debugging: slides: update dlsym example

It is way more convenient to pass directly RTLD_NEXT to dlsym instead of
a handle returned by dlopen, especially for the LD_PRELOAD use case:
developers then don't care about the exact file providing the targeted
symbol

Reported-by: Luca Ceresoli <[email protected]>
Signed-off-by: Alexis Lothoré <[email protected]>

slides/debugging-application-tracing/debugging-application-tracing.tex

@@ -102,26 +102,17 @@ \subsection{LD\_PRELOAD}
 
 ssize_t read(int fd, void *data, size_t size)
 {
-        size_t (*read_func)(int, void *, size_t);
-        void *handle;
-        char *error;
-
-        handle = dlopen("/lib/libc.so.6", RTLD_LAZY);
-        if (!handle) {
-                fprintf(stderr, "Can not find overriden library\n");
-                return 0;
-        }
-        dlerror();
-        read_func = dlsym(handle, "read");
-        error = dlerror();
-        if (error) {
-                fprintf(stderr, "Can not find overriden symbol: %s\n", error);
-                return 0;
-        }
-        fprintf(stderr, "Trying to read %lu bytes to %p from file descriptor %d\n", size, data, fd);
-        return read_func(fd, data, size);
+    size_t (*read_func)(int, void *, size_t);
+    char *error;
+
+    read_func = dlsym(RTLD_NEXT, "read");
+    if (!read_func) {
+        fprintf(stderr, "Can not find read symbol: %s\n", dlerror());
+        return 0;
+    }
+    fprintf(stderr, "Trying to read %lu bytes to %p from file descriptor %d\n", size, data, fd);
+    return read_func(fd, data, size);
 }
-
     \end{minted}
   \end{block}
 \end{frame}