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Date: Fri, 16 Feb 2024 15:54:00 +0100
From: Stefano Brivio <sbrivio@redhat.com>
To: Laurent Vivier <lvivier@redhat.com>
Subject: Re: [PATCH v2 6/8] checksum: use csum_ip4_header() in udp.c and
tcp.c
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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.
--
Stefano