Re: [PATCH v2] signal: add procfd_signal() syscall

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 29, 2018, at 11:55 AM, Christian Brauner <christian@brauner.io> wrote:
> 
>> On Thu, Nov 29, 2018 at 11:22:58AM -0800, Andy Lutomirski wrote:
>>> On Thu, Nov 29, 2018 at 11:17 AM Christian Brauner <christian@brauner.io> wrote:
>>> 
>>>> On November 30, 2018 5:54:18 AM GMT+13:00, Andy Lutomirski <luto@amacapital.net> wrote:
>>>> 
>>>> 
>>>>> On Nov 29, 2018, at 4:28 AM, Florian Weimer <fweimer@redhat.com>
>>>> wrote:
>>>>> 
>>>>> Disclaimer: I'm looking at this patch because Christian requested it.
>>>>> I'm not a kernel developer.
>>>>> 
>>>>> * Christian Brauner:
>>>>> 
>>>>>> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl
>>>> b/arch/x86/entry/syscalls/syscall_32.tbl
>>>>>> index 3cf7b533b3d1..3f27ffd8ae87 100644
>>>>>> --- a/arch/x86/entry/syscalls/syscall_32.tbl
>>>>>> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
>>>>>> @@ -398,3 +398,4 @@
>>>>>> 384    i386    arch_prctl        sys_arch_prctl
>>>> __ia32_compat_sys_arch_prctl
>>>>>> 385    i386    io_pgetevents        sys_io_pgetevents
>>>> __ia32_compat_sys_io_pgetevents
>>>>>> 386    i386    rseq            sys_rseq            __ia32_sys_rseq
>>>>>> +387    i386    procfd_signal        sys_procfd_signal
>>>> __ia32_compat_sys_procfd_signal
>>>>>> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl
>>>> b/arch/x86/entry/syscalls/syscall_64.tbl
>>>>>> index f0b1709a5ffb..8a30cde82450 100644
>>>>>> --- a/arch/x86/entry/syscalls/syscall_64.tbl
>>>>>> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
>>>>>> @@ -343,6 +343,7 @@
>>>>>> 332    common    statx            __x64_sys_statx
>>>>>> 333    common    io_pgetevents        __x64_sys_io_pgetevents
>>>>>> 334    common    rseq            __x64_sys_rseq
>>>>>> +335    64    procfd_signal        __x64_sys_procfd_signal
>>>>>> 
>>>>>> #
>>>>>> # x32-specific system call numbers start at 512 to avoid cache
>>>> impact
>>>>>> @@ -386,3 +387,4 @@
>>>>>> 545    x32    execveat        __x32_compat_sys_execveat/ptregs
>>>>>> 546    x32    preadv2            __x32_compat_sys_preadv64v2
>>>>>> 547    x32    pwritev2        __x32_compat_sys_pwritev64v2
>>>>>> +548    x32    procfd_signal        __x32_compat_sys_procfd_signal
>>>>> 
>>>>> Is there a reason why these numbers have to be different?
>>>>> 
>>>>> (See the recent discussion with Andy Lutomirski.)
>>>> 
>>>> Hah, I missed this part of the patch.  Let’s not add new x32 syscall
>>>> numbers.
>>>> 
>>>> Also, can we perhaps rework this a bit to get rid of the compat entry
>>>> point?  The easier way would be to check in_compat_syscall(). The nicer
>>>> way IMO would be to use the 64-bit structure for 32-bit as well.
>>> 
>>> Do you have a syscall which set precedence/did this before I could look at?
>>> Just if you happen to remember one.
>>> Fwiw, I followed the other signal syscalls.
>>> They all introduce compat syscalls.
>>> 
>> 
>> Not really.
>> 
>> Let me try to explain.  I have three issues with the approach in your patchset:
>> 
>> 1. You're introducing a new syscall, and it behaves differently on
>> 32-bit and 64-bit because the structure you pass in is different.
>> This is necessary for old syscalls where compatibility matters, but
>> maybe we can get rid of it for new syscalls.   Could we define a
>> siginfo64_t that is identical to the 64-bit siginfo_t and just use
>> that in all cases?
>> 
>> 2. Assuming that #1 doesn't work, then we need compat support.  But
>> you're doing it by having two different entry points.  Instead, you
>> could have a single entry point that calls in_compat_syscall() to
>> decide which structure to read.  This would simplify things because
>> x86 doesn't really support the separate compat entry points, which
>> leads me to #3.
>> 
>> 3. The separate x32 numbers are a huge turd that may have security
>> holes and certainly have comprehensibility holes.  I will object to
>> any patch that adds a new one (like yours).  Fixing #1 or #2 makes
>> this problem go away.
>> 
>> Does that make any sense?  The #2 fix would be something like:
>> 
>> if (in_compat_syscall)
>>  copy...user32();
>> else
>>  copy_from_user();
>> 
>> The #1 fix would add a copy_siginfo_from_user64() or similar.
> 
> Thanks very much! That all helped a bunch already! I'll try to go the
> copy_siginfo_from_user64() way first and see if I can make this work. If
> we do this I would however only want to use it for the new syscall first
> and not change all other signal syscalls over to it too. I'd rather keep
> this patchset focussed and small and do such conversions caused by the
> new approach later. Does that sound reasonable?

Absolutely. I don’t think we can change old syscalls — the ABI is set in stone. But for new syscalls, I think the always-64-bit behavior makes sense.