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

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Date:   Sun, 14 Jan 2018 09:31:04 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Karim Eshapa <karim.eshapa@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, ecree@...arflare.com,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] kernel:bpf Remove structure passing and assignment to
 save stack and no coping structures

On Sun, Jan 14, 2018 at 01:18:35PM +0200, Karim Eshapa wrote:
> >> 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.

1. your patch has compile time warnings
2. it doesn't reduce stack size.
   For two functions that use tnum_add:
   adjust_ptr_min_max_vals() before and after has exactly the same.
   check_ptr_alignment() after your patch _increased_ stack size.
3. text of verifier.o shrank 133 bytes while tnum.o increased 198

Please do your homework next time.
tnum code will stay as-is.