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

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Date: Sun, 23 Jun 2024 03:07:00 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Xi Ruoyao <xry111@...111.site>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 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 Sun, Jun 23, 2024 at 2:59 AM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Sat, 2024-06-22 at 15:41 -0700, Linus Torvalds wrote:
>
> > 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.
>
> Oops.  I'll try to get around of getname() too...
>
> > So the short-cut in vfs_fstatat() to never get a pathname is
> > disgusting - people should have used 'fstat()' - but it's _important_
> > disgusting.
>
> The problem is we don't have fstat() for LoongArch, and it'll be
> unusable on all 32-bit arch after 2037.
>
> And Arnd hates the idea adding fstat() for LoongArch because there would
> be one more 32-bit arch broken in 2037.
>
> Or should we just add something like "fstat_2037()"?
>

In that case fstat is out of the question, but no problem.

It was suggested to make AT_EMPTY_PATH + NULL pathname do the right
thing and have the syscalls short-circuit as needed.

for statx it would look like this (except you are going to have
implement do_statx_by_fd):

diff --git a/fs/stat.c b/fs/stat.c
index 16aa1f5ceec4..0afe72b320cc 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -710,6 +710,9 @@ SYSCALL_DEFINE5(statx,
        int ret;
        struct filename *name;

+       if (flags == AT_EMPTY_PATH && filename == NULL)
+               return do_statx_by_fd(...);
+
        name = getname_flags(filename, getname_statx_lookup_flags(flags));
        ret = do_statx(dfd, name, flags, mask, buffer);
        putname(name);

and so on

Personally I would prefer if fstatx was added instead, fwiw most
massaging in the area will be the same regardless.

-- 
Mateusz Guzik <mjguzik gmail.com>