[Bug 2082636] Re: [SRU] dpkg: backport frame-pointer enabling mechanism for Rust

[Steve Langasek]

> This is now cleared up in the updated description (LLVM <= 17 and Rust
<= 1.80, where the upstream patches landed).


Ok.

rustc in noble is 1.75, so is affected.

And it's build with llvm 17, which is also affected.

what is the impact of llvm not producing proper backchain-saving logic?

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https://bugs.launchpad.net/bugs/2082636

Title:
  [SRU] dpkg: backport frame-pointer enabling mechanism for Rust

Status in dpkg package in Ubuntu:
  New

Bug description:
  [ Impact ]

   * On Noble, dpkg scripts could not produce Rust binaries with frame-
  pointer enabled.

  [ Test Plan ]

   * Build a Rust binary package like ripgrep and you will not find frame-pointers being used in the binary.
   * Since LLVM could inline and choose to optimize frame pointers away in some cases, you will need to check the disassembly of the program around the function prologue to see if the frame pointer register is correctly saved to stack (rbp/ebp on x86 architecture). You can use the script provided in https://bugs.launchpad.net/ubuntu/+source/dpkg/+bug/2082636/+attachment/5840384/+files/check-rust-fp.py to check if the frame pointers have been enabled.

  [ Where problems could occur ]

   * On older Rust versions (Rust <= 1.80), it is known that LLVM on s390x sometimes can produce incorrect warnings about how the "backchain" feature does not exist.
   * On older LLVM versions (LLVM <= 17), LLVM may not produce correct backchain saving logic inside the function prologue.

  [ Other Info ]

   * This change is mandatory to enable Rust-related packages with
  frame-pointers during the build.

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