4 ms·

Where do I even start to understand what's going on here? I know (some) C and I have Tanenbaum's book but this seems so far beyond.

csmpltn 4y ago
The TLDR summary here is that the compiler allows you to hook into ("instrument") function calls and run custom logic of your choosing on function enter/leave. From that point on, you could use that mechanism to log how your program executes ("tracing").
- NeutralForest 4y ago
  Thanks, the overall logic is somewhat clear to me but I'd like to understand Justine's work a bit more in depth (like https://justine.lol/cosmopolitan/ https://justine.lol/cosmopolitan/) but I don't know where to start. Just reading the code or the tutorial requires some background knowledge I don't think I have.
  - jart 4y ago
    Justine here. We recently started a Discord chatroom https://discord.gg/WH25psU9 https://discord.gg/WH25psU9 for Redbean / Cosmopolitan Libc / etc. You're invited to join us! You're free to ask myself and others for help on using / understanding Cosmopolitan Libc in this chatroom. You can also just come to hang out and meet the authors.
    - NeutralForest 4y ago
      Thanks Justine that's really cool! I'll def check it out
jart 4y ago
The Cosmopolitan Libc _start() function starts by intercepting the --ftrace flag. https://github.com/jart/cosmopolitan/blob/master/libc/runtime/ftraceinit.greg.c#L35 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... If it exists, then it opens and sorts of the symbol table from the elf binary. https://github.com/jart/cosmopolitan/blob/master/libc/runtime/opensymboltable.greg.c#L109 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... Then it changes the protection of memory https://github.com/jart/cosmopolitan/blob/master/libc/runtime/morph.greg.c#L60 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... so it's able to iterate over the program's memory to look for nop instructions it can mutate. https://github.com/jart/cosmopolitan/blob/master/libc/runtime/hook.greg.c#L106 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... Those NOPs were inserted by GCC. It's easy to self-modify them in memory, since they have the same byte length as the CALL instruction. Think of it like a mini linker. It just relinks the profiling nops. Once they've been rewritten, functions will start calling https://github.com/jart/cosmopolitan/blob/master/libc/runtime/ftrace-hook.S#L22 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... which saves the CPU state to the stack. That means ftrace kind of acts like an operating system kernel. Once the assembly saved the CPU it can call the C code https://github.com/jart/cosmopolitan/blob/master/libc/runtime/ftracer.c#L113 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... that acquires a reentrant mutex and unwinds the RBP backtrace pointer (via __builtin_frame_address(0)) to determine the address of the function that called it. Once it has the address of the function, it passes it along to kprintf() which has a special syntax for turning numbers into symbols. https://github.com/jart/cosmopolitan/blob/master/libc/intrin/kprintf.greg.c#L519 https://github.com/jart/cosmopolitan/blob/master/libc/intrin... That's really all there is to it!
- NeutralForest 4y ago
  Thanks for your answer! I'll dig into the code then =)

csmpltn 4y ago

The TLDR summary here is that the compiler allows you to hook into ("instrument") function calls and run custom logic of your choosing on function enter/leave. From that point on, you could use that mechanism to log how your program executes ("tracing").

NeutralForest 4y ago
Thanks, the overall logic is somewhat clear to me but I'd like to understand Justine's work a bit more in depth (like https://justine.lol/cosmopolitan/ https://justine.lol/cosmopolitan/) but I don't know where to start. Just reading the code or the tutorial requires some background knowledge I don't think I have.
- jart 4y ago
  Justine here. We recently started a Discord chatroom https://discord.gg/WH25psU9 https://discord.gg/WH25psU9 for Redbean / Cosmopolitan Libc / etc. You're invited to join us! You're free to ask myself and others for help on using / understanding Cosmopolitan Libc in this chatroom. You can also just come to hang out and meet the authors.
  - NeutralForest 4y ago
    Thanks Justine that's really cool! I'll def check it out

jart 4y ago

The Cosmopolitan Libc _start() function starts by intercepting the --ftrace flag. https://github.com/jart/cosmopolitan/blob/master/libc/runtime/ftraceinit.greg.c#L35 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... If it exists, then it opens and sorts of the symbol table from the elf binary. https://github.com/jart/cosmopolitan/blob/master/libc/runtime/opensymboltable.greg.c#L109 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... Then it changes the protection of memory https://github.com/jart/cosmopolitan/blob/master/libc/runtime/morph.greg.c#L60 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... so it's able to iterate over the program's memory to look for nop instructions it can mutate. https://github.com/jart/cosmopolitan/blob/master/libc/runtime/hook.greg.c#L106 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... Those NOPs were inserted by GCC. It's easy to self-modify them in memory, since they have the same byte length as the CALL instruction. Think of it like a mini linker. It just relinks the profiling nops. Once they've been rewritten, functions will start calling https://github.com/jart/cosmopolitan/blob/master/libc/runtime/ftrace-hook.S#L22 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... which saves the CPU state to the stack. That means ftrace kind of acts like an operating system kernel. Once the assembly saved the CPU it can call the C code https://github.com/jart/cosmopolitan/blob/master/libc/runtime/ftracer.c#L113 https://github.com/jart/cosmopolitan/blob/master/libc/runtim... that acquires a reentrant mutex and unwinds the RBP backtrace pointer (via __builtin_frame_address(0)) to determine the address of the function that called it. Once it has the address of the function, it passes it along to kprintf() which has a special syntax for turning numbers into symbols. https://github.com/jart/cosmopolitan/blob/master/libc/intrin/kprintf.greg.c#L519 https://github.com/jart/cosmopolitan/blob/master/libc/intrin... That's really all there is to it!

NeutralForest 4y ago
Thanks for your answer! I'll dig into the code then =)