linux-kernel - Re: [PATCH] man/man2/setns.2: clarify type of nsfs fd required

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Message-ID: <viwd2c53bnfyflny7sdmumawimwjy2mcmoigk5shhkmiabtbm5@3vcv3a3664cc>
Date: Sat, 13 Dec 2025 20:37:37 +0100
From: Alejandro Colomar <alx@...nel.org>
To: Alyssa Ross <hi@...ssa.is>
Cc: Christian Brauner <brauner@...nel.org>, 
	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org, linux-man@...r.kernel.org
Subject: Re: [PATCH] man/man2/setns.2: clarify type of nsfs fd required

Hi Alyssa, Christian, Al,

On Sat, Dec 13, 2025 at 07:59:04PM +0100, Alyssa Ross wrote:
> Alejandro Colomar <alx@...nel.org> writes:
> 
> > Hi Alyssa,
> >
> > On Sat, Dec 13, 2025 at 06:58:53PM +0100, Alyssa Ross wrote:
> >> I was surprised to discover than an O_PATH file descriptor was
> >> insufficient.
> >
> > How did you discover it?  Could you please link to relevant information
> > (or kernel sources)?
> 
> By trying it!
> 
> Presumably it's the fd_empty() check at the beginning of the syscall
> implementation in nsproxy.c.

Hmm, thanks!  I don't see any documentation about this, neither in the
kernel Documentation/, nor in the commit messages that introduced this
code.  Christian, Al, would you mind checking if this is intended?  If
so, it would be useful to document why O_PATH is not accepted.  Is it
a security problem?

> >> Since the mode of nsfs files is always 0444, tell
> >> callers to always a file descriptor opened for reading.
> >
> > Missing 'use'?
> 
> Yes.  Feel free to add it.

Thanks!


Cheers,
Alex

-- 
<https://www.alejandro-colomar.es>

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