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Message-ID: <7a9915cf-f004-453a-b328-80d086c14a80@redhat.com>
Date: Fri, 16 Feb 2024 19:05:39 +0100
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Subject: Re: [PATCH v2 6/8] checksum: use csum_ip4_header() in udp.c and tcp.c
To: Stefano Brivio <sbrivio@redhat.com>
References: <20240214085628.210783-1-lvivier@redhat.com>
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<53d4a403-0d3f-4aa0-b980-27c2026a468b@redhat.com>
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From: Laurent Vivier <lvivier@redhat.com>
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On 2/16/24 15:54, Stefano Brivio wrote:
> On Fri, 16 Feb 2024 15:17:13 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
>
>> On 2/16/24 10:08, Stefano Brivio wrote:
>>> On Wed, 14 Feb 2024 09:56:26 +0100
>>> Laurent Vivier <lvivier@redhat.com> wrote:
>>>
>>>> ...
>>>> /**
>>>> * udp_update_l2_buf() - Update L2 buffers with Ethernet and IPv4 addresses
>>>> * @eth_d: Ethernet destination address, NULL if unchanged
>>>> @@ -579,6 +562,9 @@ static void udp_splice_sendfrom(const struct ctx *c, unsigned start, unsigned n,
>>>> *
>>>> * Return: size of tap frame with headers
>>>> */
>>>> +#pragma GCC diagnostic push
>>>> +/* ignore unaligned pointer value warning for &b->iph */
>>>> +#pragma GCC diagnostic ignored "-Waddress-of-packed-member"
>>>> static size_t udp_update_hdr4(const struct ctx *c, int n, in_port_t dstport,
>>>> const struct timespec *now)
>>>> {
>>>> @@ -614,13 +600,14 @@ static size_t udp_update_hdr4(const struct ctx *c, int n, in_port_t dstport,
>>>> b->iph.saddr = b->s_in.sin_addr.s_addr;
>>>> }
>>>>
>>>> - udp_update_check4(b);
>>>> + b->iph.check = csum_ip4_header(&b->iph);
>>> Similar comment as I had on v1: I don't think this is safe.
>>>
>>> If &b->iph is, say, 0x2000, it's all fine: when csum_ip4_header() needs
>>> to access, say, ip4h->tot_len, it will dereference 0x2000 and look at
>>> 16 bits, 2 bytes into it.
>>>
>>> If &b->iph is 0x2001, though, csum_ip4_header() will dereference 0x2001
>>> and, on some architectures, boom.
>> I don't understand how &b->iph cannot be aligned as b should be aligned and b is defined
>> using udp4_l2_buf_t structure with _attribute__ ((packed, aligned(__alignof__(unsigned
>> int)))).
> That's because of the size of struct tap_hdr (18 bytes). On, at least,
> x86_64, armhf, and i686:
>
> $ pahole passt
>
> [...]
>
> struct udp4_l2_buf_t {
> struct sockaddr_in s_in; /* 0 16 */
> struct tap_hdr taph; /* 16 18 */
> struct iphdr iph; /* 34 20 */
>
> [...]
>
> ...we could align the start of 'taph' by adding 2 bytes of padding before
> it, note that the size of struct sockaddr_in doesn't depend on the
> architecture. But then you can't dereference 'taph', which is probably
> even worse.
>
So I think in the worst case iph is aligned on 2.
Do you know which architectures don't support this alignment?
Do you know if we will support this architecture?
I think I will send the v3 of my series without fixing that because I don't have enough
time this week. I will address the problem later.
Thanks,
Laurent