selftests/mm/uffd: initialize char variable to Null

In "uffd-stress.c" & "uffd-unit-tests.c". address of char variable having
garbage value (uninitialized) is passed to 'write' syscall triggers
warning.

	uffd-stress.c:246:39: warning: variable 'c' is uninitialized when
	passed  as a const pointer argument here
	[-Wuninitialized-const-pointer]

	uffd-unit-tests.c:581:31: warning: variable 'c' is uninitialized
	when passed as a const pointer argument here
	[-Wuninitialized-const-pointer]

so the fix is to assign char variable to '\0' to prevent writing of
garbage value.

Link: https://lkml.kernel.org/r/20251126160830.52124-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ankitkhushwaha

tools/testing/selftests/mm/uffd-stress.c

@@ -241,7 +241,7 @@ static int stress(struct uffd_args *args)
 			return 1;
 
 	for (cpu = 0; cpu < gopts->nr_parallel; cpu++) {
-		char c;
+		char c = '\0';
 		if (bounces & BOUNCE_POLL) {
 			if (write(gopts->pipefd[cpu*2+1], &c, 1) != 1)
 				err("pipefd write error");

tools/testing/selftests/mm/uffd-unit-tests.c

@@ -543,7 +543,7 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 {
 	unsigned long p;
 	pthread_t uffd_mon;
-	char c;
+	char c = '\0';
 	struct uffd_args args = { 0 };
 	args.gopts = gopts;
 
@@ -759,7 +759,7 @@ static void uffd_sigbus_test_common(uffd_global_test_opts_t *gopts, bool wp)
 	pthread_t uffd_mon;
 	pid_t pid;
 	int err;
-	char c;
+	char c = '\0';
 	struct uffd_args args = { 0 };
 	args.gopts = gopts;
 
@@ -819,7 +819,7 @@ static void uffd_events_test_common(uffd_global_test_opts_t *gopts, bool wp)
 	pthread_t uffd_mon;
 	pid_t pid;
 	int err;
-	char c;
+	char c = '\0';
 	struct uffd_args args = { 0 };
 	args.gopts = gopts;
 
@@ -1125,7 +1125,7 @@ uffd_move_test_common(uffd_global_test_opts_t *gopts,
 {
 	unsigned long nr;
 	pthread_t uffd_mon;
-	char c;
+	char c = '\0';
 	unsigned long long count;
 	struct uffd_args args = { 0 };
 	char *orig_area_src = NULL, *orig_area_dst = NULL;