ld.so: add RTLD_LOCAL, RTLD_DEEPBIND notes + example

4 files changed, 84 insertions, 0 deletions

diff --git a/src/development/ldso/deepbind/.gitignore b/src/development/ldso/deepbind/.gitignore
new file mode 100644
index 0000000..a80e8fd
--- /dev/null
+++ b/src/development/ldso/deepbind/.gitignore
@@ -0,0 +1,2 @@
+*.so
+main
diff --git a/src/development/ldso/deepbind/Makefile b/src/development/ldso/deepbind/Makefile
new file mode 100644
index 0000000..6fc4eb1
--- /dev/null
+++ b/src/development/ldso/deepbind/Makefile
@@ -0,0 +1,19 @@
+run: build
+	LD_PRELOAD=./libprel.so ./main
+
+debug: build
+	#LD_DEBUG_OUTPUT=ldso
+	LD_DEBUG=scopes,symbols,bindings LD_PRELOAD=./libprel.so ./main
+
+build:
+	gcc -g -o libprel.so lib.c -DNAME=\"prel\" -fPIC -shared
+	gcc -g -o libdeep.so lib.c -DNAME=\"deep\" -fPIC -shared
+	gcc -g -o libnodp.so lib.c -DNAME=\"nodp\" -fPIC -shared
+	gcc -g -o liblink.so lib.c -DNAME=\"link\" -fPIC -shared
+	gcc -g -o main main.c ./liblink.so -ldl
+
+fmt:
+	clang-format -i *.c
+
+clean:
+	$(RM) *.so main
diff --git a/src/development/ldso/deepbind/lib.c b/src/development/ldso/deepbind/lib.c
new file mode 100644
index 0000000..e18a4ce
--- /dev/null
+++ b/src/development/ldso/deepbind/lib.c
@@ -0,0 +1,23 @@
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <stdio.h>
+
+#ifndef NAME
+#define NAME ""
+#endif
+
+void test() {
+  puts(NAME ":test()");
+
+  // Lookup next symbol from the libraries scope, which will search only in the
+  // libraries LOCAL scope, starting from the next object after the current one.
+  (void)dlsym(RTLD_NEXT, "next_lib" NAME);
+
+  // Global lookup from the libraries scope, which will search libraries in the
+  // GLOBAL scope and the libraries LOCAL scope. The order in which the scopes
+  // are searched depends on whether the library was loaded (a) with DEEPBIND or
+  // (b) not. whether this library was loaded with DEEPBIND). In the first case,
+  // the LOCAL scope is searched first, where in the latter, the GLOBAL scope is
+  // searched first.
+  (void)dlsym(RTLD_DEFAULT, "default_lib" NAME);
+}
diff --git a/src/development/ldso/deepbind/main.c b/src/development/ldso/deepbind/main.c
new file mode 100644
index 0000000..6d93520
--- /dev/null
+++ b/src/development/ldso/deepbind/main.c
@@ -0,0 +1,40 @@
+#include <dlfcn.h>
+#include <stdio.h>
+
+int main() {
+  puts("-- deep --");
+  // Load library into its own LOCAL scope with DEEPBINDING, meaning that
+  // symbols will first resolve to the library + its dependencies first before
+  // considering global symbols.
+  void* h1 = dlopen("./libdeep.so", RTLD_LAZY | RTLD_LOCAL | RTLD_DEEPBIND);
+  if (h1) {
+    // Lookup symbol in the libraries LOCAL scope (library + its own
+    // dependencies).
+    void (*test_fn)() = dlsym(h1, "test");
+    test_fn();
+
+    // Lookup non-existing symbol in the libraries LOCAL scope (library + its
+    // own dependencies).
+    (void)dlsym(h1, "libdeep_main");
+  }
+
+  puts("-- nodp --");
+  // Load library into its own LOCAL scope.
+  void* h2 = dlopen("./libnodp.so", RTLD_LOCAL | RTLD_LAZY);
+  if (h2) {
+    // Lookup symbol in the libraries LOCAL scope (library + its own
+    // dependencies).
+    void (*test_fn)() = dlsym(h2, "test");
+    test_fn();
+
+    // Lookup non-existing symbol in the libraries LOCAL scope (library + its
+    // own dependencies).
+    (void)dlsym(h2, "libnodp_main");
+  }
+
+  puts("-- main --");
+  // Lookup non-existing symbol in the GLOBAL scope.
+  (void)dlsym(RTLD_DEFAULT, "default_main");
+
+  return 0;
+}