ld.so: load libs with same name experiments

src/development/ld.so.md

@@ -199,6 +199,10 @@ symbol=default_main;  lookup in file=/lib64/ld-linux-x86-64.so.2 [0]
 ./main: error: symbol lookup error: undefined symbol: default_main (fatal)
 ```
 
+## Load lib with same name from different locations
+The sources in [ldso/samename][src-samename] show some experiments, loading the
+libs with the same name but potentially from different locations (paths).
+
 ## Dynamic Linking (x86_64)
 Dynamic linking basically works via one indirect jump. It uses a combination of
 function trampolines (`.plt` section) and a function pointer table (`.got.plt`
@@ -260,3 +264,4 @@ Using `radare2` we can analyze this in more detail:
   As we can see the offset from relocation at index `0` points to `GOT[3]`.
 
 [src-deepbind]: https://github.com/johannst/notes/tree/master/src/development/ldso/deepbind
+[src-samename]: https://github.com/johannst/notes/tree/master/src/development/ldso/samename

src/development/ldso/samename/Makefile

@@ -0,0 +1,13 @@
+all:
+	mkdir -p foo bar
+	gcc -g -o foo/lib.so lib.c -shared -fPIC -DNAME=\"foo\"
+	gcc -g -o bar/lib.so lib.c -shared -fPIC -DNAME=\"bar\"
+	gcc -g -o main main.c -ldl
+	./main foo/lib.so bar/lib.so
+	LD_LIBRARY_PATH=foo:bar ./main lib.so lib.so
+	LD_LIBRARY_PATH=bar:foo ./main lib.so lib.so
+	LD_LIBRARY_PATH=foo ./main lib.so foo/lib.so
+	LD_LIBRARY_PATH=foo ./main lib.so bar/lib.so
+
+clean:
+	$(RM) -r foo bar main

src/development/ldso/samename/lib.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+
+#ifndef NAME
+#define NAME __FILE_NAME__
+#endif
+
+void moose() {
+  puts(NAME " says moose");
+}

src/development/ldso/samename/main.c

@@ -0,0 +1,36 @@
+#include <assert.h>
+#include <dlfcn.h>
+#include <stdio.h>
+#include <string.h>
+
+void print_libso() {
+  FILE* m = fopen("/proc/self/maps", "r");
+  assert(m);
+
+  char line[256];
+  while (fgets(line, sizeof(line), m)) {
+    if (strstr(line, "lib.so")) {
+      printf("%s", line);
+    }
+  }
+  fclose(m);
+}
+
+int main(int argc, char* argv[]) {
+  for (int i = 1; i < argc; ++i) {
+    void* h = dlopen(argv[i], RTLD_LAZY | RTLD_GLOBAL);
+    if (!h) {
+      puts(dlerror());
+      return 1;
+    }
+
+    void (*next)() = dlsym(h, "moose");
+    assert(next);
+    next();
+
+    // leak lib, we want to priint the mmap.
+  }
+
+  print_libso();
+  return 0;
+}