Re: [PATCH bpf-next v1 08/15] bpf: Adapt copy_map_value for multiple offset case

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Sun, Feb 20, 2022 at 07:18:06PM +0530, Kumar Kartikeya Dwivedi wrote:
> The changes in this patch deserve closer look, so it has been split into
> its own independent patch. While earlier we just had to skip two objects
> at most while copying in and out of map, now we have potentially many
> objects (at most 8 + 2 = 10, due to the BPF_MAP_VALUE_OFF_MAX limit).
> 
> Hence, divide the copy_map_value function into an inlined fast path and
> function call to slowpath. The slowpath handles the case of > 3 offsets,
> while we handle the most common cases (0, 1, 2, or 3 offsets) in the
> inline function itself.
> 
> In copy_map_value_slow, we use 11 offsets, just to make the for loop
> that copies the value free of edge cases for the last offset, by using
> map->value_size as final offset to subtract remaining area to copy from.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  include/linux/bpf.h  | 43 +++++++++++++++++++++++++++++++---
>  kernel/bpf/syscall.c | 55 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 95 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index ae599aaf8d4c..5d845ca02eba 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -253,12 +253,22 @@ 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_ptr_to_btf_id(map))) {
> +		struct bpf_map_value_off *tab = map->ptr_off_tab;
> +		int i;
> +
> +		for (i = 0; i < tab->nr_off; i++)
> +			*(u64 *)(dst + tab->off[i].offset) = 0;
> +	}
>  }
>  
> +void copy_map_value_slow(struct bpf_map *map, void *dst, void *src, u32 s_off,
> +			 u32 s_sz, u32 t_off, u32 t_sz);
> +
>  /* 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;
> +	u32 s_off = 0, s_sz = 0, t_off = 0, t_sz = 0, p_off = 0, p_sz = 0;
>  
>  	if (unlikely(map_value_has_spin_lock(map))) {
>  		s_off = map->spin_lock_off;
> @@ -268,13 +278,40 @@ static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)
>  		t_off = map->timer_off;
>  		t_sz = sizeof(struct bpf_timer);
>  	}
> +	/* Multiple offset case is slow, offload to function */
> +	if (unlikely(map_value_has_ptr_to_btf_id(map))) {
> +		struct bpf_map_value_off *tab = map->ptr_off_tab;
> +
> +		/* Inline the likely common case */
> +		if (likely(tab->nr_off == 1)) {
> +			p_off = tab->off[0].offset;
> +			p_sz = sizeof(u64);
> +		} else {
> +			copy_map_value_slow(map, dst, src, s_off, s_sz, t_off, t_sz);
> +			return;
> +		}
> +	}
> +
> +	if (unlikely(s_sz || t_sz || p_sz)) {
> +		/* The order is p_off, t_off, s_off, use insertion sort */
>  
> -	if (unlikely(s_sz || t_sz)) {
> +		if (t_off < p_off || !t_sz) {
> +			swap(t_off, p_off);
> +			swap(t_sz, p_sz);
> +		}
>  		if (s_off < t_off || !s_sz) {
>  			swap(s_off, t_off);
>  			swap(s_sz, t_sz);
> +			if (t_off < p_off || !t_sz) {
> +				swap(t_off, p_off);
> +				swap(t_sz, p_sz);
> +			}
>  		}
> -		memcpy(dst, src, t_off);
> +
> +		memcpy(dst, src, p_off);
> +		memcpy(dst + p_off + p_sz,
> +		       src + p_off + p_sz,
> +		       t_off - p_off - p_sz);
>  		memcpy(dst + t_off + t_sz,
>  		       src + t_off + t_sz,
>  		       s_off - t_off - t_sz);
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index beb96866f34d..83d71d6912f5 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>
> @@ -230,6 +231,60 @@ static int bpf_map_update_value(struct bpf_map *map, struct fd f, void *key,
>  	return err;
>  }
>  
> +static int copy_map_value_cmp(const void *_a, const void *_b)
> +{
> +	const u32 a = *(const u32 *)_a;
> +	const u32 b = *(const u32 *)_b;
> +
> +	/* We only need to sort based on offset */
> +	if (a < b)
> +		return -1;
> +	else if (a > b)
> +		return 1;
> +	return 0;
> +}
> +
> +void copy_map_value_slow(struct bpf_map *map, void *dst, void *src, u32 s_off,
> +			 u32 s_sz, u32 t_off, u32 t_sz)
> +{
> +	struct bpf_map_value_off *tab = map->ptr_off_tab; /* already set to non-NULL */
> +	/* 3 = 2 for bpf_timer, bpf_spin_lock, 1 for map->value_size sentinel */
> +	struct {
> +		u32 off;
> +		u32 sz;
> +	} off_arr[BPF_MAP_VALUE_OFF_MAX + 3];
> +	int i, cnt = 0;
> +
> +	/* Reconsider stack usage when bumping BPF_MAP_VALUE_OFF_MAX */
> +	BUILD_BUG_ON(sizeof(off_arr) != 88);
> +
> +	for (i = 0; i < tab->nr_off; i++) {
> +		off_arr[cnt].off = tab->off[i].offset;
> +		off_arr[cnt++].sz = sizeof(u64);
> +	}
> +	if (s_sz) {
> +		off_arr[cnt].off = s_off;
> +		off_arr[cnt++].sz = s_sz;
> +	}
> +	if (t_sz) {
> +		off_arr[cnt].off = t_off;
> +		off_arr[cnt++].sz = t_sz;
> +	}
> +	off_arr[cnt].off = map->value_size;
> +
> +	sort(off_arr, cnt, sizeof(off_arr[0]), copy_map_value_cmp, NULL);

Ouch. sort every time we need to copy map value?
sort it once please. 88 bytes in a map are worth it.
Especially since "slow" version will trigger with just 2 kptrs.
(if I understand this correctly).