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233 lines
9.3 KiB
233 lines
9.3 KiB
Remote Debugging
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================
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Remote debugging refers to the act of debugging a process which is running on a
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different system, than the debugger itself. We shall refer to the system
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running the debugger as the local system, while the system running the debugged
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process will be the remote system.
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To enable remote debugging, LLDB employs a client-server architecture. The
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client part runs on the local system and the remote system runs the server. The
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client and server communicate using the gdb-remote protocol, usually
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transported over TCP/IP. More information on the protocol can be found here and
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the LLDB-specific extensions are documented in docs/lldb-gdb-remote.txt file
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inside LLDB source repository. Besides the gdb-remote stub, the server part of
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LLDB also consists of a platform binary, which is responsible for performing
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advanced debugging operations, like copying files from/to the remote system and
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can be used to execute arbitrary shell commands on the remote system.
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In order to reduce code complexity and improve remote debugging experience LLDB
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on Linux and macOS uses the remote debugging stub even when debugging a process
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locally. This is achieved by spawning a remote stub process locally and
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communicating with it over the loopback interface. In the case of local
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debugging this whole process is transparent to the user. The platform binary is
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not used in this case, since no file transfers are needed.
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.. contents::
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:local:
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Preparation for Remote Debugging
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---------------------------------
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While the process of actual debugging (stepping, backtraces, evaluating
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expressions) is same as in the local case, in the case of remote debugging,
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more preparation is needed as the required binaries cannot started on the
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remote system automatically. Also, if the remote system runs a different OS or
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architecture, the server component needs to be compiled separately.
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Remote system
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*************
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On Linux and Android, all required remote functionality is contained in the
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lldb-server binary. This binary combines the functionality of the platform and
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gdb-remote stub. A single binary facilitates deployment and reduces code size,
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since the two functions share a lot of code. The lldb-server binary is also
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statically linked with the rest of LLDB (unlike lldb, which dynamically links
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to liblldb.so by default), so it does not have any dependencies on the rest of
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lldb. On macOS and iOS, the remote-gdb functionality is implemented by the
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debugserver binary, which you will need to deploy alongside lldb-server.
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The binaries mentioned above need to be present on the remote system to enable
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remote debugging. You can either compile on the remote system directly or copy
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them from the local machine. If compiling locally and the remote architecture
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differs from the local one, you will need to cross-compile the correct version
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of the binaries. More information on cross-compiling LLDB can be found on the
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build page.
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Once the binaries are in place, you just need to run the lldb-server in
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platform mode and specify the port it should listen on. For example, the
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command
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::
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remote% lldb-server platform --listen "*:1234" --server
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will start the LLDB platform and wait for incoming connections from any address
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to port 1234. Specifying an address instead of * will only allow connections
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originating from that address. Adding a --server parameter to the command line
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will fork off a new process for every incoming connection, allowing multiple
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parallel debug sessions.
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Local system
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************
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On the local system, you need to let LLDB know that you intend to do remote
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debugging. This is achieved through the platform command and its sub-commands.
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As a first step you need to choose the correct platform plug-in for your remote
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system. A list of available plug-ins can be obtained through platform list.
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::
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local% lldb
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(lldb) platform list
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Available platforms:
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host: Local macOS user platform plug-in.
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remote-freebsd: Remote FreeBSD user platform plug-in.
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remote-linux: Remote Linux user platform plug-in.
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remote-netbsd: Remote NetBSD user platform plug-in.
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remote-windows: Remote Windows user platform plug-in.
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remote-android: Remote Android user platform plug-in.
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remote-ios: Remote iOS platform plug-in.
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remote-macosx: Remote macOS user platform plug-in.
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ios-simulator: iOS simulator platform plug-in.
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darwin-kernel: Darwin Kernel platform plug-in.
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tvos-simulator: Apple TV simulator platform plug-in.
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watchos-simulator: Apple Watch simulator platform plug-in.
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remote-tvos: Remote Apple TV platform plug-in.
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remote-watchos: Remote Apple Watch platform plug-in.
