1 /*
2  * Copyright (C) 2013 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 #pragma once
18 
19 #include <elf.h>
20 #include <link.h>
21 #include <stdint.h>
22 #include <sys/auxv.h>
23 
24 #include "platform/bionic/macros.h"
25 
26 // When the kernel starts the dynamic linker, it passes a pointer to a block
27 // of memory containing argc, the argv array, the environment variable array,
28 // and the array of ELF aux vectors. This class breaks that block up into its
29 // constituents for easy access.
30 class KernelArgumentBlock {
31  public:
KernelArgumentBlock(void * raw_args)32   explicit KernelArgumentBlock(void* raw_args) {
33     uintptr_t* args = reinterpret_cast<uintptr_t*>(raw_args);
34     argc = static_cast<int>(*args);
35     argv = reinterpret_cast<char**>(args + 1);
36     envp = argv + argc + 1;
37 
38     // Skip over all environment variable definitions to find the aux vector.
39     // The end of the environment block is marked by a NULL pointer.
40     char** p = envp;
41     while (*p != nullptr) {
42       ++p;
43     }
44     ++p; // Skip the NULL itself.
45 
46     auxv = reinterpret_cast<ElfW(auxv_t)*>(p);
47   }
48 
49   // Similar to ::getauxval but doesn't require the libc global variables to be set up,
50   // so it's safe to call this really early on.
getauxval(unsigned long type)51   unsigned long getauxval(unsigned long type) {
52     for (ElfW(auxv_t)* v = auxv; v->a_type != AT_NULL; ++v) {
53       if (v->a_type == type) {
54         return v->a_un.a_val;
55       }
56     }
57     return 0;
58   }
59 
60   int argc;
61   char** argv;
62   char** envp;
63   ElfW(auxv_t)* auxv;
64 
65  private:
66   BIONIC_DISALLOW_COPY_AND_ASSIGN(KernelArgumentBlock);
67 };
68