cmd/link: allow linkname reference to a TEXT symbol regardless of size

In CL 660696, we made the linker to choose the symbol of the
larger size in case there are multiple contentless declarations of
the same symbol. We also made it emit an error in the case that
there are a contentless declaration of a larger size and a
definition with content of a smaller size. In this case, we should
choose the definition with content, but the code accesses it
through the declaration of the larger size could fall into the
next symbol, potentially causing data corruption. So we disallowed
it.

There is one spcial case, though, that some code uses a linknamed
variable declaration to reference a function in assembly, in order
to take its address. The variable is often declared as uintptr.
The function symbol is the definition, which could sometimes be
shorter. This would trigger the error case above, causing existing
code failing to build.

This CL allows it as a special case. It is still not safe to
access the variable's content. But it is actually okay to just
take its address, which the existing code often do.

Fixes #73617.

Change-Id: I467381bc5f6baa16caee6752a0a824c7185422f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/676636
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 23 insertions, 0 deletions

diff --git a/src/cmd/link/testdata/linkname/textvar/asm.s b/src/cmd/link/testdata/linkname/textvar/asm.s
new file mode 100644
index 0000000000..332dcdb4e7
--- /dev/null
+++ b/src/cmd/link/testdata/linkname/textvar/asm.s
@@ -0,0 +1,6 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+TEXT	·asmfunc(SB),0,$0-0
+	RET
diff --git a/src/cmd/link/testdata/linkname/textvar/main.go b/src/cmd/link/testdata/linkname/textvar/main.go
new file mode 100644
index 0000000000..b38995e706
--- /dev/null
+++ b/src/cmd/link/testdata/linkname/textvar/main.go
@@ -0,0 +1,17 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Using a linknamed variable to reference an assembly
+// function in the same package is ok.
+
+package main
+
+import _ "unsafe"
+
+func main() {
+	println(&asmfunc)
+}
+
+//go:linkname asmfunc
+var asmfunc uintptr