On Sun, Mar 20, 2022 at 8:55 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> Since now there might be at most 10 offsets that need handling in
> copy_map_value, the manual shuffling and special case is no longer going
> to work. Hence, let's generalise the copy_map_value function by using
> a sorted array of offsets to skip regions that must be avoided while
> copying into and out of a map value.
>
> When the map is created, we populate the offset array in struct map,
> with one extra element for map->value_size, which is used as the final
> offset to subtract previous offset from. Then, copy_map_value uses this
> sorted offset array is used to memcpy while skipping timer, spin lock,
> and kptr.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> include/linux/bpf.h | 55 +++++++++++++++++++++++---------------------
> kernel/bpf/syscall.c | 52 +++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 81 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 9d424d567dd3..6474d2d44b78 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -158,6 +158,10 @@ struct bpf_map_ops {
> enum {
> /* Support at most 8 pointers in a BPF map value */
> BPF_MAP_VALUE_OFF_MAX = 8,
> + BPF_MAP_OFF_ARR_MAX = BPF_MAP_VALUE_OFF_MAX +
> + 1 + /* for bpf_spin_lock */
> + 1 + /* for bpf_timer */
> + 1, /* for map->value_size sentinel */
> };
>
> enum {
> @@ -206,9 +210,17 @@ struct bpf_map {
> char name[BPF_OBJ_NAME_LEN];
> bool bypass_spec_v1;
> bool frozen; /* write-once; write-protected by freeze_mutex */
> - /* 6 bytes hole */
> -
> - /* The 3rd and 4th cacheline with misc members to avoid false sharing
> + /* 2 bytes hole */
> + struct {
> + struct {
> + u32 off;
> + u8 sz;
So here we are wasting 11 * 3 == 33 bytes of padding, right? And it
will only increase as we add bpf_dynptr support soon.
But if we split this struct into two arrays you won't be wasting any of that:
struct {
u32 cnt;
u32 field_offs[BPF_MAP_OFF_ARR_MAX];
u8 szs[BPF_MAP_OFF_ARR_MAX]
} off_arr;
?
Further, given the majority of BPF maps in the system probably won't
use any of these special fields, would it make sense to dynamically
allocate this portion of struct bpf_map?
> + } field[BPF_MAP_OFF_ARR_MAX];
> + u32 cnt;
> + } off_arr;
> + /* 40 bytes hole */
> +
> + /* The 4th and 5th cacheline with misc members to avoid false sharing
> * particularly with refcounting.
> */
> atomic64_t refcnt ____cacheline_aligned;
> @@ -250,36 +262,27 @@ static inline void check_and_init_map_value(struct bpf_map *map, void *dst)
> memset(dst + map->spin_lock_off, 0, sizeof(struct bpf_spin_lock));
> if (unlikely(map_value_has_timer(map)))
> memset(dst + map->timer_off, 0, sizeof(struct bpf_timer));
> + if (unlikely(map_value_has_kptr(map))) {
> + struct bpf_map_value_off *tab = map->kptr_off_tab;
> + int i;
> +
> + for (i = 0; i < tab->nr_off; i++)
> + *(u64 *)(dst + tab->off[i].offset) = 0;
> + }
> }
>
> /* copy everything but bpf_spin_lock and bpf_timer. There could be one of each. */
> static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)
> {
> - u32 s_off = 0, s_sz = 0, t_off = 0, t_sz = 0;
> + int i;
>
> - if (unlikely(map_value_has_spin_lock(map))) {
> - s_off = map->spin_lock_off;
> - s_sz = sizeof(struct bpf_spin_lock);
> - }
> - if (unlikely(map_value_has_timer(map))) {
> - t_off = map->timer_off;
> - t_sz = sizeof(struct bpf_timer);
> - }
> + memcpy(dst, src, map->off_arr.field[0].off);
> + for (i = 1; i < map->off_arr.cnt; i++) {
> + u32 curr_off = map->off_arr.field[i - 1].off;
> + u32 next_off = map->off_arr.field[i].off;
>
> - if (unlikely(s_sz || t_sz)) {
> - if (s_off < t_off || !s_sz) {
> - swap(s_off, t_off);
> - swap(s_sz, t_sz);
> - }
> - memcpy(dst, src, t_off);
> - memcpy(dst + t_off + t_sz,
> - src + t_off + t_sz,
> - s_off - t_off - t_sz);
> - memcpy(dst + s_off + s_sz,
> - src + s_off + s_sz,
> - map->value_size - s_off - s_sz);
> - } else {
> - memcpy(dst, src, map->value_size);
> + curr_off += map->off_arr.field[i - 1].sz;
> + memcpy(dst + curr_off, src + curr_off, next_off - curr_off);
> }
We can also get away with value_size sentinel value if we rewrite this
logic as follows:
u32 cur_off = 0;
int i;
for (i = 0; i < map->off_arr.cnt; i++) {
memcpy(dst + cur_off, src + cur_off, map->off_arr.field[i].off - cur_off);
cur_off += map->off_arr.field[i].sz;
}
memcpy(dst + cur_off, src + cur_off, map->value_size - cur_off);
It will be as optimal but won't require value_size sentinel.
