linux-kernel - Re: Chromium sandbox on LoongArch and statx -- seccomp deep argument inspection again?

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Message-Id: <3c396b7c-adec-4762-9584-5824f310bf7b@app.fastmail.com>
Date: Mon, 26 Feb 2024 07:56:38 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Icenowy Zheng" <uwu@...nowy.me>, "Xi Ruoyao" <xry111@...111.site>,
 "Huacai Chen" <chenhuacai@...nel.org>, "WANG Xuerui" <kernel@...0n.name>
Cc: linux-api@...r.kernel.org, "Christian Brauner" <brauner@...nel.org>,
 "Kees Cook" <keescook@...omium.org>, "Xuefeng Li" <lixuefeng@...ngson.cn>,
 "Jianmin Lv" <lvjianmin@...ngson.cn>, "Xiaotian Wu" <wuxiaotian@...ngson.cn>,
 "WANG Rui" <wangrui@...ngson.cn>, "Miao Wang" <shankerwangmiao@...il.com>,
 "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
 Linux-Arch <linux-arch@...r.kernel.org>,
 "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: Chromium sandbox on LoongArch and statx -- seccomp deep argument
 inspection again?

On Mon, Feb 26, 2024, at 07:03, Icenowy Zheng wrote:
> 在 2024-02-25星期日的 15:32 +0800，Xi Ruoyao写道：
>> 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.
>> 
>> I'd not consider fstat an error as using statx for fstat has a
>> performance impact (severe for some workflows), and Linus has
>> concluded
>
> Sorry for clearance, I mean statx is an error in ABI design, not fstat.

The same has been said about seccomp(). ;-)

It's clear that the two don't go well together at the moment.

>> "if the user wants fstat, give them fstat" for the performance issue:
>> 
>> https://sourceware.org/pipermail/libc-alpha/2023-September/151365.html
>> 
>> 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.  So it seems we need to add some new #if to
>> fs/stat.c and include/uapi/asm-generic/unistd.h.
>> 
>> And no, it's not a design issue of all other syscalls.  It's just the
>> design issue of seccomp.  There's no way to design a syscall allowing
>> 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.
>
> Well my meaning is that syscalls should be designed to be simple to
> prevent this kind of circumstance.

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()
was trying to avoid.

Whichever solution we end up with should work on both
loongarch64 and on armv7 at least.

    Arnd