Re: [PATCH bpf-next] bpf: arm64: Silent "UBSAN: negation-overflow" warning

[Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx>]

On 2/18/2025 4:02 PM, Song Liu wrote:
> With UBSAN, test_bpf.ko triggers warnings like:
>
> UBSAN: negation-overflow in arch/arm64/net/bpf_jit_comp.c:1333:28
> negation of -2147483648 cannot be represented in type 's32' (aka 'int'):
>
> Silent these warnings by casting imm to u32 first.
>
> Fixes: fd868f148189 ("bpf, arm64: Optimize ADD,SUB,JMP BPF_K using arm64 add/sub immediates")

I doubt this is a bug fix since I checked the build result and found nothing changed.

> Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
> Signed-off-by: Song Liu <song@xxxxxxxxxx>
> ---
>   arch/arm64/net/bpf_jit_comp.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 8446848edddb..7409c8acbde3 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -272,7 +272,7 @@ static inline void emit_a64_add_i(const bool is64, const int dst, const int src,
>   {
>   	if (is_addsub_imm(imm)) {
>   		emit(A64_ADD_I(is64, dst, src, imm), ctx);
> -	} else if (is_addsub_imm(-imm)) {
> +	} else if (is_addsub_imm(-(u32)imm)) {
>   		emit(A64_SUB_I(is64, dst, src, -imm), ctx);
>   	} else {
>   		emit_a64_mov_i(is64, tmp, imm, ctx);
> @@ -1159,7 +1159,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
>   	case BPF_ALU64 | BPF_SUB | BPF_K:
>   		if (is_addsub_imm(imm)) {
>   			emit(A64_SUB_I(is64, dst, dst, imm), ctx);
> -		} else if (is_addsub_imm(-imm)) {
> +		} else if (is_addsub_imm(-(u32)imm)) {
>   			emit(A64_ADD_I(is64, dst, dst, -imm), ctx);
>   		} else {
>   			emit_a64_mov_i(is64, tmp, imm, ctx);
> @@ -1330,7 +1330,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
>   	case BPF_JMP32 | BPF_JSLE | BPF_K:
>   		if (is_addsub_imm(imm)) {
>   			emit(A64_CMP_I(is64, dst, imm), ctx);
> -		} else if (is_addsub_imm(-imm)) {
> +		} else if (is_addsub_imm(-(u32)imm)) {
>   			emit(A64_CMN_I(is64, dst, -imm), ctx);
>   		} else {
>   			emit_a64_mov_i(is64, tmp, imm, ctx);