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	Sat, 30 Aug 2025 20:42:01 +0000
Date: Sat, 30 Aug 2025 21:42:01 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org, jack@suse.cz,
	Siddhesh Poyarekar <siddhesh@gotplt.org>,
	Ian Kent <raven@themaw.net>, David Howells <dhowells@redhat.com>,
	Christian Brauner <brauner@kernel.org>
Subject: Re: [RFC] does # really need to be escaped in devnames?
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Sat, Aug 30, 2025 at 12:40:32PM -0700, Linus Torvalds wrote:
> On Sat, 30 Aug 2025 at 00:33, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > So...  Siddhesh, could you clarify the claim about breaking getmntent(3)?
> > Does it or does it not happen on every system that has readonly AFS
> > volumes mounted?
> 
> Hmm. Looking at various source trees using Debian code search, at
> least dietlibc doesn't treat '#' specially at all.
> 
> And glibc seems to treat only a line that *starts* with a '#'
> (possibly preceded by space/tab combinations) as an empty line.
> 
> klibc checks for '#' at the beginning of the file (without any
> potential space skipping before)
> 
> Busybox seems to do the same "skip whitespace, then skip lines
> starting with '#'" that glibc does.
> 
> So I think the '#'-escaping logic is wrong.  We should only escape '#'
> marks at the beginning of a line (since we already escape spaces and
> tabs, the "preceded by whitespace" doesn't matter).
> 
> And that means that we shouldn't do it in 'mangle()' at all - because
> it's irrelevant for any field but the first.
> 
> And the first field in /proc/mounts is that 'r->mnt_devname' (or
> show_devname), and again, that should only trigger on the first
> character, not every character.

*nod*

Amusingly enough, glibc addmntent(3) does *not* consider # for an
octal escape.

BTW, another amuzing bogosity:

	seq_escape(m, "blah", "X") => "blah"
	seq_escape(m, "blah", "b") => "\142lah"
	seq_escape(m, "blah", "") => "\142\154\141\150"

IOW, about 10 years ago an empty string switched meaning from "escape nothing"
to "escape everything"...