netdev - Re: [PATCH bpf] tools/bpftool: fix a percpu

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Date:   Sat, 28 Jul 2018 21:14:25 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, ast@...com, netdev@...r.kernel.org
Cc:     kernel-team@...com
Subject: Re: [PATCH bpf] tools/bpftool: fix a percpu_array map dump problem

On 07/28/2018 01:11 AM, Yonghong Song wrote:
> I hit the following problem when I tried to use bpftool
> to dump a percpu array.
> 
>   $ sudo ./bpftool map show
>   61: percpu_array  name stub  flags 0x0
> 	  key 4B  value 4B  max_entries 1  memlock 4096B
>   ...
>   $ sudo ./bpftool map dump id 61
>   bpftool: malloc.c:2406: sysmalloc: Assertion
>   `(old_top == initial_top (av) && old_size == 0) || \
>    ((unsigned long) (old_size) >= MINSIZE && \
>    prev_inuse (old_top) && \
>    ((unsigned long) old_end & (pagesize - 1)) == 0)'
>   failed.
>   Aborted
> 
> Further debugging revealed that this is due to
> miscommunication between bpftool and kernel.
> For example, for the above percpu_array with value size of 4B.
> The map info returned to user space has value size of 4B.
> 
> In bpftool, the values array for lookup is allocated like:
>    info->value_size * get_possible_cpus() = 4 * get_possible_cpus()
> In kernel (kernel/bpf/syscall.c), the values array size is
> rounded up to multiple of 8.
>    round_up(map->value_size, 8) * num_possible_cpus()
>    = 8 * num_possible_cpus()
> So when kernel copies the values to user buffer, the kernel will
> overwrite beyond user buffer boundary.
> 
> This patch fixed the issue by allocating and stepping through
> percpu map value array properly in bpftool.
> 
> Fixes: 71bb428fe2c19 ("tools: bpf: add bpftool")
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  tools/bpf/bpftool/map.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index 0ee3ba479d87..92bc55f98c4c 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -35,6 +35,7 @@
>  #include <errno.h>
>  #include <fcntl.h>
>  #include <linux/err.h>
> +#include <linux/kernel.h>
>  #include <stdbool.h>
>  #include <stdio.h>
>  #include <stdlib.h>
> @@ -91,7 +92,8 @@ static bool map_is_map_of_progs(__u32 type)
>  static void *alloc_value(struct bpf_map_info *info)
>  {
>  	if (map_is_per_cpu(info->type))
> -		return malloc(info->value_size * get_possible_cpus());
> +		return malloc(round_up(info->value_size, 8) *
> +			      get_possible_cpus());
>  	else
>  		return malloc(info->value_size);
>  }
> @@ -273,9 +275,10 @@ static void print_entry_json(struct bpf_map_info *info, unsigned char *key,
>  			do_dump_btf(&d, info, key, value);
>  		}
>  	} else {
> -		unsigned int i, n;
> +		unsigned int i, n, step;
>  
>  		n = get_possible_cpus();
> +		step = round_up(info->value_size, 8);
>  
>  		jsonw_name(json_wtr, "key");
>  		print_hex_data_json(key, info->key_size);
> @@ -288,7 +291,7 @@ static void print_entry_json(struct bpf_map_info *info, unsigned char *key,
>  			jsonw_int_field(json_wtr, "cpu", i);
>  
>  			jsonw_name(json_wtr, "value");
> -			print_hex_data_json(value + i * info->value_size,
> +			print_hex_data_json(value + i * step,
>  					    info->value_size);
>  
>  			jsonw_end_object(json_wtr);

Fix looks correct to me, but you would also need the same fix in print_entry_plain(), no?

Thanks,
Daniel