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虚拟地址如何访问到物理地址
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環境：32bit CPU
struct task_struct { . . . . . . int on_rq
; int prio
, static_prio
, normal_prio
; unsigned int rt_priority
; const struct sched_class * sched_class
; struct sched_entity se
; struct sched_rt_entity rt
; . . . . . . struct mm_struct * mm
, * active_mm
; struct mm_struct { . . . . . . unsigned long mmap_base
; unsigned long mmap_legacy_base
; unsigned long task_size
; unsigned long highest_vm_end
; pgd_t * pgd
; atomic_t mm_users
; atomic_t mm_count
; atomic_long_t nr_ptes
; . . . . . .
其中pgd保存的就是一級頁表的基地址。進程切換時，會把TTBRx寄存器（上圖所示）保存起來，把下一個進程的pgd內容寫到TTBRx寄存器中。
# include <linux/init_task.h> unsigned int get_phy_addr ( struct mm_struct * mm
, unsigned long addr
)
{ pgd_t * pgd
= NULL ; pud_t * pud
= NULL ; pmd_t * pmd
= NULL ; pte_t * pte
= NULL ; unsigned int phy_addr
= 0 ; printk ( "pgd_base = %p\n" , mm
-> pgd
) ; pgd
= pgd_offset ( mm
, addr
) ; printk ( "vaddr=[0x%08x] *pgd=%08x\n" , addr
, pgd_val ( * pgd
) ) ; if ( pgd_none ( * pgd
) ) goto out
; if ( pgd_bad ( * pgd
) ) { printk ( "pgd bad\n" ) ; goto out
; } pud
= pud_offset ( pgd
, addr
) ; printk ( "pud=0x%08x,*pud=%08x\n" , pud
, pud_val ( * pud
) ) ; if ( pud_none ( * pud
) ) goto out
; if ( pud_bad ( * pud
) ) { printk ( "pud bad" ) ; goto out
; } pmd
= pmd_offset ( pud
, addr
) ; printk ( "pmd=0x%08x,*pmd=%08x\n" , pmd
, pmd_val ( * pmd
) ) ; if ( pmd_none ( * pmd
) ) goto out
; if ( pmd_bad ( * pmd
) ) { printk ( "(bad)" ) ; goto out
; } pte
= pte_offset_map ( pmd
, addr
) ; printk ( "pte=0x%08x, *pte=%08x\n" , pte
, pte_val ( * pte
) ) ; if ( ! pte_none ( * pte
) && pte_present ( * pte
) ) { phy_addr
= ( pte_val ( * pte
) & 0xFFFFF000 ) | ( addr
& 0xFFF ) ; printk ( "phy_addr is 0x%x\n" , phy_addr
) ; } else { printk ( "pte is not present\n" ) ; } pte_unmap ( pte
) ; return phy_addr
;
out
: return - 1 ;
}
2.2、/dev/mem節點說明
cat /proc/iomem
# include <stdio.h> # include <unistd.h> # include <sys/mman.h> # include <sys/types.h> # include <sys/stat.h> # include <fcntl.h> # include <errno.h> int main ( int argc
, char * argv
[ ] )
{ int i
= 0 , fd
= 0 ; unsigned int addr
= 0 , content
= 0 ; unsigned int page_size
= getpagesize ( ) ; unsigned int taddr
= 0 ; unsigned int * map_base
= NULL ; unsigned int offset_in_page
= 0 ; printf ( "page_size=0x%x\n" , page_size
) ; fd
= open ( "/dev/mem" , O_RDWR
| O_SYNC
) ; if ( fd
== - 1 ) { printf ( "open /dev/mem error.\n" ) ; return - 1 ; } taddr
= strtoll ( argv
[ 1 ] , NULL , 16 ) ; offset_in_page
= taddr
& ( page_size
- 1 ) ; map_base
= mmap ( NULL , page_size
, PROT_READ
, MAP_PRIVATE
, fd
, taddr
& ( ~ ( unsigned int ) ( page_size
- 1 ) ) ) ; if ( - 1 == ( int ) map_base
) { printf ( "mmap failed!, errno=%d\n" , errno
) ; close ( fd
) ; return - 1 ; } printf ( "map_base=0x%x\n" , map_base
) ; addr
= ( unsigned int ) ( ( ( unsigned char * ) map_base
) + offset_in_page
) ; printf ( "addressx:0x%x\n" , addr
) ; printf ( "content 0x%x\n\n" , * ( unsigned int * ) ( addr
) ) ; close ( fd
) ; munmap ( map_base
, page_size
) ; return 1 ;
}
2.4、測試程序
測試程序：
int main ( int argc
, char * * argv
)
{ int fd
= - 1 ; struct param_s param
; char * buf
= "Chinaxxxxx.\n" ; memset ( & param
, 0x00 , sizeof ( struct ty_motor_param_s ) ) ; fd
= open ( "/dev/motor" , O_RDWR
) ; if ( fd
< 0 ) { printf ( "open error.\n" ) ; return 0 ; } printf ( "buf=0x%x" , buf
) ; param
. addr
= buf
; ioctl ( fd
, IOCTL_SET
, & param
) ; close ( fd
) ; sleep ( 10 ) ; printf ( "buf=0x%x" , buf
) ; return 0 ;
}
2.5、實驗開始
3、遇到的問題
正如上面描述的，通過獲取物理地址修改物理地址里面的數據，這對系統來說就毫無安全可言。因此內核通過CONFIG_STRICT_DEVMEM配置項禁止內存空間實現映射。
總結
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