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

[Алексей Сафин]

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@xxxxxxxxx> wrote:
> > On Fri, Nov 7, 2025 at 7:41 PM David Laight
> > <david.laight.linux@xxxxxxxxx> wrote:
> > > On Fri,  7 Nov 2025 13:03:05 +0300
> > > Alexei Safin <a.safin@xxxxxxx> 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 ;-)