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

[Nadia Derbey]

Pavel Machek wrote:
> On Tue 2008-07-08 16:47:21, Serge E. Hallyn wrote:
> 
>>Quoting Pavel Machek (pavel at ucw.cz):
>>
>>>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. strace will be very confusing to read, etc...
>>
>>I dunno...  if you normally open(), you get back a random fd.  If you do
>>it having set the next_id inadvertently, then as far as you know you get
>>back a random fd, no?
> 
> 
> Sorry?!
> 
> No, open does not return random fds. It allocates them bottom-up. So
> you do not need any changes in open case.
> 
> (If you want to open "/foo/bar" as fd #50, open /dev/zero 49 times,

49 times - <# of already busy fds>

Don't you think it's simpler to specify the target fd, and then open the 
file.

> then open "/foo/bar"; bash already uses that trick.) 
> 								Pavel
> 

Regards,
Nadia


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