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Re: [PATCH v6 6/9] seccomp: add "seccomp" syscall

To: Alexei Starovoitov <ast@plumgrid.com>
Subject: Re: [PATCH v6 6/9] seccomp: add "seccomp" syscall
From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 13 Jun 2014 14:25:35 -0700
Cc: Kees Cook <keescook@chromium.org>, LKML <linux-kernel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, John Johansen <john.johansen@canonical.com>, Andrew Morton <akpm@linux-foundation.org>, X86 ML <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-mips@linux-mips.org, linux-arch <linux-arch@vger.kernel.org>, LSM List <linux-security-module@vger.kernel.org>
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On Fri, Jun 13, 2014 at 2:22 PM, Alexei Starovoitov <ast@plumgrid.com> wrote:
> On Tue, Jun 10, 2014 at 8:25 PM, Kees Cook <keescook@chromium.org> wrote:
>> This adds the new "seccomp" syscall with both an "operation" and "flags"
>> parameter for future expansion. The third argument is a pointer value,
>> used with the SECCOMP_SET_MODE_FILTER operation. Currently, flags must
>> be 0. This is functionally equivalent to prctl(PR_SET_SECCOMP, ...).
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Cc: linux-api@vger.kernel.org
>> ---
>>  arch/x86/syscalls/syscall_32.tbl  |    1 +
>>  arch/x86/syscalls/syscall_64.tbl  |    1 +
>>  include/linux/syscalls.h          |    2 ++
>>  include/uapi/asm-generic/unistd.h |    4 ++-
>>  include/uapi/linux/seccomp.h      |    4 +++
>>  kernel/seccomp.c                  |   63 
>> ++++++++++++++++++++++++++++++++-----
>>  kernel/sys_ni.c                   |    3 ++
>>  7 files changed, 69 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/syscalls/syscall_32.tbl 
>> b/arch/x86/syscalls/syscall_32.tbl
>> index d6b867921612..7527eac24122 100644
>> --- a/arch/x86/syscalls/syscall_32.tbl
>> +++ b/arch/x86/syscalls/syscall_32.tbl
>> @@ -360,3 +360,4 @@
>>  351    i386    sched_setattr           sys_sched_setattr
>>  352    i386    sched_getattr           sys_sched_getattr
>>  353    i386    renameat2               sys_renameat2
>> +354    i386    seccomp                 sys_seccomp
>> diff --git a/arch/x86/syscalls/syscall_64.tbl 
>> b/arch/x86/syscalls/syscall_64.tbl
>> index ec255a1646d2..16272a6c12b7 100644
>> --- a/arch/x86/syscalls/syscall_64.tbl
>> +++ b/arch/x86/syscalls/syscall_64.tbl
>> @@ -323,6 +323,7 @@
>>  314    common  sched_setattr           sys_sched_setattr
>>  315    common  sched_getattr           sys_sched_getattr
>>  316    common  renameat2               sys_renameat2
>> +317    common  seccomp                 sys_seccomp
>>
>>  #
>>  # x32-specific system call numbers start at 512 to avoid cache impact
>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>> index b0881a0ed322..1713977ee26f 100644
>> --- a/include/linux/syscalls.h
>> +++ b/include/linux/syscalls.h
>> @@ -866,4 +866,6 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
>>  asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
>>                          unsigned long idx1, unsigned long idx2);
>>  asmlinkage long sys_finit_module(int fd, const char __user *uargs, int 
>> flags);
>> +asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
>> +                           const char __user *uargs);
>
> It looks odd to add 'flags' argument to syscall that is not even used.
> It don't think it will be extensible this way.
> 'uargs' is used only in 2nd command as well and it's not 'char __user *'
> but rather 'struct sock_fprog __user *'
> I think it makes more sense to define only first argument as 'int op' and the
> rest as variable length array.
> Something like:
> long sys_seccomp(unsigned int op, struct nlattr *attrs, int len);
> then different commands can interpret 'attrs' differently.
> if op == mode_strict, then attrs == NULL, len == 0
> if op == mode_filter, then attrs->nla_type == seccomp_bpf_filter
> and nla_data(attrs) is 'struct sock_fprog'

Eww.  If the operation doesn't imply the type, then I think we've
totally screwed up.

> If we decide to add new types of filters or new commands, the syscall 
> prototype
> won't need to change. New commands can be added preserving backward
> compatibility.
> The basic TLV concept has been around forever in netlink world. imo makes
> sense to use it with new syscalls. Passing 'struct xxx' into syscalls
> is the thing
> of the past. TLV style is more extensible. Fields of structures can become
> optional in the future, new fields added, etc.
> 'struct nlattr' brings the same benefits to kernel api as protobuf did
> to user land.

I see no reason to bring nl_attr into this.

Admittedly, I've never dealt with nl_attr, but everything
netlink-related I've even been involved in has involved some sort of
API atrocity.

--Andy

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