Re: [PATCH 4/6] selftests/bpf: libarena: Add calloc() call

> 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;
> +}

This comment was moved and reworded from asan.bpf.c, but now has three
issues. First, the sentence boundary is lost: it reads "support the
memset intrinsics. for large sequential copies" with a lowercase
"for" starting what appears to be a new sentence. The original had
"intrinsics. For large sequential copies" as a clear sentence break.

Second, "intrinsics" is plural here but only memset is provided. The
plural was accurate when the original comment listed
"memset/memcpy/memcmp", but that list was dropped.

Third, "Provide implementations for the intrinsic" doesn't match what
the code does. There's no memset symbol or lowering hook here, so
__builtin_memset(), a plain memset() call, or a large struct
assignment will still fail in the BPF backend. What's actually
provided is a helper that must be called explicitly as arena_memset().

The original comment was more accurate: "Provide a simple
implementation just for memset to use it for poisoning/unpoisoning
the map." A reader of the new comment might reasonably think the
compiler-generated intrinsic is now handled.

Could the comment be updated to reflect what this function actually
provides?

> 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

[ ... ]

> diff --git a/tools/testing/selftests/bpf/libarena/src/common.bpf.c b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
> index 41b1de3452fe..d1af60b81838 100644
> --- a/tools/testing/selftests/bpf/libarena/src/common.bpf.c
> +++ b/tools/testing/selftests/bpf/libarena/src/common.bpf.c

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32062809860