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remote-gdb-server: A platform that uses the GDB remote protocol as the communication transport.
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The default platform is the platform host which is used for local debugging.
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Apart from this, the list should contain a number of plug-ins, for debugging
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different kinds of systems. The remote plug-ins are prefixed with "remote-".
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For example, to debug a remote Linux application:
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::
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(lldb) platform select remote-linux
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After selecting the platform plug-in, you should receive a prompt which
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confirms the selected platform, and states that you are not connected. This is
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because remote plug-ins need to be connected to their remote platform
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counterpart to operate. This is achieved using the platform connect command.
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This command takes a number of arguments (as always, use the help command to
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find out more), but normally you only need to specify the address to connect
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to, e.g.:
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::
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(lldb) platform connect connect://remote:1234
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Platform: remote-linux
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Triple: x86_64-gnu-linux
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Hostname: remote
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Connected: yes
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WorkingDir: /tmp
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Note that the platform has a working directory of /tmp. This directory will be
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used as the directory that executables will be uploaded to by default when
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launching a process from local.
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After this, you should be able to debug normally. You can use the process
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attach to attach to an existing remote process or target create, process launch
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to start a new one. The platform plugin will transparently take care of
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uploading or downloading the executable in order to be able to debug. If your
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application needs additional files, you can transfer them using the platform
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commands: get-file, put-file, mkdir, etc. The environment can be prepared
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further using the platform shell command.
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Launching a locally built process on the remote machine
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-------------------------------------------------------
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Install and run in the platform working directory
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*************************************************
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To launch a locally built process on the remote system in the platform working
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directory:
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::
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(lldb) file a.out
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(lldb) run
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This will cause LLDB to create a target with the "a.out" executable that you
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cross built. The "run" command will cause LLDB to upload "a.out" to the
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platform's current working directory only if the file has changed. The platform
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connection allows us to transfer files, but also allows us to get the MD5
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checksum of the file on the other end and only upload the file if it has
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changed. LLDB will automatically launch a lldb-server in gdbremote mode to
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allow you to debug this executable, connect to it and start your debug session
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for you.
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Changing the platform working directory
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***************************************
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You can change the platform working directory while connected to the platform
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with:
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::
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(lldb) platform settings -w /usr/local/bin
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And you can verify it worked using "platform status":
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::
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(lldb) platform status
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Platform: remote-linux
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Triple: x86_64-gnu-linux
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Hostname: remote
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Connected: yes
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WorkingDir: /usr/local/bin
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If we run again, the program will be installed into ``/usr/local/bin``.
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Install and run by specifying a remote install path
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***************************************************
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If you want the "a.out" executable to be installed into "/bin/a.out" instead of
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the platform's current working directory, we can set the platform file
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specification using python:
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::
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(lldb) file a.out
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(lldb) script lldb.target.module['a.out'].SetPlatformFileSpec("/bin/a.out")
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(lldb) run
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Now when you run your program, the program will be uploaded to "/bin/a.out"
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instead of the platform current working directory. Only the main executable is
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uploaded to the remote system by default when launching the application. If you
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have shared libraries that should also be uploaded, then you can add the
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locally build shared library to the current target and set its platform file
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specification:
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::
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(lldb) file a.out
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(lldb) target module add /local/build/libfoo.so
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(lldb) target module add /local/build/libbar.so
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(lldb) script lldb.target.module['libfoo.so'].SetPlatformFileSpec("/usr/lib/libfoo.so")
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(lldb) script lldb.target.module['libbar.so'].SetPlatformFileSpec("/usr/local/lib/libbar.so")
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(lldb) run
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Attaching to a remote process
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*****************************
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If you want to attach to a remote process, you can first list the processes on
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the remote system:
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::
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(lldb) platform process list
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223 matching processes were found on "remote-linux"
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PID PARENT USER TRIPLE NAME
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====== ====== ========== ======================== ============================
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68639 90652 x86_64-apple-macosx lldb
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...
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Then attaching is as simple as specifying the remote process ID:
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::
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(lldb) attach 68639
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