利用/proc/pid/pagemap将虚拟地址转换为物理地址

内核文档： Documentation/vm/pagemap.txt

pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
userspace programs to examine the page tables and related information by
reading files in /proc.

There are four components to pagemap:

* /proc/pid/pagemap.  This file lets a userspace process find out which
   physical frame each virtual page is mapped to.  It contains one 64-bit
   value for each virtual page, containing the following data (from
   fs/proc/task_mmu.c, above pagemap_read):

* Bits 0-54  page frame number (PFN) if present
    * Bits 0-4   swap type if swapped
    * Bits 5-54  swap offset if swapped
    * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
    * Bit  56    page exclusively mapped (since 4.2)
    * Bits 57-60 zero
    * Bit  61    page is file-page or shared-anon (since 3.5)
    * Bit  62    page swapped
    * Bit  63    page present

Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
   In 4.0 and 4.1 opens by unprivileged fail with -EPERM.  Starting from
   4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.

If the page is not present but in swap, then the PFN contains an
   encoding of the swap file number and the page's offset into the
   swap. Unmapped pages return a null PFN. This allows determining
   precisely which pages are mapped (or in swap) and comparing mapped
   pages between processes.

Efficient users of this interface will use /proc/pid/maps to
   determine which areas of memory are actually mapped and llseek to
   skip over unmapped regions.

下面是一个工具：

 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <assert.h>
 #include <errno.h>
 #include <stdint.h>
 #include <string.h>

 #define PAGEMAP_ENTRY 8
 #define GET_BIT(X,Y) (X & ((uint64_t)1<<Y)) >> Y
 #define GET_PFN(X) X & 0x7FFFFFFFFFFFFF

 const int __endian_bit = ;
 #define is_bigendian() ( (*(char*)&__endian_bit) == 0 )

 int i, c, pid, status;
 unsigned long virt_addr;
 uint64_t read_val, file_offset, page_size;
 char path_buf [0x100] = {};
 FILE * f;
 char *end;

 int read_pagemap(char * path_buf, unsigned long virt_addr);

 int main(int argc, char ** argv){
     if(argc!=){
         printf("Argument number is not correct!\n pagemap PID VIRTUAL_ADDRESS\n");
         return -;
     }
     if(!memcmp(argv[],"self",sizeof("self"))){
         sprintf(path_buf, "/proc/self/pagemap");
         pid = -;
     }
     else{
         pid = strtol(argv[],&end, );
         if (end == argv[] || *end != '\0' || pid<=){
             printf("PID must be a positive number or 'self'\n");
             return -;
         }
     }
     virt_addr = strtoll(argv[], NULL, );
     if(pid!=-)
         sprintf(path_buf, "/proc/%u/pagemap", pid);

     page_size = getpagesize();
     read_pagemap(path_buf, virt_addr);
     return ;
 }

 int read_pagemap(char * path_buf, unsigned long virt_addr){
     printf("Big endian? %d\n", is_bigendian());
     f = fopen(path_buf, "rb");
     if(!f){
         printf("Error! Cannot open %s\n", path_buf);
         return -;
     }

     //Shifting by virt-addr-offset number of bytes
     //and multiplying by the size of an address (the size of an entry in pagemap file)
     file_offset = virt_addr / page_size * PAGEMAP_ENTRY;
     printf("Vaddr: 0x%lx, Page_size: %lld, Entry_size: %d\n", virt_addr, page_size, PAGEMAP_ENTRY);
     printf("Reading %s at 0x%llx\n", path_buf, (unsigned long long) file_offset);
     status = fseek(f, file_offset, SEEK_SET);
     if(status){
         perror("Failed to do fseek!");
         return -;
     }
     errno = ;
     read_val = ;
     unsigned char c_buf[PAGEMAP_ENTRY];
     for(i=; i < PAGEMAP_ENTRY; i++){
         c = getc(f);
         if(c==EOF){
             printf("\nReached end of the file\n");
             return ;
         }
         if(is_bigendian())
             c_buf[i] = c;
         else
             c_buf[PAGEMAP_ENTRY - i - ] = c;
         printf("[%d]0x%x ", i, c);
     }
     for(i=; i < PAGEMAP_ENTRY; i++){
         //printf("%d ",c_buf[i]);
         read_val = (read_val << ) + c_buf[i];
     }
     printf("\n");
     printf("Result: 0x%llx\n", (unsigned long long) read_val);
     if(GET_BIT(read_val, )) {
         uint64_t pfn = GET_PFN(read_val);
         printf("PFN: 0x%llx (0x%llx)\n", pfn, pfn * page_size + virt_addr % page_size);
     } else
         printf("Page not present\n");
     if(GET_BIT(read_val, ))
         printf("Page swapped\n");
     fclose(f);
     return ;
 }

