fix(asm): reject an out-of-range GLOBL size with a diagnostic

Assisted-by: GLM 5.3 Flash

asm/assembler_fuzz_test.go

@@ -82,6 +82,7 @@ func FuzzAssembleAMD64(f *testing.F) {
 	// above never reach.
 	f.Add("TEXT ·f(SB), $0\n\tBOGUSINSTR AX, BX\n\tRET\n")
 	f.Add("GLOBL d(SB), $-8\n")
+	f.Add("GLOBL d(SB), $0x7FFFFFFFFFFFFFFF\n")
 	f.Add("DATA d+0(SB)/9, $1\nGLOBL d(SB), $8\n")
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tADJSP $16\n\tRET\n")
 	f.Add("#define A A\nA\n")

asm/link.go

@@ -607,6 +607,14 @@ func checkDuplicateDecls(f *ast.File) error {
 	return nil
 }
 
+// maxSymbolSize is the ceiling on one GLOBL's declared size.  The image
+// materialises the symbol's bytes at assembly time, where the toolchain
+// defers the cost to its linker, so the bound is the toolchain's own: obj
+// rejects anything over 2,000,000,000 bytes as "symbol too large", and a
+// negative size, which the toolchain's int64 field tolerates and its linker
+// refuses, is diagnosed here rather than attempted.
+const maxSymbolSize = 2_000_000_000
+
 // collectData gathers the file's static symbols (GLOBL) and their initial
 // contents (DATA) into byte buffers.  Two passes: the Plan 9 convention puts
 // every DATA line before its symbol's GLOBL, so the symbols are registered
@@ -631,6 +639,9 @@ func collectData(f *ast.File) ([]dataSym, error) {
 		size := 0
 		if gd.Size != nil && gd.Size.Imm.HasVal {
 			size = int(gd.Size.Imm.Val)
+			if size < 0 || size > maxSymbolSize {
+				return nil, fmt.Errorf("GLOBL %q: symbol too large (%d bytes > %d bytes)", name, size, maxSymbolSize)
+			}
 		}
 		index[name] = len(syms)
 		ds := dataSym{