The arena memory allocator currently does not provide the
option to zero-initialize the allocated memory. Provide
a calloc() operation that calls memset() on the returned
memory. Reuse naive memset implementation already present
in the arena ASAN code for now. Subsequent patches will
optimize the function.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 .../bpf/libarena/include/libarena/common.h    | 18 +++++++++++++
 .../selftests/bpf/libarena/src/asan.bpf.c     | 21 ++-------------
 .../selftests/bpf/libarena/src/common.bpf.c   | 27 ++++++++++++++++++-
 3 files changed, 46 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/bpf/libarena/include/libarena/common.h b/tools/testing/selftests/bpf/libarena/include/libarena/common.h
index 931ace9a49e2..d32a51ff5e7f 100644
--- a/tools/testing/selftests/bpf/libarena/include/libarena/common.h
+++ b/tools/testing/selftests/bpf/libarena/include/libarena/common.h
@@ -49,6 +49,7 @@ extern volatile u64 asan_violated;
 int arena_fls(__u64 word);
 
 void __arena *arena_malloc(size_t size);
+void __arena *arena_calloc(size_t ncount, size_t size);
 void arena_free(void __arena *ptr);
 
 /*
@@ -61,6 +62,23 @@ void arena_free(void __arena *ptr);
  */
 #define arena_subprog_init() do { asm volatile ("" :: "r"(&arena)); } while (0)
 
+/*
+ * BPF does not currently support the memset intrinsics. for large
+ * sequential copies, or assignments of large data structures,
+ * the frontend will generate an intrinsic that causes the BPF
+ * backend to exit due to a missing implementation. Provide
+ * implementations for the intrinsic.
+ */
+static inline int arena_memset(s8 __arena *dst, s8 val, size_t size)
+{
+	size_t i;
+
+	for (i = zero; i < size && can_loop; i++)
+		dst[i] = val;
+
+	return 0;
+}
+
 #else /* ! __BPF__ */
 
 #include <stdint.h>
diff --git a/tools/testing/selftests/bpf/libarena/src/asan.bpf.c b/tools/testing/selftests/bpf/libarena/src/asan.bpf.c
index 5135d5c72a46..de656b69d13a 100644
--- a/tools/testing/selftests/bpf/libarena/src/asan.bpf.c
+++ b/tools/testing/selftests/bpf/libarena/src/asan.bpf.c
@@ -103,23 +103,6 @@ volatile bool asan_inited = false;
  */
 volatile bool asan_report_once = false;
 
-/*
- * BPF does not currently support the memset/memcpy/memcmp intrinsics.
- * For large sequential copies, or assignments of large data structures,
- * the frontend will generate an intrinsic that causes the BPF backend
- * to exit due to a missing implementation. Provide a simple implementation
- * just for memset to use it for poisoning/unpoisoning the map.
- */
-__weak int asan_memset(s8 __arena *dst, s8 val, size_t size)
-{
-	size_t i;
-
-	for (i = zero; i < size && can_loop; i++)
-		dst[i] = val;
-
-	return 0;
-}
-
 /* Validate a 1-byte access, always within a single byte. */
 static __always_inline bool memory_is_poisoned_1(s8 __arena *addr)
 {
@@ -422,7 +405,7 @@ __hidden __noasan int asan_poison(void __arena *addr, s8 val, size_t size)
 	shadow = mem_to_shadow(addr);
 	len = size >> ASAN_SHADOW_SHIFT;
 
-	asan_memset(shadow, val, len);
+	arena_memset(shadow, val, len);
 
 	return 0;
 }
@@ -463,7 +446,7 @@ __hidden __noasan int asan_unpoison(void __arena *addr, size_t size)
 	shadow = mem_to_shadow(addr);
 	len = size >> ASAN_SHADOW_SHIFT;
 
-	asan_memset(shadow, 0, len);
+	arena_memset(shadow, 0, len);
 
 	/*
 	 * If we are allocating a non-granule aligned region, we need to adjust
diff --git a/tools/testing/selftests/bpf/libarena/src/common.bpf.c b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
index 569f0f64d518..785d4872cb49 100644
--- a/tools/testing/selftests/bpf/libarena/src/common.bpf.c
+++ b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: LGPL-2.1 OR BSD-2-Clause
 /* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <limits.h>
+
 #include <libarena/common.h>
 #include <libarena/asan.h>
 #include <libarena/buddy.h>
@@ -48,10 +50,33 @@ __weak void __arena *arena_malloc(size_t size)
 	return buddy_alloc(&buddy, size);
 }
 
+__weak void __arena *arena_calloc(size_t ncount, size_t size)
+{
+	void __arena *mem;
+	size_t total;
+
+	/*
+	 * Ideally we'd be using __builtin_mul_overflow here,
+	 * but the BPF compiler backend doesn't implement __multi3.
+	 * There are ways to optimize the division away from the
+	 * overflow check, but any costs are dwarfed by the
+	 * buddy_alloc() call. Keep it simple for now.
+	 */
+	if (unlikely(ncount && size >= ULLONG_MAX / ncount))
+		return NULL;
+
+	total = ncount * size;
+
+	mem = buddy_alloc(&buddy, total);
+	if (likely(mem))
+		arena_memset(mem, 0, total);
+
+	return mem;
+}
+
 __weak void arena_free(void __arena *ptr)
 {
 	buddy_free(&buddy, ptr);
 }
 
-
 char _license[] SEC("license") = "GPL";
-- 
2.54.0