Remote 'g' packet reply is too long

Question

I am trying to debug Linux kernel with kvm vm. I am getting an error message "Remote 'g' packet reply is too long". My host is 64-bit and so is my vm.

My steps:

1. Start the VM with custom -kernel, -initrd and -append options.
2. Start gdb
3. Execute "set architecture i386:x86-64:intel"
4. Execute "add-symbol-file linux-3.0/vmlinux"
5. Execute "show arch" to verify its still "i386:x86-64:intel"
6. Execute "target remote localhost:1234"
7. Execute "continue"
8. Press Ctrl+C, I get the above message.

Has anyone faced this problem?

Answer 1

gdb doesn't work well against a cpu that switches between instruction sets at runtime. Wait for the kernel to leave early boot before connecting, and don't use qemu's -S flag.

Answer 2

I also faced same issue, I fixed it by modifying gdbstub.c (in qemu sources) to send 64bit registers always and hinting GDB that architecture is 64bit by passing set arch i386:x86-64

You can check the patch here: Visit [URL no longer available]

Answer 3

I found a similar problem (& this question) connecting gdb very early in the boot process – as mentioned in other answers, gdb does not very much appreciate the size of registers changing out from under it. This problem can be seen by using set debug remote 1:

(gdb) set debug remote 1
(gdb) target remote localhost:1234
Remote debugging using localhost:1234
...
Sending packet: $g#67...Ack
Packet received: 000000000000000... <~600 bytes>
(gdb) until *0x1000 # at this address we'll be in a different cpu mode
...
Sending packet: $g#67...Ack
Packet received: 10000080000000000000000000000000800000c... <~1000 bytes>
...
Remote 'g' packet reply is too long: 1000008000000000000000000...
(gdb)

Patching gdb to resize its internal buffer when it sees a too-large packet as found on this issue in the gdb bug tracker (and elsewhere), does indeed work around the problem, as does patching QEMU to only send 64-bit-sized packets. However, the latter solution breaks debugging in non-64-bit-modes, and it seems that the former fix could be incomplete:

It sounds quite wrong to be changing the target behind GDB's back when GDB is already debugging it. Not just the size of the g/G packets may change inadvertently, but the layout as well. If the target description changes with your re-configuration, it sounds to me like GDB should fetch/recompute the whole target description. Today, I think that can only be done with a disconnect/reconnect.

– https://sourceware.org/ml/gdb/2014-02/msg00005.html

The disconnect/reconnect workaround mentioned at the end of the post does appear to work:

(gdb) disconnect
Ending remote debugging.
(gdb) set architecture i386:x86-64
The target architecture is assumed to be i386:x86-64
(gdb) target remote localhost:1234
Remote debugging using localhost:1234
(gdb) info registers
rax            0x80000010   2147483664
rbx            0x0  0
...