sparse, llvm: Fix SIGSEGV for extern symbols

Jonathan Neuschäfer writes:

  compiling a little real-world program with sparse-llvm, it segfaulted.
  Using a tool called "delta"[1] and some bash scripting, I managed to
  reduce the code to this test case:

    extern struct foo *foop;
    extern void func(struct foo *f);
    int main(int argc, char **argv) {
        func(foop);
    }

The problem is that pseudo_to_value() does not know abou the extern
symbol because Sparse never calls output_data() on it which registers
globals with LLVMAddGlobal().

As explained by Linus, 'extern' symbols are just names with types. They
don't have any value associated with them, they just have the type and
the name. Therefore we need to explicitly call LLVMAddGlobal() for
symbols we have not encountered in pseudo_to_value().

Reported by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: Christopher Li <sparse@chrisli.org>
Cc: Jeff Garzik <jgarzik@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

2 files changed, 19 insertions, 0 deletions

diff --git a/sparse-llvm.c b/sparse-llvm.c
index 6b94205d..5bcc7589 100644
--- a/sparse-llvm.c
+++ b/sparse-llvm.c
@@ -335,6 +335,14 @@ static LLVMValueRef pseudo_to_value(struct function *fn, struct instruction *ins
 			default:
 				assert(0);
 			}
+		} else {
+			const char *name = show_ident(sym->ident);
+
+			result = LLVMGetNamedGlobal(fn->module, name);
+			if (!result) {
+				LLVMTypeRef type = symbol_type(fn->module, sym);
+				result = LLVMAddGlobal(fn->module, type, name);
+			}
 		}
 		break;
 	}
diff --git a/validation/backend/extern.c b/validation/backend/extern.c
new file mode 100644
index 00000000..24cbae55
--- /dev/null
+++ b/validation/backend/extern.c
@@ -0,0 +1,11 @@
+extern unsigned long foo;
+
+static unsigned long bar(void)
+{
+	return foo;
+}
+
+/*
+ * check-name: Extern symbol code generation
+ * check-command: ./sparsec -c $file -o tmp.o
+ */