On Wed, May 15, 2019 at 3:03 PM Amit Kumar <free.amit.kumar@gmail.com> wrote:
On Tue, May 14, 2019 at 5:52 PM Aruna Hewapathirane <aruna.hewapathirane@gmail.com> wrote:
Hi,
I am trying to wrap my head around the virtual to physical memory address translation. For example let's say I want to locate the sys_call_table.
objdump and vmlinux shows me this: aruna@debian:~/linux-5.1.1$ objdump -t vmlinux | grep -i sys_call_table ffffffff81c001c0 g O .rodata 0000000000001120 sys_call_table ffffffff81c01600 g O .rodata 0000000000000d60 ia32_sys_call_table
and System.map shows me this: aruna@debian:~/linux-5.1.1$ cat System.map | grep -i sys_call_table ffffffff81c001c0 R sys_call_table ffffffff81c01600 R ia32_sys_call_table
So addresses match.
And gdb shows me this: aruna@debian:~/linux-5.1.1$ gdb vmlinux GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1 Reading symbols from vmlinux...done.
(gdb) p sys_call_table $1 = {0xffffffff812317a0 <__x64_sys_read>, 0xffffffff812318b0 <__x64_sys_write>, 0xffffffff8122d980 <__x64_sys_open>, 0xffffffff8122bc40 <__x64_sys_close>, 0xffffffff81236220 <__x64_sys_newstat>, 0xffffffff812363e0 <__x64_sys_newfstat>, <snip>
Now if you take the address given by objdump and System.map which is 0xffffffff81c001c0 and ask gdb to show you I get:
(gdb) x 0xffffffff81c001c0 0xffffffff81c001c0 <sys_call_table>: 0x812317a0
My question is HOW is the address 0xffffffff81c001c0 translated to 0x812317a0 ?
At the moment I am unable to provide you a pointer, but I have read somewhere that kernel uses random numbers to relocate addresses for the sake of cracking.
https://lwn.net/Articles/569635/
I am reading up on page tables and page offsets just can't yet fully understand how it is done. A example that
breaks down the process step by step would really help.
Thanks - Aruna
Regards, Amit Kumar
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies