EntryExitInstrumenter: skip naked functions

[MaskRay]

Summary

The asm in a naked function may reasonably expect the argument registers and the
return address register (if present) to be live.

When using -pg and -finstrument-functions, functions are instrumented by adding
a function call to _mcount/__cyg_profile_func_enter/__cyg_profile_func_enter_bare/etc,
which will clobber these registers. If the return address register is clobbered,
the function will be unable to return to the caller, possibly causing an
infinite loop.

__attribute__((naked)) void g() {
#if defined(__arm__)
  __asm__("bx lr");
#else
  __asm__("ret");
#endif
}

int main() { g(); }

It seems that the only one reasonable way to handle the combination is to
disable instrumenting for naked functions.

GCC PR: https://gcc.gnu.org/PR109707
Close https://github.com/llvm/llvm-project/issues/62504

[MaskRay]

add a comment

[hans]

Seems reasonable to me.

Would -fprofile-instr-generate and -fprofile-generate have the same issue?

Do we have any docs about the naked attribute that should be updated?

[MaskRay]

In D149721#4314910, @hans wrote:

Seems reasonable to me.

Would -fprofile-instr-generate and -fprofile-generate have the same issue?

No. The two instrumentations don't insert calls, and are therefore unaffected.

-fsanitize-coverage= works with naked by skipping an entry block ending with an unreachable.

Do we have any docs about the naked attribute that should be updated?

I think a doc need isn't strong, and even if does, the doc need likely belongs to the individual instrumentation features.
An instrumentation may skip many functions and we don't document all of them...

For this patch, I think there isn't anything to update...

[hans]

lgtm

llvm/lib/Transforms/Utils/EntryExitInstrumenter.cpp

static void insertCall(Function &CurFn, StringRef Func,
   }
 
   // We only know how to call a fixed set of instrumentation functions, because
   // they all expect different arguments, etc.
   report_fatal_error(Twine("Unknown instrumentation function: '") + Func + "'");
 }
 
 static bool runOnFunction(Function &F, bool PostInlining) {
+  // The asm in a naked function may reasonably expect the argument registers
+  // and the return address register (if present) to be live. An inserted
+  // function call will clobber these registers. Simply skip naked functions for
+  // all targets.
+  if (F.hasFnAttribute(Attribute::Naked))
+    return false;
+
   StringRef EntryAttr = PostInlining ? "instrument-function-entry-inlined"
                                      : "instrument-function-entry";
 
   StringRef ExitAttr = PostInlining ? "instrument-function-exit-inlined"
                                     : "instrument-function-exit";
 
   StringRef EntryFunc = F.getFnAttribute(EntryAttr).getValueAsString();
   StringRef ExitFunc = F.getFnAttribute(ExitAttr).getValueAsString();

llvm/test/Transforms/EntryExitInstrumenter/mcount.ll

define ptr @tailcaller2() #8 {
   ret ptr %2
 ; CHECK-LABEL: define ptr @tailcaller2
 ; CHECK: call void @__cyg_profile_func_exit
 ; CHECK: musttail call ptr @tailcallee
 ; CHECK: bitcast
 ; CHECK: ret
 }
 
+;; naked functions are not instrumented, otherwise the argument registers
+;; and the return address register (if present) would be clobbered.
+define void @naked() naked { entry: ret void }
+; CHECK-LABEL:      define void @naked(
+; CHECK-LABEL-NEXT: entry:
+; CHECK-LABEL-NEXT:   ret void
+
 ; The attributes are "consumed" when the instrumentation is inserted.
 ; CHECK: attributes
 ; CHECK-NOT: instrument-function
 
 attributes #0 = { "instrument-function-entry-inlined"="mcount" "instrument-function-entry"="__cyg_profile_func_enter" "instrument-function-exit"="__cyg_profile_func_exit" }
 attributes #1 = { "instrument-function-entry-inlined"=".mcount" }
 attributes #2 = { "instrument-function-entry-inlined"="llvm.arm.gnu.eabi.mcount" }
 attributes #3 = { "instrument-function-entry-inlined"="\01_mcount" }
 attributes #4 = { "instrument-function-entry-inlined"="\01mcount" }
 attributes #5 = { "instrument-function-entry-inlined"="__mcount" }
 attributes #6 = { "instrument-function-entry-inlined"="_mcount" }
 attributes #7 = { "instrument-function-entry-inlined"="__cyg_profile_func_enter_bare" }
 attributes #8 = { "instrument-function-exit"="__cyg_profile_func_exit" }