Broken debugging support

[mkroening]

5685ca7 broke Rust-level debugging support. While it is still possible to step through assembly code using qemu or uyhve, any operation on a higher level, like setting a breakpoint in Rust code, is broken.

As this is caused by relocated code, which apparently confuses gdb in its current form, there must be a way to properly communicate the relocation details to make it work again.

[mkroening]

Related uhyve commit: hermit-os/uhyve@a9cae62

[mkroening]

This can be worked around by using symbol-file [ filename [ -o offset ]] (Files (Debugging with GDB)) with filename being the RustyHermit application and offset being the BootInfo::base set in rusty_loader::boot_kernel and uhyve::load_kernel.

Both of these currently seem to be 0x400000 but that is subject to change. That number can currently be found by running the application once and fetching it from the output. Then the application can be started again using gdb and debugged properly. As the base is planned to be randomized, this would not work anymore. We'd have to support a way of getting the offset in gdb, like having a breakpoint after relocation and then reloading the symbol-file with the at the breakpoint available offset. This is how you can debug u-boot (global_data::relocaddr). There might also be a way to automate that using _dl_debug_state(), which is how ld.so does it according to Stack Overflow.

Currently-working example:

symbol-file target/x86_64-unknown-hermit/debug/rusty_demo -o 0x400000

[mkroening]

Since hermit-os/uhyve#164 (section_offsets.rs) this works in uhyve again! 🎉