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
next prev parent 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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