draft: inline asm mode

Author: androm3da

llvm/docs/InlineAsmSafetyDirective.md

@@ -0,0 +1,110 @@
+# Inline Assembly Safety Directive
+
+## Overview
+
+The `.inline_asm_mode` directive provides enhanced safety for inline assembly blocks by warning about potentially unsafe label usage. This directive helps prevent common errors where programmers create non-local labels in inline assembly that could be inadvertently jumped to from external code.
+
+## Syntax
+
+```assembly
+.inline_asm_mode strict   # Enable strict mode - warn on non-local labels
+.inline_asm_mode relaxed  # Disable strict mode (default)
+```
+
+## Description
+
+When `.inline_asm_mode strict` is active, the assembler will emit warnings for labels that are considered potentially unsafe for inline assembly:
+
+- **Safe labels** (no warnings):
+  - Local labels starting with `.L` (e.g., `.L_loop`, `.L_end`)
+  - Numeric labels (e.g., `1:`, `42:`)
+  - Labels starting with special prefixes (`$`, `__`)
+  - Labels starting with `.` (local scope)
+
+- **Unsafe labels** (warnings emitted):
+  - Global labels without special prefixes (e.g., `my_function:`, `loop:`)
+  - Labels that could be accessed from outside the inline assembly block
+
+## Use Cases
+
+### Frontend Integration
+
+Compiler frontends can use this directive when generating inline assembly:
+
+```c++
+// Emitted by the compiler: .inline_asm_mode strict
+// C++ inline assembly example
+asm(
+    ".L_loop:\n"              // Safe - no warning
+    "  add %0, %1\n"
+    "  jne .L_loop\n"         // Safe - local jump
+    "exit:\n"                 // Warning
+    : "=r"(result) : "r"(input));
+```
+// Emitted by the compiler: .inline_asm_mode relaxed
+
+## Rationale
+
+Inline assembly blocks are often embedded within larger functions or modules. Non-local labels in these blocks can create several problems:
+
+1. **Naming conflicts**: Global labels may conflict with other symbols in the compilation unit
+2. **Unintended control flow**: External code might accidentally jump to labels intended for internal use
+3. **Maintenance issues**: Global labels make inline assembly less encapsulated
+
+The strict mode helps identify these potential issues during compilation, allowing developers to use safer local labels instead.
+
+## Error Handling
+
+Invalid directive usage will produce parse errors:
+
+```assembly
+.inline_asm_mode invalid_mode
+# Error: expected 'strict' or 'relaxed'
+
+.inline_asm_mode
+# Error: expected 'strict' or 'relaxed' after '.inline_asm_mode'
+```
+
+## Implementation Details
+
+- The directive affects only subsequent label definitions until changed
+- Default mode is `relaxed` (no additional warnings)
+- The directive state is maintained in the MC streamer
+- Warnings are emitted through the standard LLVM diagnostic system
+
+## Examples
+
+### Complete Example
+
+```assembly
+.text
+.globl example_function
+example_function:
+    # Regular function labels (outside inline asm) - no warnings
+
+    # Simulate inline assembly block with safety
+    .inline_asm_mode strict
+
+    # These are safe
+    .L_inline_start:
+        mov $1, %eax
+        test %eax, %eax
+        jz .L_inline_end
+
+    1:  # Numeric label
+        inc %eax
+        cmp $10, %eax
+        jl 1b
+
+    .L_inline_end:
+        # End of safe inline block
+
+    # This would generate a warning
+    # global_inline_label:  # Warning would be emitted
+
+    .inline_asm_mode relaxed
+
+    # Back to normal mode
+    ret
+```
+

llvm/include/llvm/MC/MCStreamer.h

@@ -260,6 +260,11 @@ class LLVM_ABI MCStreamer {
   /// discussion for future inclusion.
   bool AllowAutoPadding = false;
 
+  /// Is strict inline assembly mode enabled? When enabled, the assembler
+  /// will warn about non-local labels that could be unsafe jump targets
+  /// in inline assembly blocks.
+  bool InlineAsmStrictMode = false;
+
 protected:
   // Symbol of the current epilog for which we are processing SEH directives.
   WinEH::FrameInfo::Epilog *CurrentWinEpilog = nullptr;
@@ -325,6 +330,9 @@ class LLVM_ABI MCStreamer {
   void setAllowAutoPadding(bool v) { AllowAutoPadding = v; }
   bool getAllowAutoPadding() const { return AllowAutoPadding; }
 
+  void setInlineAsmMode(bool v) { InlineAsmStrictMode = v; }
+  bool getInlineAsmMode() const { return InlineAsmStrictMode; }
+
   MCSymbol *emitLineTableLabel();
 
   /// When emitting an object file, create and emit a real label. When emitting

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -533,6 +533,7 @@ class AsmParser : public MCAsmParser {
     DK_LTO_SET_CONDITIONAL,
     DK_CFI_MTE_TAGGED_FRAME,
     DK_MEMTAG,
+    DK_INLINE_ASM_MODE,
     DK_END
   };
 
@@ -703,6 +704,9 @@ class AsmParser : public MCAsmParser {
   // ".lto_discard"
   bool parseDirectiveLTODiscard();
 
