gdb 8.0.1连接qemu时，出现“Remote 'g' packet reply is too long”

Posted 2020-10-19 phoenix tree

1 原因

“Note that other tutorials also add a "-S" parameter so QEMU starts the kernel stopped, however this is ommitted deliberately. The "-S" parameter would allow gdb to set an initial breakpoint anywhere in the kernel before kernel execution begins. Unfortunately, a change made to the gdbserver in QEMU, to support debugging 32- and 16-bit guest code in an x86_64 session breaks the -S functionality. The symptoms are that gdb prints out "Remote ‘g‘ packet reply is too long:", and fails to interact successfully with QEMU. The suggested fix is to run the QEMU until it is in 64-bit code (i.e. after the boot loader has finished and the kernel started) before connecting from gdb (omitting the -S parameter). To debug a running kernel, this is sufficient; it is the method we will take.”

2 解决办法

gdb源码根目录/gdb/remote.c里面，将

if (buf_len > 2 * rsa->sizeof_g_packet)

error (_(“Remote ‘g’ packet reply is too long: %s”), rs->buf);

修改为

if (buf_len > 2 * rsa->sizeof_g_packet) {

    rsa->sizeof_g_packet = buf_len ;

    for (i = 0; i < gdbarch_num_regs (gdbarch); i++) {

        if (rsa->regs->pnum == -1)

            continue;

        if (rsa->regs->offset >= rsa->sizeof_g_packet)

            rsa->regs->in_g_packet = 0;

        else  

            rsa->regs->in_g_packet = 1;

    }     

}

也就是说，不是作为error来处理，而是按照下面的处理方式进行处理。

 

3 cpu是x86_64