Bug-fix in size calculation

Fixes a bug in how PARASITE_MAX_GROUPS was calculated, and adds a
compiler check to assert that parasite_dump_creds doesn't exceed
the page size.
Signed-off-by: Sophie Blee-Goldman <ableegoldman@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/parasite.h

@@ -162,7 +162,12 @@ struct parasite_dump_misc {
 	int dumpable;
 };
 
-#define PARASITE_MAX_GROUPS	(PAGE_SIZE / sizeof(unsigned int) - 2 * sizeof(unsigned))
+/*
+ * Calculate how long we can make the groups array in parasite_dump_creds
+ * and still fit the struct in one page
+ */
+#define PARASITE_MAX_GROUPS	\
+	((PAGE_SIZE - 2 * sizeof(unsigned int)) / sizeof(unsigned int))
 
 struct parasite_dump_creds {
 	unsigned int		secbits;

parasite-syscall.c

@@ -735,6 +735,8 @@ int parasite_dump_creds(struct parasite_ctl *ctl, CredsEntry *ce)
 {
 	struct parasite_dump_creds *pc;
 
+	BUILD_BUG_ON(sizeof(*pc) > PAGE_SIZE);
+
 	pc = parasite_args(ctl, struct parasite_dump_creds);
 	if (parasite_execute_daemon(PARASITE_CMD_DUMP_CREDS, ctl) < 0)
 		return -1;