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From: Josh Triplett <[email protected]>
To: Aleksa Sarai <[email protected]>
Cc: [email protected], [email protected],
	[email protected], [email protected],
	Alexander Viro <[email protected]>,
	Arnd Bergmann <[email protected]>, Jens Axboe <[email protected]>
Subject: Re: [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation
Date: Wed, 8 Apr 2020 20:17:08 -0700	[thread overview]
Message-ID: <20200409031708.GC6149@localhost> (raw)
In-Reply-To: <[email protected]>

On Wed, Apr 08, 2020 at 10:00:40PM +1000, Aleksa Sarai wrote:
> On 2020-04-07, Josh Triplett <[email protected]> wrote:
> > Some applications want to prevent the usual "lowest available fd"
> > allocation from allocating certain file descriptors. For instance, they
> > may want to prevent allocation of a closed fd 0, 1, or 2 other than via
> > dup2/dup3, or reserve some low file descriptors for other purposes.
> > 
> > Add a prctl to increase the minimum fd and return the previous minimum.
> > 
> > System calls that allocate a specific file descriptor, such as
> > dup2/dup3, ignore this minimum.
> > 
> > exec resets the minimum fd, to prevent one program from interfering with
> > another program's expectations about fd allocation.
> 
> Why is it implemented as an "increase the value" interface? It feels
> like this is meant to avoid some kind of security trap (with a library
> reducing the value) but it means that if you want to temporarily raise
> the minimum fd number it's not possible (without re-exec()ing yourself,
> which is hardly a fun thing to do).
> 
> Then again, this might've been discussed before and I missed it...

It was: the previous version was a "get" and "set" interface. That
interface didn't allow for the possibility that something else in the
process had already set a minimum. This new atomic increase interface
(which also serves as a "get" interface if you pass 0) makes it possible
for a userspace library to reserve a range. (You have no guarantee about
previously allocated descriptors in that range, but you know that no
*new* automatically allocated descriptors will appear in that range,
which suffices; userspace can do the rest.)

- Josh Triplett

  reply	other threads:[~2020-04-09  3:17 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-08  6:56 [PATCH v3 0/3] Support userspace-selected fds Josh Triplett
2020-04-08  6:57 ` [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Josh Triplett
2020-04-08 12:00   ` Aleksa Sarai
2020-04-09  3:17     ` Josh Triplett [this message]
2020-04-08  6:57 ` [PATCH v3 2/3] fs: openat2: Extend open_how to allow userspace-selected fds Josh Triplett
2020-04-08 12:23   ` Aleksa Sarai
2020-04-09  5:00     ` Josh Triplett
2020-04-09  8:10     ` Aleksa Sarai
2020-04-08  6:57 ` [PATCH v3 3/3] fs: pipe2: Support O_SPECIFIC_FD Josh Triplett
2020-04-08 12:26 ` [PATCH v3 0/3] Support userspace-selected fds Aleksa Sarai
2020-04-09  3:19   ` Josh Triplett
  -- strict thread matches above, loose matches on Subject: below --
2020-04-04  5:57 Josh Triplett
2020-04-04  5:58 ` [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Josh Triplett

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