+  // ".inline_asm_mode"
+  bool parseDirectiveInlineAsmMode(SMLoc DirectiveLoc);
+
   // Directives to support address-significance tables.
   bool parseDirectiveAddrsig();
   bool parseDirectiveAddrsigSym();
@@ -2222,6 +2226,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       return parseDirectiveLTODiscard();
     case DK_MEMTAG:
       return parseDirectiveSymbolAttribute(MCSA_Memtag);
+    case DK_INLINE_ASM_MODE:
+      return parseDirectiveInlineAsmMode(IDLoc);
     }
 
     return Error(IDLoc, "unknown directive");
@@ -5590,6 +5596,7 @@ void AsmParser::initializeDirectiveKindMap() {
   DirectiveKindMap[".lto_discard"] = DK_LTO_DISCARD;
   DirectiveKindMap[".lto_set_conditional"] = DK_LTO_SET_CONDITIONAL;
   DirectiveKindMap[".memtag"] = DK_MEMTAG;
+  DirectiveKindMap[".inline_asm_mode"] = DK_INLINE_ASM_MODE;
 }
 
 MCAsmMacro *AsmParser::parseMacroLikeBody(SMLoc DirectiveLoc) {
@@ -5912,6 +5919,27 @@ bool AsmParser::parseDirectiveLTODiscard() {
   return parseMany(ParseOp);
 }
 
+/// parseDirectiveInlineAsmMode
+///  ::= ".inline_asm_mode" ( "strict" | "relaxed" )
+bool AsmParser::parseDirectiveInlineAsmMode(SMLoc DirectiveLoc) {
+  if (getLexer().isNot(AsmToken::Identifier)) {
+    return Error(DirectiveLoc,
+                 "expected 'strict' or 'relaxed' after '.inline_asm_mode'");
+  }
+
+  StringRef Mode = getTok().getIdentifier();
+  if (Mode == "strict") {
+    getStreamer().setInlineAsmMode(true);
+  } else if (Mode == "relaxed") {
+    getStreamer().setInlineAsmMode(false);
+  } else {
+    return Error(getTok().getLoc(), "expected 'strict' or 'relaxed'");
+  }
+
+  Lex(); // consume mode identifier
+  return false;
+}
+
 // We are comparing pointers, but the pointers are relative to a single string.
 // Thus, this should always be deterministic.
 static int rewritesSort(const AsmRewrite *AsmRewriteA,

llvm/lib/MC/MCStreamer.cpp

@@ -400,6 +400,18 @@ void MCStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
     return getContext().reportError(Loc, "symbol '" + Twine(Symbol->getName()) +
                                              "' is already defined");
 
+  if (InlineAsmStrictMode) {
+    StringRef Name = Symbol->getName();
+    if (!Name.empty() && !Name.starts_with(".L") && !Name.starts_with("L..") &&
+        !Name.starts_with("$") && !Name.starts_with("__") &&
+        Name.front() != '.' && !std::isdigit(Name.front())) {
+      getContext().reportWarning(
+          Loc, "non-local label '" + Name +
+                   "' in inline assembly strict mode may be unsafe for "
+                   "external jumps; consider using local labels (.L*) instead");
+    }
+  }
+
   assert(!Symbol->isVariable() && "Cannot emit a variable symbol!");
   assert(getCurrentSectionOnly() && "Cannot emit before setting section!");
   assert(!Symbol->getFragment() && "Unexpected fragment on symbol data!");

llvm/test/MC/X86/inline-asm-mode-basic.s

@@ -0,0 +1,25 @@
+# RUN: llvm-mc -triple x86_64-unknown-unknown %s 2>&1 | FileCheck %s
+
+# Test basic .inline_asm_mode directive functionality
+
+.text
+
+# Test that the directive is parsed correctly
+.inline_asm_mode strict
+.inline_asm_mode relaxed
+
+# Test strict mode warnings
+.inline_asm_mode strict
+
+# This should produce a warning
+# CHECK: warning: non-local label 'unsafe_global' in inline assembly strict mode may be unsafe for external jumps; consider using local labels (.L*) instead
+unsafe_global:
+    nop
+
+# This should not warn (local label)
+.L_safe_local:
+    nop
+
+# Test error handling
+.inline_asm_mode invalid
+# CHECK: error: expected 'strict' or 'relaxed'

llvm/test/MC/X86/inline-asm-mode-directive.s

@@ -0,0 +1,57 @@
+# RUN: llvm-mc -triple x86_64-unknown-unknown %s 2>&1 | FileCheck %s --check-prefix=RELAX
+# RUN: llvm-mc -triple x86_64-unknown-unknown %s 2>&1 | FileCheck %s --check-prefix=STRICT
+
+# Test the .inline_asm_mode directive for safer inline assembly label handling
+
+.text
+
+# Test relaxed mode (default) - no warnings
+.inline_asm_mode relaxed
+
+# These labels should not produce warnings in relaxed mode
+my_label:
+    nop
+global_symbol:
+    nop
+.L_local_label:
+    nop
+
+# RELAX-NOT: warning
+
+# Test strict mode - should warn about non-local labels
+.inline_asm_mode strict
+
+# Local labels - should not warn
+.L_local1:
+    nop
+.L_local_with_numbers_123:
+    nop
+1:
+    nop
+42:
+    nop
+
+# Non-local labels - should warn
+# STRICT: :[[@LINE+1]]:1: warning: non-local label 'unsafe_label' in inline assembly strict mode may be unsafe for external jumps; consider using local labels (.L*) instead
+unsafe_label:
+    nop
+
+# STRICT: :[[@LINE+1]]:1: warning: non-local label 'another_global' in inline assembly strict mode may be unsafe for external jumps; consider using local labels (.L*) instead
+another_global:
+    nop
+
+# Switch back to relaxed mode
+.inline_asm_mode relaxed
+
+# This should not warn again
+yet_another_label:
+    nop
+
+# RELAX-NOT: warning
+
+# Test error cases
+.inline_asm_mode invalid_mode
+# CHECK: :[[@LINE-1]]:18: error: expected 'strict' or 'relaxed'
+
+.inline_asm_mode
+# CHECK: :[[@LINE-1]]:17: error: expected 'strict' or 'relaxed' after '.inline_asm_mode'