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Subject: Re: Chromium sandbox on LoongArch and statx -- seccomp deep
 argument inspection again?
From: Xi Ruoyao <xry111@xry111.site>
To: Arnd Bergmann <arnd@arndb.de>, Icenowy Zheng <uwu@icenowy.me>, Huacai
 Chen <chenhuacai@kernel.org>, WANG Xuerui <kernel@xen0n.name>
Cc: linux-api@vger.kernel.org, Christian Brauner <brauner@kernel.org>, Kees
 Cook <keescook@chromium.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>, 
 "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: Mon, 26 Feb 2024 15:09:29 +0800
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On Mon, 2024-02-26 at 07:56 +0100, Arnd Bergmann wrote:
> On Mon, Feb 26, 2024, at 07:03, Icenowy Zheng wrote:
> > =E5=9C=A8 2024-02-25=E6=98=9F=E6=9C=9F=E6=97=A5=E7=9A=84 15:32 +0800=EF=
=BC=8CXi Ruoyao=E5=86=99=E9=81=93=EF=BC=9A
> > > On Sun, 2024-02-25 at 14:51 +0800, Icenowy Zheng wrote:
> > > > My idea is this problem needs syscalls to be designed with deep
> > > > argument inspection in mind; syscalls before this should be
> > > > considered
> > > > as historical error and get fixed by resotring old syscalls.
> > >=20
> > > I'd not consider fstat an error as using statx for fstat has a
> > > performance impact (severe for some workflows), and Linus has
> > > concluded
> >=20
> > Sorry for clearance, I mean statx is an error in ABI design, not fstat.

I'm wondering why we decided to use AT_EMPTY_PATH/"" instead of
"AT_NULL_PATH"/nullptr in the first place?

> The same has been said about seccomp(). ;-)
>=20
> It's clear that the two don't go well together at the moment.
>=20
> > > "if the user wants fstat, give them fstat" for the performance issue:
> > >=20
> > > https://sourceware.org/pipermail/libc-alpha/2023-September/151365.htm=
l
> > >=20
> > > However we only want fstat (actually "newfstat" in fs/stat.c), and it
> > > seems we don't want to resurrect newstat, newlstat, newfstatat, etc.
> > > (or
> > > am I missing any benefit - performance or "just pleasing seccomp" -
> > > of them comparing to statx?) so we don't want to just define
> > > __ARCH_WANT_NEW_STAT.=C2=A0 So it seems we need to add some new #if t=
o
> > > fs/stat.c and include/uapi/asm-generic/unistd.h.
> > >=20
> > > And no, it's not a design issue of all other syscalls.=C2=A0 It's jus=
t the
> > > design issue of seccomp.=C2=A0 There's no way to design a syscall all=
owing
> > > seccomp to inspect a 100-character path in its argument unless
> > > refactoring seccomp entirely because we cannot fit a 100-character
> > > path
> > > into 8 registers.
> >=20
> > Well my meaning is that syscalls should be designed to be simple to
> > prevent this kind of circumstance.

But it's not irrational to pass a path to syscall, as long as we still
have the concept of file system (maybe in 2371 or some year we'll use a
128-bit UUID instead of path).

> The problem I see with the 'use use fstat' approach is that this
> does not work on 32-bit architectures, unless we define a new
> fstatat64_time64() syscall, which is one of the things that statx()

"fstat64_time64".  Using statx for fstatat should be just fine.

Or maybe we can just introduce a new AT_something to make statx
completely ignore pathname but behave like AT_EMPTY_PATH + "".

> was trying to avoid.

Oops.  I thought "newstat" should be using 64-bit time but it seems the
"new" is not what I'd expected...  The "new" actually means "newer than
Linux 0.9"! :(

Let's not use "new" in future syscall names...

> Whichever solution we end up with should work on both
> loongarch64 and on armv7 at least.
>=20
> =C2=A0=C2=A0=C2=A0 Arnd

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