Re: [PATCH] nextfd(2)

["H. Peter Anvin" <hpa@zytor.com>]

On 04/01/2012 03:03 PM, H. Peter Anvin wrote:
> On 04/01/2012 05:57 AM, Alexey Dobriyan wrote:
>>
>> * /proc/self/fd is unreliable:
>>   proc may be unconfigured or not mounted at expected place.
>>   Looking at /proc/self/fd requires opening directory
>>   which may not be available due to malicious rlimit drop or ENOMEM situations.
>>   Not opening directory is equivalent to dumb close(2) loop except slower.
>>
> 
> This is really the motivation for this... the real question is how much
> functionality is actually available in the system without /proc mounted,
> and in particular if this particular subcase is worth optimizing ...
> after all, if someone is maliciously setting rlimit, we can just abort
> (if someone can set an rlimit they can also force an abort) or revert to
> the slow path.
> 

A few more observations:

- There is a huge backwards compatibility problem with this for a
substantial transition period; using /proc/self/fd has worked for a very
long time already.

- Your nextfd() system call will require more system calls that the
typical case for reading /proc/self/fd, because each getdents() system
call handles multiple readdir() invocations.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.