linux-kernel - Re: [PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY

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Message-ID: <CAAhV-H73GpnD4hTGXDdWYBmo+Hs=088tSaVum69=4UyhZoKtOw@mail.gmail.com>
Date: Wed, 3 Jul 2024 17:35:05 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>, Xi Ruoyao <xry111@...111.site>, Mateusz Guzik <mjguzik@...il.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org, 
	Jens Axboe <axboe@...nel.dk>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	loongarch@...ts.linux.dev
Subject: Re: [PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)

Hi, Christian,

On Wed, Jul 3, 2024 at 4:46 PM Christian Brauner <brauner@...nel.org> wrote:
>
> On Tue, Jul 02, 2024 at 07:06:53PM GMT, Arnd Bergmann wrote:
> > On Tue, Jul 2, 2024, at 17:36, Huacai Chen wrote:
> > > On Mon, Jul 1, 2024 at 7:59 PM Arnd Bergmann <arnd@...db.de> wrote:
> > >> On Sun, Jun 30, 2024, at 04:39, Xi Ruoyao wrote:
> > >> > On Sun, 2024-06-30 at 09:40 +0800, Huacai Chen wrote:
> > >> >> >
> > >> >> > Yes, both Linus and Christian hates introducing a new AT_ flag for
> > >> >> > this.
> > >> >> >
> > >> >> > This patch just makes statx(fd, NULL, AT_EMPTY_PATH, ...) behave
> > >> >> > like
> > >> >> > statx(fd, "", AT_EMPTY_PATH, ...) instead.  NULL avoids the
> > >> >> > performance
> > >> >> > issue and it's also audit-able by seccomp BPF.
> > >> >> To be honest, I still want to restore __ARCH_WANT_NEW_STAT. Because
> > >> >> even if statx() becomes audit-able, it is still blacklisted now.
> > >> >
> > >> > Then patch the sandbox to allow it.
> > >> >
> > >> > The sandbox **must** be patched anyway or it'll be broken on all 32-bit
> > >> > systems after 2037.  [Unless they'll unsupport all 32-bit systems before
> > >> > 2037.]
> > >>
> > >> More importantly, the sandbox won't be able to support any 32-bit
> > >> targets that support running after 2037, regardless of how long
> > >> the sandbox supports them: if you turn off COMPAT_32BIT_TIME today
> > >> in order to be sure those don't get called by accident, the
> > >> fallback is immediately broken.
> > > Would you mind if I restore newstat for LoongArch64 even if this patch exist?
> >
> > I still prefer not add newstat back: it's easier to
> > get applications to correctly implement the statx() code
> > path if there are more architectures that only have that.
>
> I agree.
>
> We've now added AT_EMPTY_PATH support with NULL names because we want to
> allow that generically. But I clearly remember that this was requested
> to make statx() work with these sandboxes. So the kernel has done its
> part. Now it's for the sandbox to allow statx() with NULL paths and
> AT_EMPTY_PATH but certainly not for the kernel to start reenabling old
> system calls.
Linux distributions don't use latest applications, so they still need
an out-of-tree kernel patch to restore newstat. Of course they can
also patch their applications, but patching the kernel is
significantly easier.

So in my opinion LoongArch has completed its task to drive statx()
improvement, now restoring newstat is a double-insurance for
compatibility.

Huacai