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From: Josh Triplett <[email protected]>
To: "Dmitry V. Levin" <[email protected]>
Cc: [email protected], [email protected],
	[email protected], [email protected],
	Alexander Viro <[email protected]>,
	Arnd Bergmann <[email protected]>, Jens Axboe <[email protected]>,
	Aleksa Sarai <[email protected]>,
	[email protected], Linux API <[email protected]>
Subject: Re: [PATCH v5 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation
Date: Wed, 22 Apr 2020 21:51:12 -0700	[thread overview]
Message-ID: <20200423045112.GI161058@localhost> (raw)
In-Reply-To: <[email protected]>

On Thu, Apr 23, 2020 at 04:12:53AM +0300, Dmitry V. Levin wrote:
> On Tue, Apr 21, 2020 at 10:19:49PM -0700, Josh Triplett 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.
> 
> Please make this aspect properly documented in "Effect on process
> attributes" section of execve(2) manual page.

Done. I'll include updated manpage patches in v6.

> > +unsigned int increase_min_fd(unsigned int num)
> > +{
> > +	struct files_struct *files = current->files;
> > +	unsigned int old_min_fd;
> > +
> > +	spin_lock(&files->file_lock);
> > +	old_min_fd = files->min_fd;
> > +	files->min_fd += num;
> > +	spin_unlock(&files->file_lock);
> > +	return old_min_fd;
> > +}
>
> If it's "increase", there should be an overflow check.
> Otherwise it's "assign" rather than "increase".

I'll add a check in v6, to make sure that the value cannot overflow into
the errno range. (Note that this is not security-sensitive, it's just
providing a footgun-resistant interface. It should absolutely check,
though.)

- Josh Triplett

  reply	other threads:[~2020-04-23  4:51 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-22  5:19 [PATCH v5 0/3] Support userspace-selected fds Josh Triplett
2020-04-22  5:19 ` [PATCH v5 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Josh Triplett
2020-04-22  6:06   ` Michael Kerrisk (man-pages)
2020-04-23  1:12   ` Dmitry V. Levin
2020-04-23  4:51     ` Josh Triplett [this message]
2020-04-23  9:24   ` Arnd Bergmann
2020-04-22  5:20 ` [PATCH v5 2/3] fs: openat2: Extend open_how to allow userspace-selected fds Josh Triplett
2020-04-22  6:06   ` Michael Kerrisk (man-pages)
2020-04-22  7:55     ` Miklos Szeredi
2020-04-23  0:48       ` Josh Triplett
2020-04-23  4:24         ` Miklos Szeredi
2020-04-23  4:42           ` Josh Triplett
2020-04-23  6:04             ` Miklos Szeredi
2020-04-23  7:33               ` Josh Triplett
2020-04-23  7:45                 ` Miklos Szeredi
2020-04-23  7:57                   ` Miklos Szeredi
2020-04-23  9:20                     ` Miklos Szeredi
2020-04-23  9:46                       ` Miklos Szeredi
2020-04-23  8:06                   ` Josh Triplett
2020-04-22  5:20 ` [PATCH v5 3/3] fs: pipe2: Support O_SPECIFIC_FD Josh Triplett
2020-04-22  6:06   ` Michael Kerrisk (man-pages)
2020-04-22 15:44   ` Florian Weimer
2020-04-23  0:44     ` Josh Triplett
2020-04-22  6:05 ` [PATCH v5 0/3] Support userspace-selected fds Michael Kerrisk (man-pages)

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