[lld] [LLD][ELF][RISCV] Handle .note.gnu.property sections for Zicfilp/Zicfiss features (PR #127193)

[Ming-Yi Lai via llvm-commits]

mylai-mtk wrote:

> Can you change the description to link to the psABI change?

Sure. The description has a new paragraph to indicate the involved psABI PRs.

> Q: what is GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_FUNC_SIG?

A: The `GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_FUNC_SIG` bit represents the adoption of forward-edge control flow protection with the `func-sig` label scheme. The RISC-V Zicfilp ISA extension introduces the landing pad instruction to mark valid indirect jump targets in the form of `lpad <label>`. The `lpad` instruction has an additional (compared to Intel's `ENDBR32/64` and AArch64's `BTI` instructions) 20-bit label that allows it to differentiate jumps coming from different sources: If an `lpad` has a non-zero label, it expects the same (not really, but sort of) value is presented in register `X7`, otherwise an exception is raised. Here the `func-sig` scheme means that the labels are generated from the signature of the function whose entry point is protected by the `lpad` instruction. (The detailed rules for generating labels from function signatures can be found at <https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/434>)

(p.s. The `GNU_PROPERTY_RISCV_FEATURE_1_CFI_LP_UNLABELD` specifies a scheme in which the `lpad` instructions' labels are always `0`. In this case, the `lpad` instruction does not check the content of register `X7`, and thus this scheme conflicts with the `func-sig` scheme.)

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Updated: Update PR description

https://github.com/llvm/llvm-project/pull/127193