From: Christopher Faylor <cgf-use-the-mailinglist-please@cygwin.com>
To: cygwin-patches@cygwin.com
Subject: Re: fix off-by-one in dup2
Date: Thu, 05 Dec 2013 19:56:00 -0000 [thread overview]
Message-ID: <20131205195555.GA4938@ednor.casa.cgf.cx> (raw)
In-Reply-To: <52A08372.7080402@redhat.com>
On Thu, Dec 05, 2013 at 06:45:22AM -0700, Eric Blake wrote:
>On 12/04/2013 10:51 AM, Christopher Faylor wrote:
>
>>>>> One question, though. Assuming start is == size, then the current code
>>>>> in CVS extends the fd table by only 1. If that happens often, the
>>>>> current code would have to call ccalloc/memcpy/cfree a lot. Wouldn't
>>>>> it in fact be better to extend always by at least NOFILE_INCR, and to
>>>>> extend by (1 + start - size) only if start is > size + NOFILE_INCR?
>>>>> Something like
>>>>>
>>>>> size_t extendby = (start >= size + NOFILE_INCR) ? 1 + start - size : NOFILE_INCR;
>>>>>
>
>Always increasing by a minimum of NOFILE_INCR is wrong in one case - we
>should never increase beyond OPEN_MAX_MAX (currently 3200). dup2(0,
>3199) should succeed (unless it fails with EMFILE due to rlimit, but we
>already know that our handling of setrlimit(RLIMIT_NOFILE) is still a
>bit awkward); but dup2(0, 3200) must always fail with EBADF. I think
>the code in CVS is still wrong: we want to increase to the larger of the
>value specified by the user or NOFILE_INCR to minimize repeated calloc,
>but we also need to cap the increase to be at most OPEN_MAX_MAX
>descriptors, to avoid having a table larger than what the rest of our
>code base will support.
I made some more changes to CVS. Incidentally did you catch the fact
that you broke how this worked in 1.7.26? You were taking a MAX of a
signed and unsigned quantity so the signed quantity was promoted to a
huge positive number.
>Not having NOFILE_INCR free slots after a user allocation is not fatal;
No one implied it was.
>it means that the first allocation to a large number will not have tail
>padding, but the next allocation to fd+1 will allocate NOFILE_INCR slots
>rather than just one. My original idea of MAX(NOFILE_INCR, start -
>size) expresses that.
That wasn't Corinna's concern. My replacement code would have called
calloc for every one of:
dup2(0, 32);
dup2(1, 33);
dup2(2, 34);
Obviously there are different ways to avoid this and I chose to extend
the table after the "start" location.
>>> That might be helpful. Tcsh, for instance, always dup's it's std
>>> descriptors to the new fds 15-19. If it does so in this order, it would
>>> have to call extend 5 times.
>>
>> dtable.h:#define NOFILE_INCR 32
>>
>> It shouldn't extend in that scenario. The table starts with 32
>> elements.
>
>Rather, the table starts with 256 elements; which is why dup2 wouldn't
>crash until dup'ing to 256 or greater before I started touching this.
The table is initialized in dtable_init() with 32 elements. When it
enters main, it is still 32 elements, at least according to
cygheap->fdtab.size. I just checked this with gdb.
cgf
prev parent reply other threads:[~2013-12-05 19:56 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-25 23:26 Eric Blake
2013-10-15 14:06 ` Christopher Faylor
2013-10-15 20:45 ` Yaakov (Cygwin/X)
2013-10-15 22:34 ` Christopher Faylor
2013-10-16 7:40 ` Yaakov (Cygwin/X)
2013-11-23 13:19 ` Eric Blake
2013-12-04 9:32 ` Corinna Vinschen
2013-12-04 11:36 ` Corinna Vinschen
2013-12-04 12:04 ` Corinna Vinschen
2013-12-04 17:00 ` Christopher Faylor
2013-12-04 17:23 ` Corinna Vinschen
2013-12-04 17:51 ` Christopher Faylor
2013-12-04 19:44 ` Corinna Vinschen
2013-12-05 13:45 ` Eric Blake
2013-12-05 19:56 ` Christopher Faylor [this message]
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