How to print panic backtraces?

[ChocolateLoverRaj]

Idk how panic backtraces work, but it seems like part of printing a nice looking backtrace is resolving symbols. I can do that because I have a user space program in an elf and I can parse the elf to look up symbols with https://github.com/cole14/rust-elf/. What would be the steps to actually printing a nice error message on panic?

Also would this be possible in the kernel itself by parsing the elf of the kernel itself inside the kernel's code?

[bjorn3]

To get a backtrace, you can either compile with frame pointers enabled (-Cforce-frame-pointers) and then walk the stack using frame pointers. Or you can compile with unwind tables enabled (-Cforce-unwind-tables) and then use the unwinding crate to handle unwinding the stack based on the unwind tables for you. The unwinding crate supports no_std, but you will need to disable the default features and enable one of the fde-* features to tell it how it should find the unwind tables.

Also would this be possible in the kernel itself by parsing the elf of the kernel itself inside the kernel's code?

Yes. Basically you did have to parse the entire kernel image as ELF file, look up it's symbol table and then for each address you got from the previous step to get a backtrace you have to find the symbol with the highest address lower than or equal to this address in the backtrace. The name of the symbol should then match the name of the function that corresponds to the stack frame. If your kernel can be relocated (PIC/PIE) then you will have to add the actual offset by which the kernel is relocated to the addresses you get from the symbol table.

[tsatke]

The unwinding crate

https://github.com/nbdd0121/unwinding

You'll need your own linker file.

[ChocolateLoverRaj]

How do you get the line and column number after getting the function from the symbol table?

[bjorn3]

For that you need to preserve the .debug_* sections in your kernel image and then parse the DWARF line tables using eg gimli. The addr2line crate may be useful for this.