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Message-ID: <CAEf4BzZN5RZ4qfMRKz0MGgEvX19TpzbmeMyTvf3misxvHuRGOg@mail.gmail.com>
Date: Mon, 30 Nov 2020 15:04:07 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/7] bpf: remove hard-coded btf_vmlinux
assumption from BPF verifier
On Sat, Nov 28, 2020 at 5:56 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Fri, Nov 20, 2020 at 06:46:10PM -0800, Andrii Nakryiko wrote:
> >
> > @@ -52,12 +53,19 @@ struct bpf_reg_state {
> > */
> > struct bpf_map *map_ptr;
> >
> > - u32 btf_id; /* for PTR_TO_BTF_ID */
> > + /* for PTR_TO_BTF_ID */
> > + struct {
> > + struct btf *btf;
> > + u32 btf_id;
> > + };
>
> bpf_reg_state is the main structure contributing to the verifier memory consumption.
> Is it possible to do the tracking without growing it?
The only way to keep this at 8 bytes in the existing union is to use
ID for BTF, but that has tons of problems: need to do look up all the
time, plus there is now a possibility of that BTF instance going away
(e.g., if kernel module is unloaded), etc. Pain.
But, I just looked at bpf_reg_state with pahole, and there are two
4-byte holes: before this union and after ref_obj_id. So if I move
"off" to before the union, the overall size of the struct won't
change, even if the union itself grows to 16 bytes. And it won't break
states_equal() logic, from what I can see.
So with that, bpf_reg_state BEFORE:
struct bpf_reg_state {
enum bpf_reg_type type; /* 0 4 */
/* XXX 4 bytes hole, try to pack */
union {
int range; /* 8 4 */
struct bpf_map * map_ptr; /* 8 8 */
u32 btf_id; /* 8 4 */
u32 mem_size; /* 8 4 */
long unsigned int raw; /* 8 8 */
}; /* 8 8 */
s32 off; /* 16 4 */
u32 id; /* 20 4 */
u32 ref_obj_id; /* 24 4 */
/* XXX 4 bytes hole, try to pack */
struct tnum var_off; /* 32 16 */
s64 smin_value; /* 48 8 */
s64 smax_value; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
u64 umin_value; /* 64 8 */
u64 umax_value; /* 72 8 */
s32 s32_min_value; /* 80 4 */
s32 s32_max_value; /* 84 4 */
u32 u32_min_value; /* 88 4 */
u32 u32_max_value; /* 92 4 */
struct bpf_reg_state * parent; /* 96 8 */
u32 frameno; /* 104 4 */
s32 subreg_def; /* 108 4 */
enum bpf_reg_liveness live; /* 112 4 */
bool precise; /* 116 1 */
/* size: 120, cachelines: 2, members: 19 */
/* sum members: 109, holes: 2, sum holes: 8 */
/* padding: 3 */
/* last cacheline: 56 bytes */
};
And with BTF pointer AFTER:
struct bpf_reg_state {
enum bpf_reg_type type; /* 0 4 */
s32 off; /* 4 4 */
union {
int range; /* 8 4 */
struct bpf_map * map_ptr; /* 8 8 */
struct {
struct btf * btf; /* 8 8 */
u32 btf_id; /* 16 4 */
}; /* 8 16 */
u32 mem_size; /* 8 4 */
struct {
long unsigned int raw1; /* 8 8 */
long unsigned int raw2; /* 16 8 */
} raw; /* 8 16 */
}; /* 8 16 */
u32 id; /* 24 4 */
u32 ref_obj_id; /* 28 4 */
struct tnum var_off; /* 32 16 */
s64 smin_value; /* 48 8 */
s64 smax_value; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
u64 umin_value; /* 64 8 */
u64 umax_value; /* 72 8 */
s32 s32_min_value; /* 80 4 */
s32 s32_max_value; /* 84 4 */
u32 u32_min_value; /* 88 4 */
u32 u32_max_value; /* 92 4 */
struct bpf_reg_state * parent; /* 96 8 */
u32 frameno; /* 104 4 */
s32 subreg_def; /* 108 4 */
enum bpf_reg_liveness live; /* 112 4 */
bool precise; /* 116 1 */
/* size: 120, cachelines: 2, members: 19 */
/* padding: 3 */
/* last cacheline: 56 bytes */
};
No more holes, but the same overall size. Does that work?
>
> >
> > u32 mem_size; /* for PTR_TO_MEM | PTR_TO_MEM_OR_NULL */
> >
> > /* Max size from any of the above. */
> > - unsigned long raw;
> > + struct {
> > + unsigned long raw1;
> > + unsigned long raw2;
> > + } raw;
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