Debug xv6

The original text can be found in debugging Xv6 Check out , Subsequent updates will be updated to Personal blog

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Debugging is a troublesome thing , There are a lot of things to toss , And I don't know some problems now

Debugging tools

There are three known debugging tools , I haven't figured out the specific difference between them yet .

riscv64-unknown-elf-gdb

stay {% post_link 6s081/ function xv6 System “ start-up ” %} xv6 The article introduced SiFive A tool chain is provided to start the system , You can find it in this tool chain riscv64-unknown-elf-gdb For debugging

gdb-multiarch

Official tool page The tools provided are gdb-multiarch. This can be done through apt-get Direct installation . Besides , Provided on the tools page GitHub Can be found in , The bags provided by the warehouse are also riscv64-unknown-elf-gdb

riscv64-linux-gnu-gdb

stay video Can be seen in , The debugging tool used by the teacher is riscv64-linux-gnu-gdb, Because the previous tool has solved the debugging problem , So I didn't find the installation method of this package

.gdbinit file

Every time you start debugging ,Makefile Will use .gdbinit.tmpl-riscv Generate .gdbinit file . from Makefile You can see , What will change is the debugging port number . The purpose of this file is to initialize the debugging environment ,gdb When starting, it will read the commands and execute . See Help document

Difficult miscellaneous diseases

Undefined item: "riscv:rv64"

Use gdb-multiarch You will see this error prompt at the beginning , At this point, through the command set architecture View all currently supported system architectures , Found that there was no riscv64 Support for , No wonder there is this error prompt . stay Stack Overflow The answer found on says that the version needs to be upgraded to 8.3 That's all . But my system is Ubuntu 18.04, System source The maximum supported version is 8.1.1. So there is no way to use this tool for debugging .

I don't know why the course official recommends this as a debugging tool , Maybe I haven't got something right

no symbol table is loaded. use the file command

This problem occurs because symbols are not loaded correctly , At first, there was doubt that it was not used when compiling -g This option , As a result, the compliance table was not entered . But look at it. Makefile After the discovery , It USES -ggdb This command , Can also type the symbol table into the executable file . It's very easy to solve this problem , Only need gdb In the command line file kernel/kernel You can load the symbols of the kernel

auto-loading has been declined by your `auto-load safe-path’ set to “$debugdir:$datadir/auto-load”

This is also strange , This warning will be output every time you start . Later I realized , In order to avoid some safety problem ,gdb Will no longer actively execute any files . So you need to declare in the user directory ,xv6 This warehouse's .gdbinit The file is secure

adopt echo add-auto-load-safe-path {PATH}/.gdbinit > ~/.gdbinit Command is enough . among {PATH} Change to your own warehouse Path

This problem incidentally affects the second problem . If not added as a trusted file , It can lead to gdb Do not actively load kernel files , Affect symbol resolution .

Reference resources

GDB Configuration and .gdbinit Compiling

Security restriction for auto-loading

GDB Debugging guide 1

MIT 6.S081 xv6 Debugging doesn't point to the North completely