linux-kernel - Re: [PATCH v2 00/11] Start porting UML to nolibc

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Message-ID: <4354d88c2ff7a57a7324cc39b4ce5ed4ebe5277d.camel@sipsolutions.net>
Date: Mon, 22 Sep 2025 09:41:36 +0200
From: Johannes Berg <johannes@...solutions.net>
To: Christoph Hellwig <hch@...radead.org>, Benjamin Berg
	 <benjamin@...solutions.net>
Cc: linux-um@...ts.infradead.org, Willy Tarreau <w@....eu>, Thomas
 Weißschuh	 <linux@...ssschuh.net>,
 linux-kselftest@...r.kernel.org, Arnaldo Carvalho de Melo
 <acme@...hat.com>, linux-kernel@...r.kernel.org, Benjamin Berg
 <benjamin.berg@...el.com>
Subject: Re: [PATCH v2 00/11] Start porting UML to nolibc

On Fri, 2025-09-19 at 08:40 -0700, Christoph Hellwig wrote:
> On Fri, Sep 19, 2025 at 05:34:09PM +0200, Benjamin Berg wrote:
> > From: Benjamin Berg <benjamin.berg@...el.com>
> > 
> > This patchset is an attempt to start a nolibc port of UML.
> 
> It would be useful to explain why that is desirable.

Agree, it should be here, but FWIW it's been discussed elsewhere on the
linux-um list in the past and basically there are various issues around
it. Off the top of my head:
 - glibc enabling new features such as rseq that interact badly with how
   UML manages memory (there were fixes for this, it worked sometimes
   and sometimes not)
 - allocation placement for TLS is problematic (see the SMP series)
 - it's (too) easy to accidentally call glibc functions that require
   huge amounts of stack space

There are probably other reasons, but the mixed nature of UML being both
kernel and "hypervisor" code in a single place doesn't mix well with
glibc.

johannes