> }
> void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 5990d6fa97ab..7b32537bd81f 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -30,6 +30,7 @@
> #include <linux/pgtable.h>
> #include <linux/bpf_lsm.h>
> #include <linux/poll.h>
> +#include <linux/sort.h>
> #include <linux/bpf-netns.h>
> #include <linux/rcupdate_trace.h>
> #include <linux/memcontrol.h>
> @@ -851,6 +852,55 @@ int map_check_no_btf(const struct bpf_map *map,
> return -ENOTSUPP;
> }
>
> +static int map_off_arr_cmp(const void *_a, const void *_b)
> +{
> + const u32 a = *(const u32 *)_a;
> + const u32 b = *(const u32 *)_b;
> +
> + if (a < b)
> + return -1;
> + else if (a > b)
> + return 1;
> + return 0;
> +}
> +
> +static void map_populate_off_arr(struct bpf_map *map)
> +{
> + u32 i;
> +
> + map->off_arr.cnt = 0;
> + if (map_value_has_spin_lock(map)) {
> + i = map->off_arr.cnt;
> +
> + map->off_arr.field[i].off = map->spin_lock_off;
> + map->off_arr.field[i].sz = sizeof(struct bpf_spin_lock);
> + map->off_arr.cnt++;
> + }
> + if (map_value_has_timer(map)) {
> + i = map->off_arr.cnt;
> +
> + map->off_arr.field[i].off = map->timer_off;
> + map->off_arr.field[i].sz = sizeof(struct bpf_timer);
> + map->off_arr.cnt++;
> + }
> + if (map_value_has_kptr(map)) {
> + struct bpf_map_value_off *tab = map->kptr_off_tab;
> + u32 j = map->off_arr.cnt;
> +
> + for (i = 0; i < tab->nr_off; i++) {
> + map->off_arr.field[j + i].off = tab->off[i].offset;
> + map->off_arr.field[j + i].sz = sizeof(u64);
> + }
> + map->off_arr.cnt += tab->nr_off;
> + }
> +
> + map->off_arr.field[map->off_arr.cnt++].off = map->value_size;
Using a pointer for map->off_arr.field[j + i] and incrementing it
along the cnt would make this code more succinct, and possibly even a
bit more efficient. With my above suggestion to split offs from szs,
you'll need two pointers, but still might be cleaner.
> + if (map->off_arr.cnt == 1)
> + return;
> + sort(map->off_arr.field, map->off_arr.cnt, sizeof(map->off_arr.field[0]),
> + map_off_arr_cmp, NULL);
See how Jiri is using sort_r() to sort two related arrays and keep
them in sync w.r.t. order.
> +}
> +
> static int map_check_btf(struct bpf_map *map, const struct btf *btf,
> u32 btf_key_id, u32 btf_value_id)
> {
> @@ -1018,6 +1068,8 @@ static int map_create(union bpf_attr *attr)
> attr->btf_vmlinux_value_type_id;
> }
>
> + map_populate_off_arr(map);
> +
> err = security_bpf_map_alloc(map);
> if (err)
> goto free_map;
> --
> 2.35.1
>
