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/

1 files changed, 6 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index dcf523f..b55003b 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -965,18 +965,24 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
   }
 
   unsigned Count, Offset;
+  StringRef Section;
   if (const auto *Attr =
           D ? D->getAttr<PatchableFunctionEntryAttr>() : nullptr) {
     Count = Attr->getCount();
     Offset = Attr->getOffset();
+    Section = Attr->getSection();
   } else {
     Count = CGM.getCodeGenOpts().PatchableFunctionEntryCount;
     Offset = CGM.getCodeGenOpts().PatchableFunctionEntryOffset;
   }
+  if (Section.empty())
+    Section = CGM.getCodeGenOpts().PatchableFunctionEntrySection;
   if (Count && Offset <= Count) {
     Fn->addFnAttr("patchable-function-entry", std::to_string(Count - Offset));
     if (Offset)
       Fn->addFnAttr("patchable-function-prefix", std::to_string(Offset));
+    if (!Section.empty())
+      Fn->addFnAttr("patchable-function-entry-section", Section);
   }
   // Instruct that functions for COFF/CodeView targets should start with a
   // patchable instruction, but only on x86/x64. Don't forward this to ARM/ARM64