tools/bpftool: fix a percpu_array map dump problem

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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/bpf/bpftool/map.c

···
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <linux/kernel.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
···
 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);
 }
···
 		jsonw_name(json_wtr, "value");
 		print_hex_data_json(value, info->value_size);
 	} 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);
···
 			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);
···
 
 		printf("\n");
 	} else {
-		unsigned int i, n;
+		unsigned int i, n, step;
 
 		n = get_possible_cpus();
+		step = round_up(info->value_size, 8);
 
 		printf("key:\n");
 		fprint_hex(stdout, key, info->key_size, " ");
···
 		for (i = 0; i < n; i++) {
 			printf("value (CPU %02d):%c",
 			       i, info->value_size > 16 ? '\n' : ' ');
-			fprint_hex(stdout, value + i * info->value_size,
+			fprint_hex(stdout, value + i * step,
 				   info->value_size, " ");
 			printf("\n");
 		}