测试：

用Qemu+vexpress-ca9：

内存： 1GB， 物理地址范围： 0x60000000->0x9FFFFFFF

通过查看/proc/pid/maps获得进程的地址空间的内存映射情况：

 [root@vexpress ~]# cat /proc//maps
 -001f3000 r-xp  b3:          /bin/busybox
 001fa000-001fc000 rw-p 001ea000 b3:          /bin/busybox
 001fc000- rw-p  :           [heap]
 b6c7f000-b6c80000 rw-p  :
 b6c80000-b6c8d000 r-xp  b3:         /lib/libnss_files-2.18.so
 b6c8d000-b6c94000 ---p 0000d000 b3:         /lib/libnss_files-2.18.so
 b6c94000-b6c95000 r--p 0000c000 b3:         /lib/libnss_files-2.18.so
 b6c95000-b6c96000 rw-p 0000d000 b3:         /lib/libnss_files-2.18.so
 b6c96000-b6ca1000 r-xp  b3:         /lib/libnss_nis-2.18.so
 b6ca1000-b6ca8000 ---p 0000b000 b3:         /lib/libnss_nis-2.18.so
 b6ca8000-b6ca9000 r--p 0000a000 b3:         /lib/libnss_nis-2.18.so
 b6ca9000-b6caa000 rw-p 0000b000 b3:         /lib/libnss_nis-2.18.so
 b6caa000-b6daa000 rw-p  :
 b6daa000-b6dca000 r-xp  b3:         /lib/ld-2.18.so
 b6dca000-b6dd1000 ---p  b3:         /lib/ld-2.18.so
 b6dd1000-b6dd2000 r--p 0001f000 b3:         /lib/ld-2.18.so
 b6dd2000-b6dd3000 rw-p  b3:         /lib/ld-2.18.so
 b6dd3000-b6f06000 r-xp  b3:         /lib/libc-2.18.so
 b6f06000-b6f0d000 ---p  b3:         /lib/libc-2.18.so
 b6f0d000-b6f0f000 r--p  b3:         /lib/libc-2.18.so
 b6f0f000-b6f10000 rw-p  b3:         /lib/libc-2.18.so
 b6f10000-b6f13000 rw-p  :
 b6f13000-b6f26000 r-xp  b3:         /lib/libnsl-2.18.so
 b6f26000-b6f2d000 ---p  b3:         /lib/libnsl-2.18.so
 b6f2d000-b6f2e000 r--p  b3:         /lib/libnsl-2.18.so
 b6f2e000-b6f2f000 rw-p  b3:         /lib/libnsl-2.18.so
 b6f2f000-b6f31000 rw-p  :
 b6f31000-b6f39000 r-xp  b3:         /lib/libnss_compat-2.18.so
 b6f39000-b6f40000 ---p  b3:         /lib/libnss_compat-2.18.so
 b6f40000-b6f41000 r--p  b3:         /lib/libnss_compat-2.18.so
 b6f41000-b6f42000 rw-p  b3:         /lib/libnss_compat-2.18.so
 be958000-be979000 rw-p  :           [stack]
 bed04000-bed05000 r-xp  :           [sigpage]
 bed05000-bed06000 r--p  :           [vvar]
 bed06000-bed07000 r-xp  :           [vdso]
 ffff0000-ffff1000 r-xp  :           [vectors]

可以看看0x8000这个虚拟地址对应的物理地址：

 [root@vexpress ~]# ./translate  0x8000
 Big endian?
 Vaddr: 0x8000, Page_size: , Entry_size:
 Reading /proc//pagemap at 0x40
 []0x0 []0xf8 []0x9 []0x0 []0x0 []0x0 []0x0 []0xa0
 Result: 0xa00000000009f800
 PFN: 0x9f800 (0x9f800000)

可以看到， 对应的物理页帧是0x9F800，那么物理地址就是0x9F800000.

下面我们再做一个实验， 进程746的地址空间有一部分用来映射libc：

 b6dd3000-b6f06000 r-xp  b3:         /lib/libc-2.18.so
 b6f06000-b6f0d000 ---p  b3:         /lib/libc-2.18.so
 b6f0d000-b6f0f000 r--p  b3:         /lib/libc-2.18.so
 b6f0f000-b6f10000 rw-p  b3:         /lib/libc-2.18.so

此外， 进程835也会用到libc：

 [root@vexpress ~]# cat /proc//maps
 ... ...
 b6e0b000-b6f3e000 r-xp  b3:         /lib/libc-2.18.so
 b6f3e000-b6f45000 ---p  b3:         /lib/libc-2.18.so
 b6f45000-b6f47000 r--p  b3:         /lib/libc-2.18.so
 b6f47000-b6f48000 rw-p  b3:         /lib/libc-2.18.so
 ... ...

可以看到， 进程746和835虽然都用了libc，但是对应的虚拟地址却不同，前者是0xb6dd3000， 而后者是0xb6e0b000, 我们知道对于共享库， 在内存只会存在一份代码， 那么物理地址也就是唯一的（代码段是唯一的，所有调用libc的进程共享，而数据段每个进程一个）， 那么进程746的虚拟地址空间的0xb6dd3000（代码段）跟进程835的虚拟地址空间的0xb6e0b000（代码段）对应的物理地址应该是同一个， 下面验证一下：

进程746：

 [root@vexpress ~]# ./translate  0xb6dd3000
 virt_addr: 0xb6dd3000
 Big endian?
 Vaddr: 0xb6dd3000, Page_size: , Entry_size:
 Reading /proc//pagemap at 0x5b6e98
 []0x68 []0xfa []0x9 []0x0 []0x0 []0x0 []0x0 []0xa0
 Result: 0xa00000000009fa68
 PFN: 0x9fa68 (0x9fa68000)

可以看到，物理地址是0x9FA68000

进程835：

 [root@vexpress ~]# ./translate  0xb6e0b000
 virt_addr: 0xb6e0b000
 Big endian?
 Vaddr: 0xb6e0b000, Page_size: , Entry_size:
 Reading /proc//pagemap at 0x5b7058
 []0x68 []0xfa []0x9 []0x0 []0x0 []0x0 []0x0 []0xa0
 Result: 0xa00000000009fa68
 PFN: 0x9fa68 (0x9fa68000)

可以看到， 物理地址也是0x9FA68000， 从而证明了我们的猜想。

完。