[all-commits] [llvm/llvm-project] acc6bc: Support alternative sections for patchable functio...

[Sami Tolvanen via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: acc6bcdc504ad2e8c09a628dc18de0067f7344b8
      https://github.com/llvm/llvm-project/commit/acc6bcdc504ad2e8c09a628dc18de0067f7344b8
  Author: Sami Tolvanen <samitolvanen at google.com>
  Date:   2025-04-02 (Wed, 02 Apr 2025)

  Changed paths:
    M clang/include/clang/Basic/Attr.td
    M clang/include/clang/Basic/AttrDocs.td
    M clang/include/clang/Basic/CodeGenOptions.h
    M clang/include/clang/Basic/DiagnosticSemaKinds.td
    M clang/include/clang/Driver/Options.td
    M clang/lib/CodeGen/CodeGenFunction.cpp
    M clang/lib/Driver/ToolChains/Clang.cpp
    M clang/lib/Sema/SemaDeclAttr.cpp
    A clang/test/CodeGen/patchable-function-entry-section.c
    M clang/test/Driver/fpatchable-function-entry.c
    M clang/test/Sema/patchable-function-entry-attr.c
    M llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
    M llvm/lib/IR/Verifier.cpp
    M llvm/test/CodeGen/X86/patchable-function-entry.ll
    M llvm/test/Verifier/invalid-patchable-function-entry.ll

  Log Message:
  -----------
  Support alternative sections for patchable function entries (#131230)

With -fpatchable-function-entry (or the patchable_function_entry
function attribute), we emit records of patchable entry locations to the
__patchable_function_entries section. Add an additional parameter to the
command line option that allows one to specify a different default
section name for the records, and an identical parameter to the function
attribute that allows one to override the section used.

The main use case for this change is the Linux kernel using prefix NOPs
for ftrace, and thus depending on__patchable_function_entries to locate
traceable functions. Functions that are not traceable currently disable
entry NOPs using the function attribute, but this creates a
compatibility issue with -fsanitize=kcfi, which expects all indirectly
callable functions to have a type hash prefix at the same offset from
the function entry.

Adding a section parameter would allow the kernel to distinguish between
traceable and non-traceable functions by adding entry records to
separate sections while maintaining a stable function prefix layout for
all functions. LKML discussion:

https://lore.kernel.org/lkml/Y1QEzk%2FA41PKLEPe@hirez.programming.kicks-ass.net/



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