Re: [PATCH bpf-next] bpf, x86: Simplify the parsing logic of structure parameters

[Pu Lehui <pulehui@huaweicloud.com>]

On 2023/1/5 2:24, Yonghong Song wrote:
> 
> 
> On 1/2/23 5:31 PM, Pu Lehui wrote:
>> From: Pu Lehui <pulehui@huawei.com>
>>
>> Extra_nregs of structure parameters and nr_args can be
>> added directly at the beginning, and using a flip flag
>> to identifiy structure parameters. Meantime, renaming
>> some variables to make them more sense.
>>
>> Signed-off-by: Pu Lehui <pulehui@huawei.com>
> 
> Thanks for refactoring. Using nr_regs instead of nr_args indeed
> making things easier to understand. Ack with a few nits below.
> 
> Acked-by: Yonghong Song <yhs@fb.com>
> 
>> ---
>>   arch/x86/net/bpf_jit_comp.c | 99 +++++++++++++++++--------------------
>>   1 file changed, 46 insertions(+), 53 deletions(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index e3e2b57e4e13..e7b72299f5a4 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -1839,62 +1839,57 @@ st:            if (is_imm8(insn->off))
>>       return proglen;
>>   }
>> -static void save_regs(const struct btf_func_model *m, u8 **prog, int 
>> nr_args,
>> +static void save_regs(const struct btf_func_model *m, u8 **prog, int 
>> nr_regs,
>>                 int stack_size)
>>   {
>> -    int i, j, arg_size, nr_regs;
>> +    int i, j, arg_size;
>> +    bool is_struct = false;
>> +
>>       /* Store function arguments to stack.
>>        * For a function that accepts two pointers the sequence will be:
>>        * mov QWORD PTR [rbp-0x10],rdi
>>        * mov QWORD PTR [rbp-0x8],rsi
>>        */
>> -    for (i = 0, j = 0; i < min(nr_args, 6); i++) {
>> -        if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
>> -            nr_regs = (m->arg_size[i] + 7) / 8;
>> +    for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
>> +        arg_size = m->arg_size[j];
>> +        if (arg_size > 8) {
>>               arg_size = 8;
>> -        } else {
>> -            nr_regs = 1;
>> -            arg_size = m->arg_size[i];
>> +            is_struct ^= 1;
>>           }
>> -        while (nr_regs) {
>> -            emit_stx(prog, bytes_to_bpf_size(arg_size),
>> -                 BPF_REG_FP,
>> -                 j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
>> -                 -(stack_size - j * 8));
>> -            nr_regs--;
>> -            j++;
>> -        }
>> +        emit_stx(prog, bytes_to_bpf_size(arg_size),
>> +             BPF_REG_FP,
>> +             i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
>> +             -(stack_size - i * 8));
>> +
>> +        j = is_struct ? j : j + 1;
>>       }
>>   }
>> -static void restore_regs(const struct btf_func_model *m, u8 **prog, 
>> int nr_args,
>> +static void restore_regs(const struct btf_func_model *m, u8 **prog, 
>> int nr_regs,
>>                int stack_size)
>>   {
>> -    int i, j, arg_size, nr_regs;
>> +    int i, j, arg_size;
>> +    bool is_struct = false;
> 
> Maybe
>      bool next_same_struct = false
> to better characterize what it means?
> 

agree, will do as suggested bellow.

>>       /* Restore function arguments from stack.
>>        * For a function that accepts two pointers the sequence will be:
>>        * EMIT4(0x48, 0x8B, 0x7D, 0xF0); mov rdi,QWORD PTR [rbp-0x10]
>>        * EMIT4(0x48, 0x8B, 0x75, 0xF8); mov rsi,QWORD PTR [rbp-0x8]
>>        */
>> -    for (i = 0, j = 0; i < min(nr_args, 6); i++) {
>> -        if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
>> -            nr_regs = (m->arg_size[i] + 7) / 8;
>> +    for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
> 
> Let us put a comment here so the later users can understand the logic
> behind 'is_struct ^= 1'.
> 
> /* The arg_size is at most 16 bytes, enforced by the verifier. */
> 
>> +        arg_size = m->arg_size[j];
>> +        if (arg_size > 8) {
>>               arg_size = 8;
>> -        } else {
>> -            nr_regs = 1;
>> -            arg_size = m->arg_size[i];
>> +            is_struct ^= 1;
> 
> next_same_struct = !next_same_struct;
> 
> The same for above save_regs().
> 
>>           }
>> -        while (nr_regs) {
>> -            emit_ldx(prog, bytes_to_bpf_size(arg_size),
>> -                 j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
>> -                 BPF_REG_FP,
>> -                 -(stack_size - j * 8));
>> -            nr_regs--;
>> -            j++;
>> -        }
>> +        emit_ldx(prog, bytes_to_bpf_size(arg_size),
>> +             i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
>> +             BPF_REG_FP,
>> +             -(stack_size - i * 8));
>> +
>> +        j = is_struct ? j : j + 1;
>>       }
>>   }
> [...]