selftests/bpf: libarena: Fix can-loop zero variable definition

BPF can_loop based loops require the index variable to stay imprecise.
This means we must initialize them from a currently imprecise variable
instead of directly assigning 0 to them, like so:

static volatile u32 zero = 0;

for (i = zero; i < NUM_LOOPS; i++) {
	/* loop body */
}

The libarena implementation of this technique is currently faulty. For
the technique to work, the variable must not be in a map. This includes
the .rodata DATASEC map used for const variables. However, libarena
still defines the zero variable as constant.

Modify the zero variable definition into a volatile variable. This
change adds a complication caused by the compiler optimizing array
derefences from

for (i = zero; i < NUM_LOOPS; i++) {
	val = *(ptr + i);
}

into

for (i = zero; i < NUM_LOOPS; i++) {
	val = *ptr++;
}

and causing verification failures. Use the barrier_var() clobber macro
to prevent this optimization from taking place. Using barrier_var() is
the only way to break the optimization, as annotating the index as
volatile does not suffice.

After that, remove the bpf_for() invocations introduced in libarena for
parallel spmc testing.

Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260706181730.21731-3-emil@etsalapatis.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

tools/testing/selftests/bpf/libarena/include/libarena/common.h

@@ -43,7 +43,7 @@ struct {
  * imprecise. To force the variable to be imprecise, initialize it with
  * the opaque volatile variable 0 instead of the constant 0.
  */
-extern const volatile u32 zero;
+volatile u32 zero __weak;
 extern volatile u64 asan_violated;
 
 int arena_fls(__u64 word);

tools/testing/selftests/bpf/libarena/selftests/test_asan_buddy.bpf.c

@@ -154,7 +154,8 @@ __weak int asan_test_buddy_oob(void)
 	size_t sizes[] = {
 		7, 8, 17, 18, 64, 256, 317, 512, 1024,
 	};
-	int ret, i;
+	int ret;
+	u32 i;
 
 	ret = buddy_init(&buddy);
 	if (ret) {
@@ -163,6 +164,7 @@ __weak int asan_test_buddy_oob(void)
 	}
 
 	for (i = zero; i < sizeof(sizes) / sizeof(sizes[0]) && can_loop; i++) {
+		barrier_var(i);
 		ret = asan_test_buddy_oob_single(sizes[i]);
 		if (ret) {
 			arena_stdout("%s:%d Failed for size %lu", __func__,
@@ -190,7 +192,8 @@ __stderr("Call trace:\n"
 __weak int asan_test_buddy_uaf(void)
 {
 	size_t sizes[] = { 16, 32, 64, 128, 256, 512, 1024, 16384 };
-	int ret, i;
+	int ret;
+	u32 i;
 
 	ret = buddy_init(&buddy);
 	if (ret) {
@@ -199,6 +202,7 @@ __weak int asan_test_buddy_uaf(void)
 	}
 
 	for (i = zero; i < sizeof(sizes) / sizeof(sizes[0]) && can_loop; i++) {
+		barrier_var(i);
 		ret = asan_test_buddy_uaf_single(sizes[i]);
 		if (ret) {
 			arena_stdout("%s:%d Failed for size %lu", __func__,

tools/testing/selftests/bpf/libarena/selftests/test_buddy.bpf.c

@@ -171,7 +171,8 @@ __weak int test_buddy_alloc_multiple(void)
 SEC("syscall")
 __weak int test_buddy_alignment(void)
 {
-	int ret, i;
+	int ret;
+	u32 i;
 
 	ret = buddy_init(&buddy);
 	if (ret)
@@ -179,6 +180,7 @@ __weak int test_buddy_alignment(void)
 
 	/* Allocate various sizes and check alignment */
 	for (i = zero; i < 17 && can_loop; i++) {
+		barrier_var(i);
 		ptrs[i] = buddy_alloc(&buddy, alignment_sizes[i]);
 		if (!ptrs[i]) {
 			arena_stdout("alignment test: alloc failed for size %lu",
@@ -198,8 +200,10 @@ __weak int test_buddy_alignment(void)
 	}
 
 	/* Free all allocations */
-	for (i = zero; i < 17 && can_loop; i++)
+	for (i = zero; i < 17 && can_loop; i++) {
+		barrier_var(i);
 		buddy_free(&buddy, ptrs[i]);
+	}
 
 	buddy_destroy(&buddy);
 

tools/testing/selftests/bpf/libarena/selftests/test_parallel_spmc.bpf.c

@@ -155,7 +155,7 @@ int spmc_quiesce_on_owner(u64 epoch)
 {
 	u64 i;
 
-	bpf_for(i, 0, TEST_SPMC_SYNC_SPINS) {
+	for (i = zero; i < TEST_SPMC_SYNC_SPINS && can_loop; i++) {
 		if (test_abort)
 			return -EINTR;
 		if (smp_load_acquire(&owner_epoch) >= epoch)
@@ -175,8 +175,7 @@ int spmc_quiesce_on_stealer(u64 epoch)
 	int err = -ETIMEDOUT;
 
 	target = STEALER_EPOCH(epoch);
-	bpf_for(i, 0, TEST_SPMC_SYNC_SPINS) {
-
+	for (i = zero; i < TEST_SPMC_SYNC_SPINS && can_loop; i++) {
 		if (test_abort) {
 			err = -EINTR;
 			break;
@@ -391,7 +390,7 @@ int spmc_wait_for_stealers_to_start(u64 target)
 {
 	u64 i;
 
-	bpf_for(i, 0, TEST_SPMC_SYNC_SPINS) {
+	for (i = zero; i < TEST_SPMC_SYNC_SPINS && can_loop; i++) {
 		if (test_abort)
 			return -EINTR;
 		if (READ_ONCE(stealers_started) >= target)
@@ -537,7 +536,7 @@ static int spmc_wait_for_round_steals(u64 target)
 
 	arena_subprog_init();
 
-	bpf_for(i, 0, TEST_SPMC_SYNC_SPINS) {
+	for (i = zero; i < TEST_SPMC_SYNC_SPINS && can_loop; i++) {
 		if (test_abort)
 			return -EINTR;
 		if (round_steals >= target)

tools/testing/selftests/bpf/libarena/src/common.bpf.c

@@ -4,9 +4,8 @@
 #include <libarena/asan.h>
 #include <libarena/buddy.h>
 
-const volatile u32 zero = 0;
-
 struct buddy __arena buddy;
+volatile u32 zero = 0;
 
 int arena_fls(__u64 word)
 {