[glib] Detect "empty" socket credentials on Linux

[Alexander Larsson <alexl src gnome org>]

commit ec91ed00f14c70cca9749347b8ebc19d72d9885b
Author: Alexander Larsson <alexl redhat com>
Date:   Thu Apr 12 16:43:49 2012 +0200

    Detect "empty" socket credentials on Linux
    
    Linux uses struct ucred to pass over socket credentials. Historically
    this has always worked in recievemsg, if SO_PASSCRED was set on the socket,
    even if the remote side didn't pass any credits. But this change broke that:
    
    http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=16e5726269611b71c930054ffe9b858c1cea88eb;hp=a9e9fd7182332d0cf5f3e601df3e71dd431b70d7
    
    However, it doesn't actually fail getting the credentials, it just returns
    an "empty" one, as initialized by cred_to_ucred() at:
    
    http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=net/core/sock.c;h=b29ab61b029cf7f76fda992ecfcb8dcaa06b0483;#l756
    
    So, we detect this and fail the credentials reading.
    
    This actually happened in real life with gdbus acting as a server, as
    gdbus expected an ucred but libdbus didn't send one.

 gio/gunixcredentialsmessage.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/gio/gunixcredentialsmessage.c b/gio/gunixcredentialsmessage.c
index 47d5173..7cbbab9 100644
--- a/gio/gunixcredentialsmessage.c
+++ b/gio/gunixcredentialsmessage.c
@@ -144,6 +144,13 @@ g_unix_credentials_message_deserialize (gint     level,
 
     ucred = data;
 
+    if (ucred->uid == (uid_t)-1 &&
+	ucred->gid == (gid_t)-1)
+      {
+	/* This happens if the remote side didn't pass the credentials */
+	goto out;
+      }
+
     credentials = g_credentials_new ();
     g_credentials_set_native (credentials, G_CREDENTIALS_TYPE_LINUX_UCRED, ucred);
     message = g_unix_credentials_message_new_with_credentials (credentials);