tree-wide: dlopen libpam in pam plugins

Same reasoning as for cryptsetup tokens. It means we can include
the pam plugins in the main systemd package without the package 
manager introducing a dependency on libpam. It also makes things
more consistent and makes writing the upcoming linking test script
a lot simpler.

At the same time, we get rid of the libpam_misc dependency as the
one symbol we were using from it is trivial to reimplement ourselves.

src/shared/pam-util.c

@@ -25,10 +25,15 @@ static void *libpam_dl = NULL;
 DLSYM_PROTOTYPE(pam_acct_mgmt) = NULL;
 DLSYM_PROTOTYPE(pam_close_session) = NULL;
 DLSYM_PROTOTYPE(pam_end) = NULL;
+DLSYM_PROTOTYPE(pam_get_authtok_noverify) = NULL;
+DLSYM_PROTOTYPE(pam_get_authtok_verify) = NULL;
 DLSYM_PROTOTYPE(pam_get_data) = NULL;
 DLSYM_PROTOTYPE(pam_get_item) = NULL;
+DLSYM_PROTOTYPE(pam_get_user) = NULL;
+DLSYM_PROTOTYPE(pam_getenv) = NULL;
 DLSYM_PROTOTYPE(pam_getenvlist) = NULL;
 DLSYM_PROTOTYPE(pam_open_session) = NULL;
+DLSYM_PROTOTYPE(pam_prompt) = NULL;
 DLSYM_PROTOTYPE(pam_putenv) = NULL;
 DLSYM_PROTOTYPE(pam_set_data) = NULL;
 DLSYM_PROTOTYPE(pam_set_item) = NULL;
@@ -361,6 +366,19 @@ int pam_prompt_graceful(pam_handle_t *pamh, int style, char **ret_response, cons
         return PAM_SUCCESS;
 }
 
+int pam_putenv_assign(pam_handle_t *pamh, const char *name, const char *value) {
+        _cleanup_(erase_and_freep) char *s = NULL;
+
+        assert(pamh);
+        assert(name);
+        assert(value);
+
+        if (asprintf(&s, "%s=%s", name, value) < 0)
+                return PAM_BUF_ERR;
+
+        return sym_pam_putenv(pamh, s);
+}
+
 #endif
 
 int dlopen_libpam(int log_level) {
@@ -378,10 +396,15 @@ int dlopen_libpam(int log_level) {
                         DLSYM_ARG(pam_acct_mgmt),
                         DLSYM_ARG(pam_close_session),
                         DLSYM_ARG(pam_end),
+                        DLSYM_ARG(pam_get_authtok_noverify),
+                        DLSYM_ARG(pam_get_authtok_verify),
                         DLSYM_ARG(pam_get_data),
                         DLSYM_ARG(pam_get_item),
+                        DLSYM_ARG(pam_get_user),
+                        DLSYM_ARG(pam_getenv),
                         DLSYM_ARG(pam_getenvlist),
                         DLSYM_ARG(pam_open_session),
+                        DLSYM_ARG(pam_prompt),
                         DLSYM_ARG(pam_putenv),
                         DLSYM_ARG(pam_set_data),
                         DLSYM_ARG(pam_set_item),

src/shared/pam-util.h

@@ -6,7 +6,7 @@
 #if HAVE_PAM
 #include <security/pam_appl.h>
 #include <security/pam_ext.h>
-#include <security/pam_modules.h> /* IWYU pragma: export */
+#include <security/pam_modules.h>       /* IWYU pragma: export */
 #include <syslog.h>
 
 #include "dlfcn-util.h"
@@ -14,10 +14,15 @@
 extern DLSYM_PROTOTYPE(pam_acct_mgmt);
 extern DLSYM_PROTOTYPE(pam_close_session);
 extern DLSYM_PROTOTYPE(pam_end);
+extern DLSYM_PROTOTYPE(pam_get_authtok_noverify);
+extern DLSYM_PROTOTYPE(pam_get_authtok_verify);
 extern DLSYM_PROTOTYPE(pam_get_data);
 extern DLSYM_PROTOTYPE(pam_get_item);
+extern DLSYM_PROTOTYPE(pam_get_user);
+extern DLSYM_PROTOTYPE(pam_getenv);
 extern DLSYM_PROTOTYPE(pam_getenvlist);
 extern DLSYM_PROTOTYPE(pam_open_session);
+extern DLSYM_PROTOTYPE(pam_prompt);
 extern DLSYM_PROTOTYPE(pam_putenv);
 extern DLSYM_PROTOTYPE(pam_set_data);
 extern DLSYM_PROTOTYPE(pam_set_item);
@@ -27,6 +32,11 @@ extern DLSYM_PROTOTYPE(pam_strerror);
 extern DLSYM_PROTOTYPE(pam_syslog);
 extern DLSYM_PROTOTYPE(pam_vsyslog);
 
+/* sym_pam_prompt() replacement for the pam_info() macro from <security/pam_ext.h>, which expands to a direct
+ * pam_prompt() call. */
+#define sym_pam_info(pamh, fmt, ...) \
+        sym_pam_prompt((pamh), PAM_TEXT_INFO, NULL, (fmt), ## __VA_ARGS__)
+
 void pam_log_setup(void);
 
 int errno_to_pam_error(int error) _const_;
@@ -90,6 +100,10 @@ int pam_get_data_many_internal(pam_handle_t *pamh, ...) _sentinel_;
 
 int pam_prompt_graceful(pam_handle_t *pamh, int style, char **ret_response, const char *fmt, ...) _printf_(4,5);
 
+/* Equivalent of pam_misc_setenv(pamh, name, value, 0), without the libpam_misc dep — builds "name=value"
+ * and hands it to sym_pam_putenv(), then erases the buffer before freeing in case it carried a secret. */
+int pam_putenv_assign(pam_handle_t *pamh, const char *name, const char *value);
+
 #endif
 
 int dlopen_libpam(int log_level);