Virtual To Physical Address Translation

[Aruna Hewapathirane]

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 at 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 at 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 at 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 ? 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
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