1 files changed, 75 insertions, 0 deletions
diff --git a/02_process_init/entry.c b/02_process_init/entry.c
new file mode 100644
index 0000000..a6b0918
--- /dev/null
+++ b/02_process_init/entry.c
@@ -0,0 +1,75 @@
+// Copyright (c) 2020 Johannes Stoelp
+
+#include <asm/unistd.h>
+#include <elf.h>
+#include <fmt.h>
+#include <stdint.h>
+#include <syscall.h>
+
+#if !defined(__linux__) || !defined(__x86_64__)
+#    error "Only supported in linux(x86_64)!"
+#endif
+
+int dynld_printf(const char* fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    char buf[64];
+    int ret = dynld_vsnprintf(buf, sizeof(buf), fmt, ap);
+    va_end(ap);
+    syscall3(__NR_write, 1 /* stdout */, buf, ret);
+    return ret;
+}
+
+void entry(long* prctx) {
+    // Interpret data on the stack passed by the OS kernel as specified in the
+    // x86_64 SysV ABI.
+
+    long argc = *prctx;
+    const char** argv = (const char**)(prctx + 1);
+    const char** envv = (const char**)(argv + argc + 1);
+
+    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;
+    }
+
+    const uint64_t* auxvp = (const uint64_t*)(envv + envc + 1);
+    for (unsigned i = 0; auxvp[i] != AT_NULL; i += 2) {
+        if (auxvp[i] < AT_MAX_CNT) {
+            auxv[auxvp[i]] = auxvp[i + 1];
+        }
+    }
+
+    // Print for demonstration
+
+    dynld_printf("Got %d arg(s)\n", argc);
+    for (const char** arg = argv; *arg; ++arg) {
+        dynld_printf("\targ = %s\n", *arg);
+    }
+
+    const int max_env = 10;
+    dynld_printf("Print first %d env var(s)\n", max_env - 1);
+    for (const char** env = envv; *env && (env - envv < max_env); ++env) {
+        dynld_printf("\tenv = %s\n", *env);
+    }
+
+    dynld_printf("Print auxiliary vector\n");
+    dynld_printf("\tAT_EXECFD: %ld\n", auxv[AT_EXECFD]);
+    dynld_printf("\tAT_PHDR  : %p\n", auxv[AT_PHDR]);
+    dynld_printf("\tAT_PHENT : %ld\n", auxv[AT_PHENT]);
+    dynld_printf("\tAT_PHNUM : %ld\n", auxv[AT_PHNUM]);
+    dynld_printf("\tAT_PAGESZ: %ld\n", auxv[AT_PAGESZ]);
+    dynld_printf("\tAT_BASE  : %lx\n", auxv[AT_BASE]);
+    dynld_printf("\tAT_FLAGS : %ld\n", auxv[AT_FLAGS]);
+    dynld_printf("\tAT_ENTRY : %p\n", auxv[AT_ENTRY]);
+    dynld_printf("\tAT_NOTELF: %lx\n", auxv[AT_NOTELF]);
+    dynld_printf("\tAT_UID   : %ld\n", auxv[AT_UID]);
+    dynld_printf("\tAT_EUID  : %ld\n", auxv[AT_EUID]);
+    dynld_printf("\tAT_GID   : %ld\n", auxv[AT_GID]);
+    dynld_printf("\tAT_EGID  : %ld\n", auxv[AT_EGID]);
+}