Re: [Qemu-devel] [PATCH] net: fix misaligned member access

[Philippe Mathieu-Daudé]

Hi Marc-André,

On 02/09/2018 04:03 PM, Marc-André Lureau wrote:
> Fixes the following ASAN warnings:
> 
> /home/elmarco/src/qemu/hw/net/net_tx_pkt.c:201:27: runtime error: member 
> access within misaligned address 0x631000028846 for type 'struct ip_header', 
> which requires 4 byte alignment
> 0x631000028846: note: pointer points here
>  01 00 00 00 45 00  01 a9 01 00 00 00 40 11  78 45 00 00 00 00 ff ff  ff ff 
> 00 00 00 00 00 00  00 00
>              ^
> /home/elmarco/src/qemu/hw/net/net_tx_pkt.c:208:63: runtime error: member 
> access within misaligned address 0x631000028846 for type 'struct ip_header', 
> which requires 4 byte alignment
> 0x631000028846: note: pointer points here
>  01 00 00 00 45 00  01 a9 01 00 00 00 40 11  78 45 00 00 00 00 ff ff  ff ff 
> 00 00 00 00 00 00  00 00
>              ^
> /home/elmarco/src/qemu/hw/net/net_tx_pkt.c:210:13: runtime error: member 
> access within misaligned address 0x631000028846 for type 'struct ip_header', 
> which requires 4 byte alignment
> 0x631000028846: note: pointer points here
>  01 00 00 00 45 00  01 a9 01 00 00 00 40 11  78 45 00 00 00 00 ff ff  ff ff 
> 00 00 00 00 00 00  00 00
> 
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
>  include/net/eth.h   | 4 +++-
>  hw/net/net_tx_pkt.c | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/eth.h b/include/net/eth.h
> index 09054a506d..e6dc8a7ba0 100644
> --- a/include/net/eth.h
> +++ b/include/net/eth.h
> @@ -194,7 +194,9 @@ struct tcp_hdr {
>  #define PKT_GET_IP_HDR(p)         \
>      ((struct ip_header *)(((uint8_t *)(p)) + eth_get_l2_hdr_length(p)))
>  #define IP_HDR_GET_LEN(p)         \
> -    ((((struct ip_header *)(p))->ip_ver_len & 0x0F) << 2)
> +    ((ldub_p(p + offsetof(struct ip_header, ip_ver_len)) & 0x0F) << 2)

I like the idea of using the ldst API and offsetof() macro,
however I think it would be cleaner to use inlined static functions.

> +#define IP_HDR_GET_P(p)                                           \
> +    (ldub_p(p + offsetof(struct ip_header, ip_p)))
>  #define PKT_GET_IP_HDR_LEN(p)     \
>      (IP_HDR_GET_LEN(PKT_GET_IP_HDR(p)))
>  #define PKT_GET_IP6_HDR(p)        \
> diff --git a/hw/net/net_tx_pkt.c b/hw/net/net_tx_pkt.c
> index e29c881bc2..162f802dd7 100644
> --- a/hw/net/net_tx_pkt.c
> +++ b/hw/net/net_tx_pkt.c
> @@ -205,7 +205,7 @@ static bool net_tx_pkt_parse_headers(struct NetTxPkt *pkt)
>              return false;
>          }
>  
> -        pkt->l4proto = ((struct ip_header *) l3_hdr->iov_base)->ip_p;
> +        pkt->l4proto = IP_HDR_GET_P(l3_hdr->iov_base);
>  
>          if (IP_HDR_GET_LEN(l3_hdr->iov_base) != sizeof(struct ip_header)) {
>              /* copy optional IPv4 header data if any*/
>