Re: [RFC PATCH 0/5] Resend - Use procfs to change a syscall behavior

[Nadia Derbey <Nadia.Derbey-6ktuUTfB/bM@public.gmane.org>]

Pavel Machek wrote:
> Hi!
> 
> 
>>>>An alternative to this solution consists in defining a new field in the
>>>>task structure (let's call it next_syscall_data) that, if set, would change
>>>>the behavior of next syscall to be called. The sys_fork_with_id() previously
>>>>cited can be replaced by
>>>>1) set next_syscall_data to a target upid nr
>>>>2) call fork().
>>>
>>>
>>>...bloat task struct and
>>>
>>>
>>>
>>>>A new file is created in procfs: /proc/self/task/<my_tid>/next_syscall_data.
>>>>This makes it possible to avoid races between several threads belonging to
>>>>the same process.
>>>
>>>
>>>...introducing this kind of uglyness.
>>>
>>>Actually, there were proposals for sys_indirect(), which is slightly
>>>less ugly, but IIRC we ended up with adding syscalls, too.
> 
> 
>>I had a look at the lwn.net article that describes the sys_indirect() 
>>interface.
>>It does exactly what we need here, so I do like it, but it has the same 
>>drawbacks as the one you're complaining about:
>>. a new field is needed in the task structure
>>. looks like many people found it ugly...
> 
> 
>>Now, coming back to what I'm proposing: what we need is actually to change 
>>the behavior of *existing* syscalls, since we are in a very particular 
>>context (restarting an application).
> 
> 
> Changing existing syscalls is _bad_: for backwards compatibility
> reasons.

I'm sorry but I don't see a backward compatibility problem: same 
interface, same functionality provided. The only change is in the way 
ids are assigned.

Actually, one drawback I'm seeing is that we are adding a test to the 
classical syscall path (the test on the current->next_syscall_data being 
set or not).

> strace will be very confusing to read, etc...

We'll have the 3 following lines added to an strace output each time we 
fill the proc file:

open("/proc/15084/task/15084/next_syscall_data", O_RDWR) = 4
write(4, "LONG1 100", 9)                = 9
close(4)                                = 0

I don't see anthing confusing here ;-)

Regards,
Nadia

> 
> 
>>Defining brand new syscalls is very touchy: needs to be careful about the 
>>interface + I can't imagine the number of syscalls that would be
>>needed.
> 
> 
> Of course new syscalls is touchy... modifying _existing_ should be
> even more touchy.
> 
> 									Pavel
>