Use an embedded linker for host bionic

The linux kernel requires that the ELF interpreter (runtime linker)
that's referenced by PT_INTERP be either an absolute path, or a relative
path from the current working directory. We'd prefer a relative path
from the binary, similarly to how we handle looking up shared libraries,
but that's not supported.

Instead, extract the LOAD segments from the runtime linker ELF binary
and embed them into each host bionic binary, omitting the PT_INTERP
declaration. The kernel will treat it as a static binary, and we'll use
a special entry point (linker_wrapper) to fix up the arguments passed by
the kernel before jumping to the embedded linker. From the linker's
point of view, it looks like the kernel loaded the linker like normal.

Bug: 31559095
Test: Enable host bionic, build and run libdemangle_test
Change-Id: I1753401ef91eecbf0ae3376faca31eec1c53842b

1 files changed, 42 insertions, 0 deletions

diff --git a/linker/Android.bp b/linker/Android.bp
index 5775426a3..9125fd60e 100644
--- a/linker/Android.bp
+++ b/linker/Android.bp
@@ -13,6 +13,48 @@ cc_library_static {
     static_libs: ["libasync_safe"],
 }
 
+// This is used for bionic on (host) Linux to bootstrap our linker embedded into
+// a binary.
+//
+// Host bionic binaries do not have a PT_INTERP section, instead this gets
+// embedded as the entry point, and the linker is embedded as ELF sections in
+// each binary. There's a linker script that sets all of that up (generated by
+// extract_linker), and defines the extern symbols used in this file.
+cc_object {
+    name: "linker_wrapper",
+    host_supported: true,
+    device_supported: false,
+    target: {
+        linux_bionic: { enabled: true },
+        linux: { enabled: false },
+        darwin: { enabled: false },
+    },
+
+    cflags: [
+        "-fno-stack-protector",
+        "-Wstrict-overflow=5",
+        "-fvisibility=hidden",
+        "-Wall",
+        "-Wextra",
+        "-Wno-unused",
+        "-Werror",
+    ],
+
+    srcs: [
+        "linker_wrapper.cpp",
+    ],
+    arch: {
+        x86_64: {
+            srcs: ["arch/x86_64/begin.S"],
+        },
+    },
+
+    prefix_symbols: "__dlwrap_",
+
+    // We need to access Bionic private headers in the linker.
+    include_dirs: ["bionic/libc"],
+}
+
 cc_binary {
     defaults: ["linux_bionic_supported"],
     srcs: [