Virtual To Physical Address Translation

[Amit Kumar]

On Wed, May 15, 2019 at 3:03 PM Amit Kumar <free.amit.kumar at gmail.com> wrote:
>
> On Tue, May 14, 2019 at 5:52 PM Aruna Hewapathirane
> <aruna.hewapathirane at 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 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 ?
> 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
> >
> >
> >
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