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Subject: Re: Chromium sandbox on LoongArch and statx -- seccomp deep
 argument inspection again?
From: Xi Ruoyao <xry111@xry111.site>
To: WANG Xuerui <kernel@xen0n.name>, linux-api@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>, Christian Brauner <brauner@kernel.org>, 
 Kees Cook <keescook@chromium.org>, Huacai Chen <chenhuacai@kernel.org>,
 Xuefeng Li <lixuefeng@loongson.cn>, Jianmin Lv <lvjianmin@loongson.cn>,
 Xiaotian Wu <wuxiaotian@loongson.cn>, WANG Rui <wangrui@loongson.cn>, Miao
 Wang <shankerwangmiao@gmail.com>, Icenowy Zheng <uwu@icenowy.me>, 
 "loongarch@lists.linux.dev" <loongarch@lists.linux.dev>, linux-arch
 <linux-arch@vger.kernel.org>, Linux Kernel Mailing List
 <linux-kernel@vger.kernel.org>
Date: Wed, 21 Feb 2024 20:03:16 +0800
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On Wed, 2024-02-21 at 18:49 +0800, WANG Xuerui wrote:
>=20
> On 2/21/24 18:31, Xi Ruoyao wrote:
> > On Wed, 2024-02-21 at 14:31 +0800, Xi Ruoyao wrote:
> > > On Wed, 2024-02-21 at 14:09 +0800, WANG Xuerui wrote:
> > >=20
> > > > - just restore fstat and be done with it;
> > > > - add a flag to statx so we can do the equivalent of just fstat(fd,
> > > > &out) with statx, and ensuring an error happens if path is not empt=
y in
> > > > that case;
> > > It's worse than "just restore fstat" considering the performance.=C2=
=A0 Read
> > > this thread:
> > > https://sourceware.org/pipermail/libc-alpha/2023-September/151320.htm=
l
> > Hmm, but it looks like statx already suffers the same performance issue=
.
> > And in this libc-alpha discussion Linus said:
> >=20
> > =C2=A0=C2=A0=C2=A0 If the user asked for 'fstat()', just give the user =
'fstat()'.
> > =C2=A0=C2=A0=C2=A0=20
> > So to me we should just add fstat (and use it in Glibc for LoongArch, o=
nly
> > falling back to statx if fstat returns -ENOSYS), or am I missing someth=
ing?
>=20
> Or we could add a AT_STATX_NULL_PATH flag and mandate that `path` must
> be NULL if this flag is present -- then with simple checks we could have=
=20
> statx(fd, NULL, AT_STATX_NULL_PATH, STATX_BASIC_STATS, &out) that's both=
=20
> fstat-like and fast.

But then to take the advantage in Glibc fstat() implementation, we'll
need to try AT_STATX_NULL_PATH first, then check for EFAULT (not
ENOSYS!) and fall back to AT_EMPTY_PATH.  The EFAULT checking seems
nasty to me...

--=20
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University