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

[Christian Brauner]

On May 16, 2019 6:50:22 PM GMT+02:00, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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?

Very much in favor of that!
That'd be a neat new addition.