Re: Re: [musl] [QUESTION] how to gdb dynamic linker for musl?

["Chen Wang" <unicorn_wang@outlook.com>]

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Regarding the setup of gdb cross-debugging env, I find a solution and summarize here just FYI. If you have more questions or better idea please feel free let me know.

First compile musl：
```
$ CFLAGS=-g CROSS_COMPILE=riscv64-unknown-linux-gnu- ./configure --prefix=/home/u/ws/test-musl-shared/install --exec-prefix=/home/u/ws/test-musl-shared/install
```
notice I use `CFLAGS=-g` to enable debug info, but not `--enable-debug`, I just find if I use `--enable-debug` instead, when I install the musl, the folder bin would not be created. I'm testing with musl-1.2.1, not sue if it is a bug.

After installation is completed, enter install/lib folder and create a symbol link for libc.so, the command you can input is `ln -s ./libc.so ld-musl-riscv64.so.1`. Use this link-name just match the interpreter path name in executale we would create later. The purpose of what I do here is to use the install folder as sysroot for later gdb debugging. 

Build a simple hello a.out
`./install/bin/musl-gcc -g hello.c`

Check it's elf info and look at the interpreter info here.
```
$ riscv64-unknown-linux-gnu-readelf -l a.out
......
  INTERP         0x00000000000001c8 0x00000000000101c8 0x00000000000101c8
                 0x000000000000001a 0x000000000000001a  R      0x1
      [Requesting program interpreter: /lib/ld-musl-riscv64.so.1]
......
```

Now we can launch gdb to do cross-debug:
Run gdb-server: `$ qemu-riscv64 -L /home/u/ws/test-musl-shared/install -g 1234 ./a.out`
Run gdb-client and set sysroot as the install folder so gdb can load symbols from our libc.so successfully.
``` 
$ riscv64-unknown-linux-gnu-gdb a.out
GNU gdb (GDB) 9.1
......
Reading symbols from a.out...
(gdb) show sysroot
The current system root is "/opt/riscv64/sysroot".
(gdb) set sysroot /home/u/ws/test-musl-shared/install
(gdb) target remote:1234
Remote debugging using :1234
Reading symbols from /home/u/ws/test-musl-shared/install/lib/ld-musl-riscv64.so.1...
0x0000004000850ae2 in _dlstart () from /home/u/ws/test-musl-shared/install/lib/ld-musl-riscv64.so.1
```

That's all.





Best Regards

Wang Chen
 
From: Rich Felker
Date: 2020-12-08 00:46
To: Chen Wang
CC: musl
Subject: Re: [musl] [QUESTION] how to gdb dynamic linker for musl?
On Mon, Dec 07, 2020 at 09:37:45PM +0800, Chen Wang wrote:
> Hello,
> I confugure and make musl with following:
> 
> CFLAGS=-g CROSS_COMPILE=riscv64-unknown-linux-gnu- ./configure --prefix=/home/u/ws/musl/install --exec-prefix=/home/u/ws/musl/install --syslibdir=/home/u/ws/musl/install
> 
> create the executable with: ./install/bin/musl-gcc -g test.c
 
I can't say conclusively that you can't make this work, but you really
don't want to do it this way. musl-gcc is for repurposing the host gcc
on a glibc-based system for "trying out" musl and making minimal
programs. It doesn't make sense to build a glibc-targeting cross
compiler then use musl-gcc to try to repurpose it. Just build a proper
musl-targeting cross-compiler.
 
However this probably isn't your problem.
 
> Then I tried to debug:
> $ qemu-riscv64 -g 1234 ./a.out
> then launch gdb:
> $ riscv64-unknown-linux-gnu-gdb a.out
> 
> but after I tried to target remote, gdb oops and tell me:
> ```
> gdb) target remote:1234
> Remote debugging using :1234
> warning: Unable to find dynamic linker breakpoint function.
> GDB will be unable to debug shared library initializers
> and track explicitly loaded dynamic code.
> 0x0000004000850ae2 in ?? ()
> ```
> 
> I tried to set sysroot but does NOT take effect:
> ```
> (gdb) set sysroot /home/u/ws/musl/install/
> warning: Unable to find dynamic linker breakpoint function.
> GDB will be unable to debug shared library initializers
> and track explicitly loaded dynamic code.
> (gdb) set sysroot /home/u/ws/musl/install/lib
> warning: Unable to find dynamic linker breakpoint function.
> GDB will be unable to debug shared library initializers
> and track explicitly loaded dynamic code.
> ```
> I checked the a.out
> ```
> $ riscv64-unknown-linux-gnu-readelf -l a.out
> ......
>   INTERP         0x00000000000001c8 0x00000000000101c8 0x00000000000101c8
>                  0x000000000000002d 0x000000000000002d  R      0x1
>       [Requesting program interpreter: /home/u/ws/musl/install/ld-musl-riscv64.so.1]
> ......
> ```
> And I can run with qemu directly: $ qemu-riscv64 ./a.out, this
> works, but why I can not debug with gdb? Who can help me out? Thanks
> in adv.
 
I haven't used gdb remote with qemu so I'm not sure what to expect
here in terms of how it finds/loads the binary and dynamic linker
(does it do that over the remote interface or via the local fs?). If
you can enable more output from qemu or strace one or both of gdb and
qemu it might help figure out what's going on.
 
Hopefully someone else who's use gdb this way can offer more useful
help.
 
Rich

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