On 07/30/2018 05:49 PM, 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 <[email protected]>
Applied to bpf, thanks Yonghong!
