Commit 7de8587b53 for qemu.org

commit 7de8587b5317f0bb5f839e2f55855d6c9ee62835
Author: Peter Maydell <peter.maydell@linaro.org>
Date:   Thu Feb 12 14:09:17 2026 +0000

    net: mark struct tcp_header as QEMU_PACKED

    The eth_header is not actually guaranteed to be aligned.  We attempt
    to deal with this in some places such as net_checksum_calculate() by
    using lduw_be_p() and so on to access the fields, but this is not
    sufficient to be correct, because even accessing a byte member within
    a misaligned struct is undefined behaviour.  The clang sanitizer will
    emit an error like this if you run the sifive_u_mmc functional test
    with sanitizers enabled:

    ../../net/checksum.c:144:24: runtime error: member access within misaligned address 0x619a74c32033 for type 'tcp_header' (aka 'struct tcp_header'), which requires 4 byte alignment
    0x619a74c32033: note: pointer points here
     0a  00 02 02 86 aa 00 16 52  c1 d3 70 00 00 00 00 a0  02 fa f0 00 00 00 00 02  04 05 b4 04 02 08 0a
                  ^
        #0 0x619a6ba84794 in net_checksum_calculate /home/pm215/qemu/build/clang/../../net/checksum.c:144:24
        #1 0x619a6b5940da in gem_transmit /home/pm215/qemu/build/clang/../../hw/net/cadence_gem.c:1386:21
        #2 0x619a6b592141 in gem_write /home/pm215/qemu/build/clang/../../hw/net/cadence_gem.c:1650:13

    Fix this by marking the tcp_header struct as QEMU_PACKED.

    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
    Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
    Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
    Message-ID: <20260212140917.1443253-6-peter.maydell@linaro.org>
    Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

diff --git a/include/net/eth.h b/include/net/eth.h
index df90ff0837..efe270dbfe 100644
--- a/include/net/eth.h
+++ b/include/net/eth.h
@@ -68,7 +68,7 @@ typedef struct tcp_header {
     uint16_t th_win;            /* window */
     uint16_t th_sum;            /* checksum */
     uint16_t th_urp;            /* urgent pointer */
-} tcp_header;
+} QEMU_PACKED tcp_header;

 #define TCP_FLAGS_ONLY(flags) ((flags) & 0x3f)