cmd/link: fix GC data reading from shared library

When linking against a Go shared library, when a global variable
in the main module has a type defined in the shared library, the
linker needs to pull the GC data from the shared library to build
the GC program for the global variable. Currently, this fails
silently, as the shared library file is closed too early and the
read failed (with no error check), causing a zero GC map emitted
for the variable, which in turn causes the runtime to treat the
variable as pointerless.

For now, fix this by keeping the file open. In the future we may
want to use mmap to read from the shared library instead.

Also add error checking. And fix a (mostly harmless) mistake in
size caluculation.

Also remove an erroneous condition for ARM64. ARM64 used to have
a special case to get the addend from the relocation on the
gcdata field. That was removed, but the new code accidentally
returned 0 unconditionally. It's no longer necessary to have any
special case, since the addend is now applied directly to the
gcdata field on ARM64, like on all the other platforms.

Fixes #39927.

Change-Id: Iecd32315b326c7059587fdc190e2fa99426e497e
Reviewed-on: https://go-review.googlesource.com/c/go/+/240462
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/cmd/link/internal/ld/lib.go b/src/cmd/link/internal/ld/lib.go
index 61ccc28a1d..802bfb2b24 100644
--- a/src/cmd/link/internal/ld/lib.go
+++ b/src/cmd/link/internal/ld/lib.go
@@ -2124,7 +2124,9 @@ func ldshlibsyms(ctxt *Link, shlib string) {
 		Errorf(nil, "cannot open shared library: %s", libpath)
 		return
 	}
-	defer f.Close()
+	// Keep the file open as decodetypeGcprog needs to read from it.
+	// TODO: fix. Maybe mmap the file.
+	//defer f.Close()
 
 	hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
 	if err != nil {