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

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Date: Sun, 30 Jun 2024 09:40:08 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xi Ruoyao <xry111@...111.site>
Cc: Mateusz Guzik <mjguzik@...il.com>, brauner@...nel.org, viro@...iv.linux.org.uk, 
	jack@...e.cz, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	io-uring@...r.kernel.org, axboe@...nel.dk, torvalds@...ux-foundation.org, 
	loongarch@...ts.linux.dev
Subject: Re: [PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)

On Tue, Jun 25, 2024 at 11:00 PM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Tue, 2024-06-25 at 22:09 +0800, Huacai Chen wrote:
> > On Tue, Jun 25, 2024 at 7:01 PM Mateusz Guzik <mjguzik@...il.com>
> > wrote:
> > >
> > > The newly used helper also checks for 0-sized buffers.
> > >
> > > This avoids path lookup code, lockref management, memory allocation
> > > and
> > > in case of NULL path userspace memory access (which can be quite
> > > expensive with SMAP on x86_64).
> > >
> > > statx with AT_EMPTY_PATH paired with "" or NULL argument as
> > > appropriate
> > > issued on Sapphire Rapids (ops/s):
> > > stock:     4231237
> > > 0-check:   5944063 (+40%)
> > > NULL path: 6601619 (+11%/+56%)
> > >
> > > Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> > Hi, Ruoyao,
> >
> > I'm a bit confused. Ii this patch a replacement of your recent patch?
>
> 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.
Restoring __ARCH_WANT_NEW_STAT is a very small change that doesn't
introduce any complexity, but it makes life easier. And I think libLoL
also likes __ARCH_WANT_NEW_STAT, though it isn't an upstream
project...

Huacai

>
> --
> Xi Ruoyao <xry111@...111.site>
> School of Aerospace Science and Technology, Xidian University
>