platform: introduce a way to hook new session start

Previously this was possible via post_fork_child, but ever since sshd
was split into multiple binaries, this is now no longer possible becase
of execv.

Makefile.in

@@ -140,7 +140,7 @@ SSHD_SESSION_OBJS=sshd-session.o auth-rhosts.o auth-passwd.o \
 	auth2-gss.o gss-serv.o gss-serv-krb5.o \
 	loginrec.o auth-pam.o auth-shadow.o auth-sia.o \
 	sftp-server.o sftp-common.o \
-	uidswap.o $(SKOBJS)
+	uidswap.o platform-listen.o $(SKOBJS)
 
 SSHD_AUTH_OBJS=sshd-auth.o \
 	auth2-methods.o \

platform-listen.c

@@ -82,3 +82,6 @@ platform_post_fork_child(void)
 #endif
 }
 
+void platform_pre_session_start(void)
+{
+}

platform.h

@@ -24,6 +24,7 @@ void platform_pre_restart(void);
 void platform_post_listen(void);
 void platform_post_fork_parent(pid_t child_pid);
 void platform_post_fork_child(void);
+void platform_pre_session_start(void);
 int  platform_privileged_uidswap(void);
 void platform_setusercontext(struct passwd *);
 void platform_setusercontext_post_groups(struct passwd *);

sshd-session.c

@@ -999,6 +999,8 @@ main(int ac, char **av)
 
 	closefrom(REEXEC_MIN_FREE_FD);
 
+	platform_pre_session_start();
+
 	/* Reserve fds we'll need later for reexec things */
 	if ((devnull = open(_PATH_DEVNULL, O_RDWR)) == -1)
 		fatal("open %s: %s", _PATH_DEVNULL, strerror(errno));