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

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Date: Sun, 30 Jun 2024 21:18:26 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xi Ruoyao <xry111@...111.site>, Arnd Bergmann <arnd@...db.de>
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 Sun, Jun 30, 2024 at 10:40 AM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Sun, 2024-06-30 at 09:40 +0800, Huacai Chen wrote:
> > 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.
>
> 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.]
Yes, but it will not happen immediately.

>
> > 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...
>
> At least you should not restore it for 32-bit.  libLoL also has nothing
> to do with 32-bit systems anyway.  Maybe conditional it with a #if
> checking __BITS_PER_LONG.
Agree, but currently LoongArch only support 64bit, so we don't need
#ifdef now (Many Kconfig options also need to depend on 64bit, but
dependencies are removed when LoongArch get upstream).

>
> And the vendors should really port their software to the upstreamed ABI
> instead of relying on liblol.  <rant>Is a recompiling so difficult, or
> are the programmers so stupid to invoke plenty of low-level syscalls
> directly (bypassing Glibc) in their code?</rant>
Unfortunately, libLoL may exist for a very long time. Recompiling
isn't difficult, the real problem is "I have already ported to
LoongArch, why should I port again?".

Huacai

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