netdev - [PATCH] kernel:bpf Remove structure passing and assignment to save stack and no coping structures

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Date:   Sun, 14 Jan 2018 13:18:35 +0200
From:   Karim Eshapa <karim.eshapa@...il.com>
To:     ast@...nel.org
Cc:     daniel@...earbox.net, ecree@...arflare.com, davem@...emloft.net,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Karim Eshapa <karim.eshapa@...il.com>
Subject: [PATCH] kernel:bpf Remove structure passing and assignment to save stack and no coping structures

>> Use pointers to structure as arguments to function instead of coping
>> structures and less stack size. Also transfer TNUM(_v, _m) to
>> tnum.h file to be used in differnet files for creating anonymous structures
>> statically.
>>
>> Signed-off-by: Karim Eshapa <karim.eshapa@...il.com>
...
>> +/* Statically tnum constant */
>> +#define TNUM(_v, _m) (struct tnum){.value = _v, .mask = _m}
>>  /* Represent a known constant as a tnum. */
>>  struct tnum tnum_const(u64 value);
>>  /* A completely unknown value */
>> @@ -26,7 +28,7 @@ struct tnum tnum_lshift(struct tnum a, u8 shift);
>>  /* Shift a tnum right (by a fixed shift) */
>>  struct tnum tnum_rshift(struct tnum a, u8 shift);
>>  /* Add two tnums, return @a + @b */
>> -struct tnum tnum_add(struct tnum a, struct tnum b);
>> +void tnum_add(struct tnum *res, struct tnum *a, struct tnum *b);
...
>> -     reg_off = tnum_add(reg->var_off, tnum_const(ip_align + reg->off + off));
>> +     tnum_add(&reg_off, &reg->var_off, &TNUM(ip_align + reg->off + off, 0));
>>       if (!tnum_is_aligned(reg_off, size)) {
>>               char tn_buf[48];
>>
>> @@ -1023,8 +1023,7 @@ static int check_generic_ptr_alignment(struct bpf_verifier_env *env,
>>       /* Byte size accesses are always allowed. */
>>       if (!strict || size == 1)
>>               return 0;
>> -
>> -     reg_off = tnum_add(reg->var_off, tnum_const(reg->off + off));
>> +     tnum_add(&reg_off, &reg->var_off, &TNUM(reg->off + off, 0));
...
>> -             dst_reg->var_off = tnum_add(ptr_reg->var_off, off_reg->var_off);
>> +             tnum_add(&dst_reg->var_off, &ptr_reg->var_off,
>> +                     &off_reg->var_off);

>Is it gnu or intel style of argumnets ? where is src or dest ?
>Can the same pointer be used as src and as dst ? etc, etc
>I don't think it saves stack either.
>I'd rather leave things as-is.

It's not specific style but it's recommended when passing structure specially if
the structures have large sizes.
and (dest, src0, src1) respectively.Although tnum structure isn't large but it saves
stack,we have 2 structure passed before calling and 1 returned to receive the return value.

>I think that looks much worse and error prone.

I don't actually see errors unless inentionally passing wrong parameters.

Thanks,
Karim