linux-kernel - Re: [PATCH v9 07/13] um: nommu: configure fs register on host syscall invocation

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Message-ID: <2231a777eef3cdb339fe98af4618f958ed658df6.camel@sipsolutions.net>
Date: Thu, 19 Jun 2025 14:38:47 +0200
From: Benjamin Berg <benjamin@...solutions.net>
To: Hajime Tazaki <thehajime@...il.com>
Cc: linux-um@...ts.infradead.org, ricarkol@...gle.com,
 Liam.Howlett@...cle.com, 	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 07/13] um: nommu: configure fs register on host
 syscall invocation

On Thu, 2025-06-19 at 21:22 +0900, Hajime Tazaki wrote:
> 
> On Thu, 19 Jun 2025 19:40:49 +0900,
> Benjamin Berg wrote:
> > 
> > On Thu, 2025-06-19 at 10:04 +0900, Hajime Tazaki wrote:
> > > As userspace on UML/!MMU also need to configure %fs register when
> > > it is
> > > running to correctly access thread structure, host syscalls
> > > implemented
> > > in os-Linux drivers may be puzzled when they are called.  Thus it
> > > has to
> > > configure %fs register via arch_prctl(SET_FS) on every host
> > > syscalls.
> > 
> > Really, I still think that we should "just" get rid of libc
> > entirely
> > inside UML. That would avoid so many weird/potential issues …
> 
> I'm not sure if I understand your point.
> 
> Q1) what do you mean by 'get rid of libc entirely' here ?
>     do you mean the following code block adds the dependency ?
> + int os_arch_prctl(int pid, int option, unsigned long *arg2)
> + {
> + 	return syscall(SYS_arch_prctl, option, arg2);
> + }
> 
> I guess this can be replaced with inline assembly instead of using
> libc's one.  but this is the code under os-Linux, which I thought
> we're
> allowed to use the host code ?
> 
> Q2) "That would avoid so many weird/potential issues …"
> I'm new to this;  I'm wondering what kind of issues did you see ?

Oh, I am just being annoyed by libc in general in UM. It isn't specific
to this patchset.

An example is that we need to keep malloc() working for libc. Which I
would think is kind of weird. Or we had issues because libc turned on
rseq and that was inherited into userspace, causing random crashes and
such.

> > Doesn't change the fact that FS/GS needs to be restored when doing
> > thread switches and such. Though one might be able to do it
> > entirely
> > within arch_switch_to then.
> 
> I believe this is already done in arch_switch_to.  This particular
> patch does the control to the host context.

OK, need to look at that again a bit. I haven't really wrapped my mind
around how everything fits together, so I probably got some stuff
wrong.

Benjamin