linux-kernel - Re: [PATCH 0/4] uapi, vfs: Change the mount API UAPI [ver #2]

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Message-ID: <D41D33CA-ADFC-4E79-9C9C-79FE19E068CA@brauner.io>
Date:   Fri, 17 May 2019 08:54:52 +0200
From:   Christian Brauner <christian@...uner.io>
To:     "Dmitry V. Levin" <ldv@...linux.org>,
        Al Viro <viro@...iv.linux.org.uk>
CC:     David Howells <dhowells@...hat.com>, torvalds@...ux-foundation.org,
        Arnd Bergmann <arnd@...db.de>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH 0/4] uapi, vfs: Change the mount API UAPI [ver #2]

On May 16, 2019 10:23:31 PM GMT+02:00, "Dmitry V. Levin" <ldv@...linux.org> wrote:
>[looks like linux-abi is a typo, Cc'ed linux-api instead]
>
>On Thu, May 16, 2019 at 05:50:22PM +0100, Al Viro wrote:
>> [linux-abi cc'd]
>> 
>> On Thu, May 16, 2019 at 06:31:52PM +0200, Christian Brauner wrote:
>> > On Thu, May 16, 2019 at 05:22:59PM +0100, Al Viro wrote:
>> > > On Thu, May 16, 2019 at 12:52:04PM +0100, David Howells wrote:
>> > > > 
>> > > > Hi Linus, Al,
>> > > > 
>> > > > Here are some patches that make changes to the mount API UAPI
>and two of
>> > > > them really need applying, before -rc1 - if they're going to be
>applied at
>> > > > all.
>> > > 
>> > > I'm fine with 2--4, but I'm not convinced that cloexec-by-default
>crusade
>> > > makes any sense.  Could somebody give coherent arguments in
>favour of
>> > > abandoning the existing conventions?
>> > 
>> > So as I said in the commit message. From a userspace perspective
>it's
>> > more of an issue if one accidently leaks an fd to a task during
>exec.
>> > 
>> > Also, most of the time one does not want to inherit an fd during an
>> > exec. It is a hazzle to always have to specify an extra flag.
>> > 
>> > As Al pointed out to me open() semantics are not going anywhere.
>Sure,
>> > no argument there at all.
>> > But the idea of making fds cloexec by default is only targeted at
>fds
>> > that come from separate syscalls. fsopen(), open_tree_clone(), etc.
>they
>> > all return fds independent of open() so it's really easy to have
>them
>> > cloexec by default without regressing anyone and we also remove the
>need
>> > for a bunch of separate flags for each syscall to turn them into
>> > cloexec-fds. I mean, those for syscalls came with 4 separate flags
>to be
>> > able to specify that the returned fd should be made cloexec. The
>other
>> > way around, cloexec by default, fcntl() to remove the cloexec bit
>is way
>> > saner imho.
>> 
>> Re separate flags - it is, in principle, a valid argument.  OTOH, I'm
>not
>> sure if they need to be separate - they all have the same value and
>> I don't see any reason for that to change...
>> 
>> Only tangentially related, but I wonder if something like
>close_range(from, to)
>> would be a more useful approach...  That kind of open-coded loops is
>not
>> rare in userland and kernel-side code can do them much cheaper. 
>Something
>> like
>> 	/* that exec is sensitive */
>> 	unshare(CLONE_FILES);
>> 	/* we don't want anything past stderr here */
>> 	close_range(3, ~0U);
>> 	execve(....);
>> on the userland side of thing.  Comments?
>
>glibc people need a syscall to implement closefrom properly, see
>https://sourceware.org/bugzilla/show_bug.cgi?id=10353#c14

I have a prototype for close_range().
I'll send it out after rc1.

Christian