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

[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:

quoted

On Thu, May 16, 2019 at 05:22:59PM +0100, Al Viro wrote:

quoted

On Thu, May 16, 2019 at 12:52:04PM +0100, David Howells wrote:

quoted

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


-- 
ldv