pasta, util: Align stack area for clones to maximum natural alignment

Given that we use this stack pointer as a location to store arbitrary
data types from the cloned process, we need to guarantee that its
alignment matches any of those possible data types.

runsisi reports that pasta gets a SIGBUS in pasta_open_ns() on
aarch64, where the alignment requirement for stack pointers is a
16 bytes (same as the size of a long double), and similar requirements
actually apply to most architectures we run on.

Reported-by: runsisi <runsisi@hust.edu.cn>
Link: https://bugs.passt.top/show_bug.cgi?id=85
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

2 files changed, 5 insertions, 2 deletions

diff --git a/pasta.c b/pasta.c
index e73b5af..31e1e00 100644
--- a/pasta.c
+++ b/pasta.c
@@ -211,12 +211,13 @@ static int pasta_spawn_cmd(void *arg)
 void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
 		    int argc, char *argv[])
 {
+	char ns_fn_stack[NS_FN_STACK_SIZE]
+	__attribute__ ((aligned(__alignof__(max_align_t))));
 	struct pasta_spawn_cmd_arg arg = {
 		.exe = argv[0],
 		.argv = argv,
 	};
 	char uidmap[BUFSIZ], gidmap[BUFSIZ];
-	char ns_fn_stack[NS_FN_STACK_SIZE];
 	char *sh_argv[] = { NULL, NULL };
 	char sh_arg0[PATH_MAX + 1];
 	sigset_t set;
diff --git a/util.h b/util.h
index 7c261d7..8c37729 100644
--- a/util.h
+++ b/util.h
@@ -9,6 +9,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <stdbool.h>
+#include <stddef.h>
 #include <string.h>
 #include <signal.h>
 
@@ -116,7 +117,8 @@ int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
 	     void *arg);
 #define NS_CALL(fn, arg)						\
 	do {								\
-		char ns_fn_stack[NS_FN_STACK_SIZE];			\
+		char ns_fn_stack[NS_FN_STACK_SIZE]			\
+		__attribute__ ((aligned(__alignof__(max_align_t))));	\
 									\
 		do_clone((fn), ns_fn_stack, sizeof(ns_fn_stack),	\
 			 CLONE_VM | CLONE_VFORK | CLONE_FILES | SIGCHLD,\