linux-kernel - Re: [PATCH v2] bpf: hashtab: fix 32-bit overflow in memory usage calculation

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Message-ID: <8a4aae40-46d3-403a-a1cf-117343c584f6@rosa.ru>
Date: Sun, 9 Nov 2025 14:00:16 +0300
From: Алексей Сафин <a.safin@...a.ru>
To: Yafang Shao <laoar.shao@...il.com>,
 David Laight <david.laight.linux@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, lvc-patches@...uxtesting.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v2] bpf: hashtab: fix 32-bit overflow in memory usage
 calculation

Thanks for the follow-up.

Just to clarify: the overflow happens before the multiplication by
num_entries. In C, the * operator is left-associative, so the expression is
evaluated as (value_size * num_possible_cpus()) * num_entries. Since
value_size was u32 and num_possible_cpus() returns int, the first product is
performed in 32-bit arithmetic due to usual integer promotions. If that
intermediate product overflows, the result is already incorrect before it is
promoted when multiplied by u64 num_entries.

A concrete example within allowed limits:
value_size = 1,048,576 (1 MiB), num_possible_cpus() = 4096
=> 1,048,576 * 4096 = 2^32 => wraps to 0 in 32 bits, even with 
num_entries = 1.

This isn’t about a single >4GiB allocation - it’s about aggregated memory
usage (percpu), which can legitimately exceed 4GiB in total.

v2 promotes value_size to u64 at declaration, which avoids the 32-bit
intermediate overflow cleanly.

09.11.2025 11:20, Yafang Shao пишет:
> On Sun, Nov 9, 2025 at 11:00 AM Yafang Shao <laoar.shao@...il.com> wrote:
>> On Fri, Nov 7, 2025 at 7:41 PM David Laight
>> <david.laight.linux@...il.com> wrote:
>>> On Fri,  7 Nov 2025 13:03:05 +0300
>>> Alexei Safin <a.safin@...a.ru> wrote:
>>>
>>>> The intermediate product value_size * num_possible_cpus() is evaluated
>>>> in 32-bit arithmetic and only then promoted to 64 bits. On systems with
>>>> large value_size and many possible CPUs this can overflow and lead to
>>>> an underestimated memory usage.
>>>>
>>>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>>> That code is insane.
>>> The size being calculated looks like a kernel memory size.
>>> You really don't want to be allocating single structures that exceed 4GB.
>> I failed to get your point.
>> The calculation `value_size * num_possible_cpus() * num_entries` can
>> overflow. While the creation of a hashmap limits `value_size *
>> num_entries` to U32_MAX, this new formula can easily exceed that
>> limit. For example, on my test server with just 64 CPUs, the following
>> operation will trigger an overflow:
>>
>>            map_fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_HASH, "count_map", 4, 4,
>>                                                       1 << 27, &map_opts)
> Upon reviewing the code, I see that `num_entries` is declared as u64,
> which prevents overflow in the calculation `value_size *
> num_possible_cpus() * num_entries`. Therefore, this change is
> unnecessary.
>
> It seems that the Linux Verification Center (linuxtesting.org) needs
> to be improved ;-)
>