Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   David Laight <David.Laight@ACULAB.COM>
To:     "'Eric W. Biederman'" <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     Andy Lutomirski <luto@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        "Namhyung Kim" <namhyung@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        "Sergey Senozhatsky" <sergey.senozhatsky@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Kees Cook <keescook@chromium.org>,
        Greg Ungerer <gerg@linux-m68k.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        "Mike Rapoport" <rppt@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Vincenzo Frascino" <vincenzo.frascino@arm.com>,
        Chinwen Chang <chinwen.chang@mediatek.com>,
        Michel Lespinasse <walken@google.com>,
        "Catalin Marinas" <catalin.marinas@arm.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Huang Ying <ying.huang@intel.com>,
        Jann Horn <jannh@google.com>, Feng Tang <feng.tang@intel.com>,
        Kevin Brodsky <Kevin.Brodsky@arm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Shawn Anastasio" <shawn@anastas.io>,
        Steven Price <steven.price@arm.com>,
        "Nicholas Piggin" <npiggin@gmail.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jens Axboe <axboe@kernel.dk>,
        "Gabriel Krisman Bertazi" <krisman@collabora.com>,
        Peter Xu <peterx@redhat.com>,
        "Suren Baghdasaryan" <surenb@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        "Marco Elver" <elver@google.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Nicolas Viennot <Nicolas.Viennot@twosigma.com>,
        Thomas Cedeno <thomascedeno@google.com>,
        Collin Fijalkovich <cfijalkovich@google.com>,
        Michal Hocko <mhocko@suse.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Chengguang Xu <cgxu519@mykernel.net>,
        =?iso-8859-1?Q?Christian_K=F6nig?= 
        <ckoenig.leichtzumerken@gmail.com>,
        "linux-unionfs@vger.kernel.org" <linux-unionfs@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: RE: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE
Thread-Topic: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE
Thread-Index: AQHXj6qO2FP1eIrarUmjR9CxLZPkE6txIhrA
Date:   Fri, 13 Aug 2021 09:05:43 +0000
Message-ID: <5b0d7c1e73ca43ef9ce6665fec6c4d7e@AcuMS.aculab.com>
References: <20210812084348.6521-1-david@redhat.com> <87o8a2d0wf.fsf@disp2133>
        <60db2e61-6b00-44fa-b718-e4361fcc238c@www.fastmail.com>
        <87lf56bllc.fsf@disp2133>
        <CAHk-=wgru1UAm3kAKSOdnbewPXQMOxYkq9PnAsRadAC6pXCCMQ@mail.gmail.com>
 <87eeay8pqx.fsf@disp2133>
In-Reply-To: <87eeay8pqx.fsf@disp2133>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

From: Eric W. Biederman
> Sent: 12 August 2021 19:47
...
> So today the best advice I can give to userspace is to mark their
> executables and shared libraries as read-only and immutable.  Otherwise
> a change to the executable file can change what is mapped into memory.
> MAP_PRIVATE does not help.

While 'immutable' might be ok for files installed by distributions
it would be a PITA in development.

ETXTBUSY is a useful reminder that the file you are copying from
machine A to machine B (etc) is still running and probably ought
to be killed/stopped before you get confused.

I've never really understood why it doesn't stop shared libraries
being overwritten - but they do tend to be updated less often.

Overwriting an in-use shared library could be really confusing.
It is likely that all the code is actually in memory.
So everything carries on running as normal.
Until the kernel gets under memory pressure and discards a page.
Then a page from the new version is faulted in and random
programs start getting SEGVs.
This could be days after the borked update.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)