MCParse: Disallow @ specifier in symbol equating

Relocation specifiers are attached to an instruction and cannot be used
in equating. GAS rejects `a = b@plt`. For now, handle just
MCSymbolRefExpr.

Author: MaskRay

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -6334,6 +6334,11 @@ bool parseAssignmentExpression(StringRef Name, bool allow_redef,
     return Parser.TokError("missing expression");
   if (Parser.parseEOL())
     return true;
+  // Relocation specifiers are not permitted. For now, handle just
+  // MCSymbolRefExpr.
+  if (auto *S = dyn_cast<MCSymbolRefExpr>(Value); S && S->getSpecifier())
+    return Parser.Error(
+        EqualLoc, "relocation specifier not permitted in symbol equating");
 
   // Validate that the LHS is allowed to be a variable (either it has not been
   // used as a symbol, or it is an absolute symbol).

llvm/test/MC/ELF/relocation-alias.s

@@ -2,6 +2,7 @@
 # RUN: llvm-objdump --no-print-imm-hex -dr %t | FileCheck %s
 # RUN: llvm-readelf -s %t | FileCheck %s --check-prefix=SYM
 
+# RUN: not llvm-mc -filetype=obj -triple x86_64 --defsym ERR0=1 %s 2>&1 | FileCheck %s --check-prefix=ERR0 --implicit-check-not=error:
 # RUN: not llvm-mc -filetype=obj -triple x86_64 --defsym ERR=1 %s 2>&1 | FileCheck %s --check-prefix=ERR
 
 ## If a fixup symbol is equated to an undefined symbol, convert the fixup
@@ -39,6 +40,14 @@ movabsq $data_alias_l, %rbx
 .globl data
 data:
 
+.ifdef ERR0
+# ERR0: {{.*}}.s:[[#@LINE+1]]:15: error: relocation specifier not permitted in symbol equating
+memcpy_spec = __GI_memcpy@PLT
+
+## Should be rejected as well
+memcpy_spec1 = __GI_memcpy@PLT+1
+.endif
+
 .ifdef ERR
 .text
 ## Note, GNU as emits a relocation for this erroneous fixup.