gu]id functions when it would be a noop.
---
 shell.c | 62 +++++++++++++++++++++++++++++++++++++++++----------------
 shell.h |  6 ++++++
 2 files changed, 51 insertions(+), 17 deletions(-)

diff --git a/shell.c b/shell.c
index 9f1934d2..9b41203f 100644
--- a/shell.c
+++ b/shell.c
@@ -117,7 +117,14 @@ COMMAND *global_command = (COMMAND *)NULL;
 /* Information about the current user. */
 struct user_info current_user =
 {
-  (uid_t)-1, (uid_t)-1, (gid_t)-1, (gid_t)-1,
+  (uid_t)-1, (uid_t)-1,
+#if HAVE_SETRESUID
+  (uid_t)-1,
+#endif
+  (gid_t)-1, (gid_t)-1,
+#if HAVE_SETRESGID
+  (gid_t)-1,
+#endif
   (char *)NULL, (char *)NULL, (char *)NULL
 };
 
@@ -1298,7 +1305,21 @@ uidget (void)
 {
   uid_t u;
 
-  u = getuid ();
+  u = current_user.uid;
+
+#if HAVE_SETRESUID
+  (void) getresuid (&current_user.uid, &current_user.euid, &current_user.suid);
+#else
+  current_user.uid = getuid ();
+  current_user.euid = geteuid ();
+#endif
+#if HAVE_SETRESGID
+  (void) getresgid (&current_user.gid, &current_user.egid, &current_user.sgid);
+#else
+  current_user.gid = getgid ();
+  current_user.egid = getegid ();
+#endif
+
   if (current_user.uid != u)
     {
       FREE (current_user.user_name);
@@ -1306,10 +1327,6 @@ uidget (void)
       FREE (current_user.home_dir);
       current_user.user_name = current_user.shell = current_user.home_dir = NULL;
     }
-  current_user.uid = u;
-  current_user.gid = getgid ();
-  current_user.euid = geteuid ();
-  current_user.egid = getegid ();
 
   /* See whether or not we are running setuid or setgid. */
   return (current_user.uid != current_user.euid) ||
@@ -1322,27 +1339,38 @@ disable_priv_mode (void)
   int e;
 
 #if HAVE_SETRESUID
-  if (setresuid (current_user.uid, current_user.uid, current_user.uid) < 0)
+  if (current_user.euid != current_user.uid || current_user.suid != current_user.uid)
+    if (setresuid (current_user.uid, current_user.uid, current_user.uid) < 0)
 #else
-  if (setuid (current_user.uid) < 0)
+  if (current_user.euid != current_user.uid)
+    if (setuid (current_user.uid) < 0)
 #endif
-    {
-      e = errno;
-      sys_error (_("cannot set uid to %d: effective uid %d"), current_user.uid, current_user.euid);
+      {
+	e = errno;
+	sys_error (_("cannot set uid to %d: effective uid %d"), current_user.uid, current_user.euid);
 #if defined (EXIT_ON_SETUID_FAILURE)
-      if (e == EAGAIN)
-	exit (e);
+	if (e == EAGAIN)
+	  exit (e);
 #endif
-    }
+      }
+
 #if HAVE_SETRESGID
-  if (setresgid (current_user.gid, current_user.gid, current_user.gid) < 0)
+  if (current_user.egid != current_user.gid || current_user.sgid != current_user.gid)
+    if (setresgid (current_user.gid, current_user.gid, current_user.gid) < 0)
 #else
-  if (setgid (current_user.gid) < 0)
+  if (current_user.egid != current_user.gid)
+    if (setgid (current_user.gid) < 0)
 #endif
-    sys_error (_("cannot set gid to %d: effective gid %d"), current_user.gid, current_user.egid);
+      sys_error (_("cannot set gid to %d: effective gid %d"), current_user.gid, current_user.egid);
 
   current_user.euid = current_user.uid;
+#if defined (HAVE_SETRESUID)
+  current_user.suid = current_user.uid;
+#endif
   current_user.egid = current_user.gid;
+#if defined (HAVE_SETRESUID)
+  current_user.sgid = current_user.gid;
+#endif
 }
 
 #if defined (WORDEXP_OPTION)
diff --git a/shell.h b/shell.h
index 055ba9f5..6315085d 100644
--- a/shell.h
+++ b/shell.h
@@ -149,7 +149,13 @@ struct fd_bitmap {
 /* Information about the current user. */
 struct user_info {
   uid_t uid, euid;
+#if defined (HAVE_SETRESUID)
+  uid_t suid;
+#endif
   gid_t gid, egid;
+#if defined (HAVE_SETRESGID)
+  gid_t sgid;
+#endif
   char *user_name;
   char *shell;		/* shell from the password file */
   char *home_dir;
-- 
2.41.0