Re: [PATCH bpf-next v3 4/8] selftests/bpf: make arena_base() opaque to GCC's -Warray-bounds

[Vineet Gupta <vineet.gupta@linux.dev>]

On 11/2/26 2:38 PM, Alexei Starovoitov wrote:
> On Mon, Nov 02, 2026 at 02:07 PM Vineet Gupta <vineet.gupta@linux.dev> wrote:
>> The barrier is behind #ifdef __clang__ so clang keeps the plain macro.
>> It does not need it, and the barrier is not free: it blocks reuse of
>> the map address, so each call site reloads it.  Across the five progs
>> and 20 call sites that is +35 instructions under GCC
>> (2528 -> 2563), about 1.4% on those objects and nothing elsewhere.
>> Clang output is unchanged, bit for bit.
> stream.c already has barrier_var() for both compilers.
>
> How about this one instead:

Yeah definitely much better. Just to be clear the intent of series is to 
not make current compiler look better o anything. As I mentioned in the 
cover, this was during in my in-works arena compiler testing, ....

Thx for the quick feedback.
-Vineet


> diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
> index 82aafe8..0eba529 100644
> --- a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
> +++ b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
> @@ -49,7 +49,16 @@ void __arena* bpf_arena_alloc_pages(void *map, void __arena *addr, __u32 page_cn
>   int bpf_arena_reserve_pages(void *map, void __arena *addr, __u32 page_cnt) __ksym __weak;
>   void bpf_arena_free_pages(void *map, void __arena *ptr, __u32 page_cnt) __ksym __weak;
>
> -#define arena_base(map) ((void __arena *)((struct bpf_arena *)(map))->user_vm_start)
> +/*
> + * The map definition is smaller than struct bpf_arena.
> + * barrier_var() hides that from -Warray-bounds of GCC.
> + */
> +#define arena_base(map)							\
> +	({								\
> +		struct bpf_arena *___arena = (void *)(map);		\
> +		barrier_var(___arena);					\
> +		(void __arena *)___arena->user_vm_start;		\
> +	})
>
>   #else /* when compiled as user space code */
>
> diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
> index 1ae015c..b4d88d0 100644
> --- a/tools/testing/selftests/bpf/progs/stream.c
> +++ b/tools/testing/selftests/bpf/progs/stream.c
> @@ -178,16 +178,8 @@ __stderr("Call trace:\n"
>   "|[ \t]+[^\n]+\n)*}}")
>   int stream_arena_write_fault(void *ctx)
>   {
> -	struct bpf_arena *ptr = (void *)&arena;
> -	u64 user_vm_start;
> +	u64 user_vm_start = (u64)arena_base(&arena);
>
> -	/* Prevent GCC bounds warning: casting &arena to struct bpf_arena *
> -	 * triggers bounds checking since the map definition is smaller than struct
> -	 * bpf_arena. barrier_var() makes the pointer opaque to GCC, preventing the
> -	 * bounds analysis
> -	 */
> -	barrier_var(ptr);
> -	user_vm_start = ptr->user_vm_start;
>   	fault_addr = user_vm_start + 0x7fff;
>   	bpf_addr_space_cast(user_vm_start, 0, 1);
>   	asm volatile (
> @@ -212,16 +204,8 @@ __stderr("Call trace:\n"
>   "|[ \t]+[^\n]+\n)*}}")
>   int stream_arena_read_fault(void *ctx)
>   {
> -	struct bpf_arena *ptr = (void *)&arena;
> -	u64 user_vm_start;
> +	u64 user_vm_start = (u64)arena_base(&arena);
>
> -	/* Prevent GCC bounds warning: casting &arena to struct bpf_arena *
> -	 * triggers bounds checking since the map definition is smaller than struct
> -	 * bpf_arena. barrier_var() makes the pointer opaque to GCC, preventing the
> -	 * bounds analysis
> -	 */
> -	barrier_var(ptr);
> -	user_vm_start = ptr->user_vm_start;
>   	fault_addr = user_vm_start + 0x7fff;
>   	bpf_addr_space_cast(user_vm_start, 0, 1);
>   	asm volatile (
> @@ -252,17 +236,9 @@ int stream_arena_load_acquire_fault(void *ctx)
>   		.off	 = 0x7fff,
>   		.imm	 = 0x100,	/* BPF_LOAD_ACQ */
>   	};
> -	struct bpf_arena *ptr = (void *)&arena;
> -	u64 user_vm_start, val;
> +	u64 user_vm_start = (u64)arena_base(&arena);
> +	u64 val;
>
> -	/*
> -	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
> -	 * triggers bounds checking since the map definition is smaller than
> -	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
> -	 * preventing the bounds analysis.
> -	 */
> -	barrier_var(ptr);
> -	user_vm_start = ptr->user_vm_start;
>   	fault_addr = user_vm_start + 0x7fff;
>   	bpf_addr_space_cast(user_vm_start, 0, 1);
>   	asm volatile (
> @@ -296,17 +272,9 @@ int stream_arena_xchg_fault(void *ctx)
>   		.off	 = 0x7fff,
>   		.imm	 = 0xe1,	/* BPF_XCHG */
>   	};
> -	struct bpf_arena *ptr = (void *)&arena;
> -	u64 user_vm_start, val;
> +	u64 user_vm_start = (u64)arena_base(&arena);
> +	u64 val;
>
> -	/*
> -	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
> -	 * triggers bounds checking since the map definition is smaller than
> -	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
> -	 * preventing the bounds analysis.
> -	 */
> -	barrier_var(ptr);
> -	user_vm_start = ptr->user_vm_start;
>   	fault_addr = user_vm_start + 0x7fff;
>   	bpf_addr_space_cast(user_vm_start, 0, 1);
>   	/*
> @@ -346,17 +314,9 @@ int stream_arena_cmpxchg_fault(void *ctx)
>   		.off	 = 0x7fff,
>   		.imm	 = 0xf1,	/* BPF_CMPXCHG */
>   	};
> -	struct bpf_arena *ptr = (void *)&arena;
> -	u64 user_vm_start, val;
> +	u64 user_vm_start = (u64)arena_base(&arena);
> +	u64 val;
>
> -	/*
> -	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
> -	 * triggers bounds checking since the map definition is smaller than
> -	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
> -	 * preventing the bounds analysis.
> -	 */
> -	barrier_var(ptr);
> -	user_vm_start = ptr->user_vm_start;
>   	fault_addr = user_vm_start + 0x7fff;
>   	bpf_addr_space_cast(user_vm_start, 0, 1);
>   	/*