Re: [PATCH bpf] tools/bpftool: fix a percpu_array map dump problem

[Yonghong Song]

On 7/28/18 12:14 PM, Daniel Borkmann wrote:

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 <y...@fb.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 for pointing this out. will submit v2 soon to fix the issue.

Thanks,
Daniel