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

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Date: Wed, 21 Feb 2024 20:03:16 +0800
From: Xi Ruoyao <xry111@...111.site>
To: WANG Xuerui <kernel@...0n.name>, linux-api@...r.kernel.org
Cc: Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>, 
 Kees Cook <keescook@...omium.org>, Huacai Chen <chenhuacai@...nel.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>, Icenowy Zheng <uwu@...nowy.me>, 
 "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 Wed, 2024-02-21 at 18:49 +0800, WANG Xuerui wrote:
> 
> 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:
> > > 
> > > > - 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 empty in
> > > > that case;
> > > It's worse than "just restore fstat" considering the performance.  Read
> > > this thread:
> > > https://sourceware.org/pipermail/libc-alpha/2023-September/151320.html
> > Hmm, but it looks like statx already suffers the same performance issue.
> > And in this libc-alpha discussion Linus said:
> > 
> >     If the user asked for 'fstat()', just give the user 'fstat()'.
> >     
> > So to me we should just add fstat (and use it in Glibc for LoongArch, only
> > falling back to statx if fstat returns -ENOSYS), or am I missing something?
> 
> 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 
> statx(fd, NULL, AT_STATX_NULL_PATH, STATX_BASIC_STATS, &out) that's both 
> 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...

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