Read and write process' memory through /dev/mem, text segment works but data segment can not, why?

问题

I want to read to and write from process' memory through /dev/mem.

First, I get process' memory map through a linux kernel module coded by myself, output is like this:

start_code_segment      4000000000000000
end_code_segment        4000000000019c38
start_data_segment      6000000000009c38
end_data_segment        600000000000b21d
start_brk               6000000000010000
brk                     6000000000034000
start_stack             60000fffffde7b00

Second, I can convert virtual address(VA) to PA thorough the linux kernel module, for example, I can convert VA:0x4000000000000008 to PA:0x100100c49f8008

Third, function read_phy_mem can get memory data in PA:0x100100c49f8008，code at the final.

Problem: My problem is when I read text segment PA memory, everything is OK, but if I read data segment PA memory, *((long *)mapAddr) in line 243 will cause system to go down. Also, I tried

memcpy( &data, (void *)mapAddr, sizeof(long) )

but it still make the system go down.

other info: my computer is IA64, OS is Linux 2.6.18, when system is down, I can get output Info from console like this, then system will restart.

Entered OS MCA handler. PSP=20010000fff21320 cpu=0 monarch=1
cpu 0, MCA occurred in user space, original stack not modified
All OS MCA slaves have reached rendezvous
MCA: global MCA
mlogbuf_finish: printing switched to urgent mode, MCA/INIT might be dodgy or fail.
Delaying for 5 seconds...

code of function read_phy_mem

    /*
     * pa:   physical address
     * data: memory data in pa
     *
     * return int: success or failed
    */
188 int read_phy_mem(unsigned long pa,long *data)
189 {
190     int memfd;
191     int pageSize;
192     int shift;
193     int do_mlock;
194     void volatile *mapStart;
195     void volatile *mapAddr;
196     unsigned long pa_base;
197     unsigned long pa_offset;
198 
199     memfd = open("/dev/mem", O_RDWR | O_SYNC);
200     if(memfd == -1)
201     {
202         perror("Failed to open /dev/mem");
203         return FAIL;
204     }
205 
206     shift = 0;
207     pageSize = PAGE_SIZE; //#define PAGE_SIZE 16384
208     while(pageSize > 0)
209     {
210         pageSize = pageSize >> 1;
211         shift ++;
212     }
213     shift --;
214     pa_base = (pa >> shift) << shift;
215     pa_offset = pa - pa_base;
224     mapStart = (void volatile *)mmap(0, PAGE_SIZE, PROT_READ | PROT_WRITE,MAP_SHARED | MAP_LOCKED, memfd, pa_base);
226     if(mapStart == MAP_FAILED)
227     {
228         perror("Failed to mmap /dev/mem");
229         close(memfd);
230         return FAIL;
231     }
232     if(mlock((void *)mapStart, PAGE_SIZE) == -1)
233     {
234         perror("Failed to mlock mmaped space");
235         do_mlock = 0;
236     }
237     do_mlock = 1;
238 
239     mapAddr = (void volatile *)((unsigned long)mapStart + pa_offset);
243     printf("mapAddr %p %d\n", mapAddr, *((long *)mapAddr));
256     if(munmap((void *)mapStart, PAGE_SIZE) != 0)
257     {
258         perror("Failed to munmap /dev/mem");
259     }
260     close(memfd);
269     return OK;
270 }

Can anyone understand why text segment works well but data segment does not?

回答1:

I guess, its happening because code-section remain in memory while process executes(if not a DLL code), Whereas data section leave in & out continuously.
Try with stack-Segment. And check if its working?
Write your own test program and allocate memory dynamically in KBs and keep that memory in use within a loop. Than try it with your code to read memory segments of test program. I think it will work.
I have done similar work in windows to replace BIOS address from IVT.
Should be root user.

来源：https://stackoverflow.com/questions/10733816/read-and-write-process-memory-through-dev-mem-text-segment-works-but-data-seg

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