Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

[Andrew Lutomirski <luto@mit.edu>]

On Tue, Jan 17, 2012 at 9:05 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 01/17, Andrew Lutomirski wrote:
>>
>> (is_compat_task says whether the executable was marked as 32-bit.  The
>> actual execution mode is determined by the cs register, which the user
>> can control.
>
> Confused... Afaics, TIF_IA32 says that the binary is 32-bit (this comes
> along with TS_COMPAT).
>
> TS_COMPAT says that, say, the task did "int 80" to enters the kernel.
> 64-bit or not, we should treat is as 32-bit in this case.

I think you're right, and checking which entry was used is better than
checking the cs register (since 64-bit code can use int80).  That's
what I get for insufficiently careful reading of the assembly.  (And
for going from memory from when I wrote the vsyscall emulation code --
that code is entered from a page fault, so the entry point used is
irrelevant.)

--Andy