linux-kernel - Re: [PATCH] vfs: Add AT_EMPTY_PATH_NOCHECK as unchecked AT_EMPTY

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Date: Sat, 22 Jun 2024 15:41:19 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Xi Ruoyao <xry111@...111.site>, Christian Brauner <brauner@...nel.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Alejandro Colomar <alx@...nel.org>, Arnd Bergmann <arnd@...db.de>, Huacai Chen <chenhuacai@...ngson.cn>, 
	Xuerui Wang <kernel@...0n.name>, Jiaxun Yang <jiaxun.yang@...goat.com>, 
	Icenowy Zheng <uwu@...nowy.me>, linux-fsdevel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
	loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: Add AT_EMPTY_PATH_NOCHECK as unchecked AT_EMPTY_PATH

On Sat, 22 Jun 2024 at 14:25, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> +cc Linus

Thanks.

> To sum up the problem: stat and statx met with "" + AT_EMPTY_PATH have
> more work to do than fstat and its hypotethical statx counterpart:
> - buf alloc/free for the path
> - userspace access (very painful on x86_64 + SMAP)
> - lockref acquire/release

Yes. That LOOKUP_EMPTY_NOCHECK is *not* the fix.

I do think that we should make AT_EMPTY_PATH with a NULL path
"JustWork(tm)", because the stupid "look if the pathname is empty" is
horrible.

But moving that check into getname() is *NOT* the right answer,
because by the time you get to getname(), you have already lost.

There's a very real reason why vfs_fstatat() catches this empty case
early, and never goes to filename lookup at all. You don't want to
generate a 'struct path' from the 'int fd', because you want to never
get anywhere close to that path, and instead only ever need a 'struct
fd' that can be looked up much more cheaply (particularly if not in a
threaded environment).

So the short-cut in vfs_fstatat() to never get a pathname is
disgusting - people should have used 'fstat()' - but it's _important_
disgusting.

This thing that tries to short-circuit things at the path level is too late.

              Linus