netdev - Re: [PATCH bpf v2] tools/bpftool: fix a percpu

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Message-ID: <a8218a13-89a1-c9af-2c38-1f8a14a8b820@iogearbox.net>
Date:   Mon, 30 Jul 2018 11:21:37 +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 v2] tools/bpftool: fix a percpu_array map dump problem

Hi Yonghong,

On 07/29/2018 07:20 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 <yhs@...com>
> ---
>  tools/bpf/bpftool/map.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> Changelogs:
>  v1 -> v2:
>    . Added missing fix in function print_entry_plain().

The patch does not apply against bpf tree. I think you've rebased that against
bpf-next instead, but the fix really should go into bpf. Please respin against
correct tree.

Thanks,
Daniel