agent: Add envvar "gnupg_SSH_AUTH_SOCK_by"

* agent/gpg-agent.c (main): Pass new envar gnupg_SSH_AUTH_SOCK_by to
an invoked process.
--

This environment variable is useful for debugging if
--use-standard-socket is used (which is the default since 2.1).
Commonly you should have this in your init script (e.g. ~/.bashrc):

    unset GPG_AGENT_INFO
    unset SSH_AGENT_PID
    SSH_AUTH_SOCK="${HOME}/.gnupg/S.gpg-agent.ssh"
    export SSH_AUTH_SOCK

The problem is that gpg-agent won't be able to override the
SSH_AUTH_SOCK envvar if gpg-agent has been invoked as

  gpg-agent --enable-ssh-support --daemon /bin/bash

To fix this you should instead use this code in the init script:

  unset GPG_AGENT_INFO
  unset SSH_AGENT_PID
  if [ ${gnupg_SSH_AUTH_SOCK_by:-0} -ne $$ ]; then
    export SSH_AUTH_SOCK="${HOME}/.gnupg/S.gpg-agent.ssh"
  fi

This will work in all cases and thus allows to start gpg-agent for
testing purposes with a different homedir and use this gpg-agent as an
ssh-agent.  Example:

  GNUPGHOME=$(pwd) gpg-agent --enable-ssh-support --daemon /bin/bash

gnupg_SSH_AUTH_SOCK_by is set to the PID of the exec-ed process and
thus will work safely if called recursively.

1 files changed, 11 insertions, 2 deletions

diff --git a/agent/gpg-agent.c b/agent/gpg-agent.c
index 32da578f0..469011493 100644
--- a/agent/gpg-agent.c
+++ b/agent/gpg-agent.c
@@ -1067,7 +1067,7 @@ main (int argc, char **argv )
         }
       else if (pid)
         { /* We are the parent */
-          char *infostr, *infostr_ssh_sock;
+          char *infostr, *infostr_ssh_sock, *infostr_ssh_valid;
 
           /* Close the socket FD. */
           close (fd);
@@ -1104,6 +1104,13 @@ main (int argc, char **argv )
 		  kill (pid, SIGTERM);
 		  exit (1);
 		}
+	      if (asprintf (&infostr_ssh_valid, "gnupg_SSH_AUTH_SOCK_by=%lu",
+			    (unsigned long)getpid()) < 0)
+		{
+		  log_error ("out of core\n");
+		  kill (pid, SIGTERM);
+		  exit (1);
+		}
 	    }
 
           *socket_name = 0; /* Don't let cleanup() remove the socket -
@@ -1142,7 +1149,8 @@ main (int argc, char **argv )
                   kill (pid, SIGTERM );
                   exit (1);
                 }
-              if (opt.ssh_support && putenv (infostr_ssh_sock))
+              if (opt.ssh_support && (putenv (infostr_ssh_sock)
+                                      || putenv (infostr_ssh_valid)))
                 {
                   log_error ("failed to set environment: %s\n",
                              strerror (errno) );
@@ -1189,6 +1197,7 @@ main (int argc, char **argv )
 	      if (opt.ssh_support)
 		{
 		  xfree (infostr_ssh_sock);
+		  xfree (infostr_ssh_valid);
 		}
               exit (0);
             }