1 files changed, 82 insertions, 0 deletions
diff --git a/03_hello_dynld/dynld.c b/03_hello_dynld/dynld.c
new file mode 100644
index 0000000..1805b33
--- /dev/null
+++ b/03_hello_dynld/dynld.c
@@ -0,0 +1,82 @@
+// Copyright (c) 2021 Johannes Stoelp
+
+#include <syscall.h>
+#include <auxv.h>
+#include <io.h>
+
+#include <stdint.h>
+#include <asm/unistd.h>
+
+#if !defined(__linux__) || !defined(__x86_64__)
+#    error "Only supported in linux(x86_64)!"
+#endif
+
+void dl_entry(const uint64_t* prctx) {
+    // Interpret data on the stack passed by the OS kernel as specified in the
+    // x86_64 SysV ABI.
+    uint64_t argc = *prctx;
+    const char** argv = (const char**)(prctx + 1);
+    const char** envv = (const char**)(argv + argc + 1);
+
+    // Count the number of environment variables in the `ENVP` segment.
+    int envc = 0;
+    for (const char** env = envv; *env; ++env) {
+        ++envc;
+    }
+
+    uint64_t auxv[AT_MAX_CNT];
+    for (unsigned i = 0; i < AT_MAX_CNT; ++i) {
+        auxv[i] = 0;
+    }
+
+    // Read the `AUXV` auxiliary vector segment.
+    const Auxv64Entry* auxvp = (const Auxv64Entry*)(envv + envc + 1);
+    for (; auxvp->tag != AT_NULL; ++auxvp) {
+        if (auxvp->tag < AT_MAX_CNT) {
+            auxv[auxvp->tag] = auxvp->val;
+        }
+    }
+
+    // Get address of the entrypoint for the user executable and
+    // transfer control.
+    // Requirements for the user executable:
+    //   - no dependencies
+    //   - no relocations
+
+    pfmt("[dynld]: Running %s @ %s\n", __FUNCTION__, __FILE__);
+
+    // Either `AT_EXECFD` or `AT_PHDR` must be specified, we only
+    // support `AT_PHDR` here.
+    //
+    // From the X86_64 SystemV ABI:
+    // AT_EXECFD
+    //   At process creation the system may pass control to an
+    //   interpreter program. When this happens, the system places
+    //   either an entry of type `AT_EXECFD` or one of type `AT_PHDR`
+    //   in the auxiliary vector. The entry for type `AT_EXECFD`
+    //   contains a file descriptor open to read the application
+    //   program’s object file.
+    //
+    // AT_PHDR
+    //   The system may create the memory image of the application
+    //   program before passing control to the interpreter
+    //   program. When this happens the `AT_PHDR` entry tells the
+    //   interpreter where to find the program header table in the
+    //   memory image.
+    if (auxv[AT_PHDR] == 0 || auxv[AT_EXECFD] != 0) {
+        pfmt("[dynld]: ERROR, expected Linux Kernel to map user executable!\n");
+        syscall1(__NR_exit, 1);
+    }
+
+    if (auxv[AT_ENTRY] == 0) {
+        pfmt("[dynld]: ERROR, AT_ENTRY not found in auxiliary vector!\n");
+        syscall1(__NR_exit, 1);
+    }
+
+    // Transfer control to user executable.
+    void (*user_entry)() = (void (*)())auxv[AT_ENTRY];
+    pfmt("[dynld]: Got user entrypoint @0x%x\n", user_entry);
+    user_entry();
+
+    syscall1(__NR_exit, 0);
+}