Re: [PATCH bpf-next 8/9] bpf: Introduce bpf register BPF_REG_PARAMS

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue Apr 21, 2026 at 5:09 PM PDT, Yonghong Song wrote:
>
>
> On 4/21/26 3:10 PM, Alexei Starovoitov wrote:
>> On Tue Apr 21, 2026 at 10:20 AM PDT, Yonghong Song wrote:
>>>   
>>>   /* Kernel hidden auxiliary/helper register. */
>>> -#define BPF_REG_AX		MAX_BPF_REG
>>> -#define MAX_BPF_EXT_REG		(MAX_BPF_REG + 1)
>>> +#define BPF_REG_PARAMS		MAX_BPF_REG
>>> +#define BPF_REG_AX		(MAX_BPF_REG + 1)
>>> +#define MAX_BPF_EXT_REG		(MAX_BPF_REG + 2)
>> ...
>>
>>> --- a/tools/testing/selftests/bpf/progs/verifier_bpf_fastcall.c
>>> +++ b/tools/testing/selftests/bpf/progs/verifier_bpf_fastcall.c
>>> @@ -630,13 +630,13 @@ __xlated("...")
>>>   __xlated("4: r0 = &(void __percpu *)(r0)")
>>>   __xlated("...")
>>>   /* may_goto expansion starts */
>>> -__xlated("6: r11 = *(u64 *)(r10 -24)")
>>> -__xlated("7: if r11 == 0x0 goto pc+6")
>>> -__xlated("8: r11 -= 1")
>>> -__xlated("9: if r11 != 0x0 goto pc+2")
>>> -__xlated("10: r11 = -24")
>>> +__xlated("6: r12 = *(u64 *)(r10 -24)")
>>> +__xlated("7: if r12 == 0x0 goto pc+6")
>>> +__xlated("8: r12 -= 1")
>>> +__xlated("9: if r12 != 0x0 goto pc+2")
>>> +__xlated("10: r12 = -24")
>> maybe shift it to r15 right away, so we don't need to touch this code
>> if/when true r12 is introduced?
>
> We can do this. Do you think the following is okay:
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index b77d0b06db6e..fe7b6b943ea4 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -59,9 +59,9 @@ struct ctl_table_header;
>   
>   /* Kernel hidden auxiliary/helper register. */
>   #define BPF_REG_PARAMS         MAX_BPF_REG
> -#define BPF_REG_AX             (MAX_BPF_REG + 1)
> -#define MAX_BPF_EXT_REG                (MAX_BPF_REG + 2)
> +#define MAX_BPF_EXT_REG                (MAX_BPF_REG + 1)
>   #define MAX_BPF_JIT_REG                MAX_BPF_EXT_REG
> +#define BPF_REG_AX             (MAX_BPF_REG + 4)
>   
>   /* unused opcode to mark special call to bpf_tail_call() helper */
>   #define BPF_TAIL_CALL  0xf0
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index ae10b9ca018d..3ad286ef3085 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1299,8 +1299,8 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
>          u32 imm_rnd = get_random_u32();
>          s16 off;
>   
> -       BUILD_BUG_ON(BPF_REG_PARAMS + 2 != MAX_BPF_JIT_REG);
> -       BUILD_BUG_ON(BPF_REG_AX + 1 != MAX_BPF_JIT_REG);
> +       BUILD_BUG_ON(BPF_REG_PARAMS + 1 != MAX_BPF_JIT_REG);
> +       BUILD_BUG_ON(BPF_REG_AX != MAX_BPF_JIT_REG + 3);

Ohh. We have:
static unsigned int PROG_NAME(stack_size)(const void *ctx, const struct bpf_insn *insn) \
{ \
        u64 stack[stack_size / sizeof(u64)]; \
        u64 regs[MAX_BPF_EXT_REG] = {}; \


Please double check that BPF_REG_AX is not used by the interpreter.

but it's starting to feel that my suggestion was premature.
Probably better to keep this patchset as-is.