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From: "NeilBrown" <neilb@suse.de>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: "Andy Lutomirski" <luto@kernel.org>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"David Laight" <David.Laight@aculab.com>,
	"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>
Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE
Date: Fri, 20 Aug 2021 13:44:51 +1000	[thread overview]
Message-ID: <162943109106.9892.7426782042253067338@noble.neil.brown.name> (raw)
In-Reply-To: <20210818154217.GB24115@fieldses.org>

On Thu, 19 Aug 2021, J. Bruce Fields wrote:
> On Fri, Aug 13, 2021 at 05:49:19PM -0700, Andy Lutomirski wrote:
> > I’ll bite.  How about we attack this in the opposite direction: remove
> > the deny write mechanism entirely.
> 
> For what it's worth, Windows has open flags that allow denying read or
> write opens.  They also made their way into the NFSv4 protocol, but
> knfsd enforces them only against other NFSv4 clients.  Last I checked,
> Samba attempted to emulate them using flock (and there's a comment to
> that effect on the flock syscall in fs/locks.c).  I don't know what Wine
> does.
> 
> Pavel Shilovsky posted flags adding O_DENY* flags years ago:
> 
> 	https://lwn.net/Articles/581005/
> 
> I keep thinking I should look back at those some day but will probably
> never get to it.

O_DENYREAD is an insane flag.  If a process reads a file that some other
process is working on, then the only which could be hurt is the reader.
So allowing a process to ask for the open to fail if someone is writing
might make sense.  Insisting that all opens fail does not.
Any code wanting O_DENYREAD *should* use advisory locking, and any code
wanting to know about read denial should too.

O_DENYWRITE can make sense.  When combined with a O_RDONLY open it is
effectively what happens when you exec a program.  You can no longer
open that file for write - you get ETXTBSY.

It would be nice to be able to combine O_DENYWRITE with O_RDWR.  This
combination is exactly what the kernel *should* do for swap files.
Unfortunately it doesn't.  The "i_writecount" field that is used to
trigger ETXTBSY for executables doesn't work for the single-writer
model.

I'm not sure about O_DENYDELETE.  It is a lock on the name.  Unix has
traditionally used lock-files to lock a name.  The functionality makes
sense for processes with write-access to the directory...

NeilBrown

  parent reply	other threads:[~2021-08-20  3:45 UTC|newest]

Thread overview: 86+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-12  8:43 [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE David Hildenbrand
2021-08-12  8:43 ` [PATCH v1 1/7] binfmt: don't use MAP_DENYWRITE when loading shared libraries via uselib() David Hildenbrand
2021-08-12  8:43 ` [PATCH v1 2/7] kernel/fork: factor out atomcially replacing the current MM exe_file David Hildenbrand
2021-08-12  9:17   ` Christian Brauner
2021-08-12  8:43 ` [PATCH v1 3/7] kernel/fork: always deny write access to " David Hildenbrand
2021-08-12 10:05   ` Christian Brauner
2021-08-12 10:13     ` David Hildenbrand
2021-08-12 12:32       ` Christian Brauner
2021-08-12 12:38         ` David Hildenbrand
2021-08-12 16:51   ` Linus Torvalds
2021-08-12 19:38     ` David Hildenbrand
2021-08-12  8:43 ` [PATCH v1 4/7] binfmt: remove in-tree usage of MAP_DENYWRITE David Hildenbrand
2021-08-12  8:43 ` [PATCH v1 5/7] mm: remove VM_DENYWRITE David Hildenbrand
2021-08-12  8:43 ` [PATCH v1 6/7] mm: ignore MAP_DENYWRITE in ksys_mmap_pgoff() David Hildenbrand
2021-08-12  8:43 ` [PATCH v1 7/7] fs: update documentation of get_write_access() and friends David Hildenbrand
2021-08-12 12:20 ` [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE Florian Weimer
2021-08-12 12:47   ` David Hildenbrand
2021-08-12 16:17   ` Eric W. Biederman
2021-08-12 17:32 ` Eric W. Biederman
2021-08-12 17:35   ` Andy Lutomirski
2021-08-12 17:48     ` Eric W. Biederman
2021-08-12 18:01       ` Andy Lutomirski
2021-08-12 18:10       ` Linus Torvalds
2021-08-12 18:47         ` Eric W. Biederman
2021-08-13  9:05           ` David Laight
2021-08-13 20:17             ` Eric W. Biederman
2021-08-13 20:51               ` Florian Weimer
2021-08-14  0:31               ` Linus Torvalds
2021-08-14  0:49                 ` Andy Lutomirski
2021-08-14  0:54                   ` Linus Torvalds
2021-08-14  0:58                     ` Linus Torvalds
2021-08-14  1:57                       ` Al Viro
2021-08-14  2:02                         ` Al Viro
2021-08-14  9:06                           ` David Hildenbrand
2021-08-14  7:53                         ` Christian Brauner
2021-08-14 19:52                     ` David Laight
2021-08-26 17:48                     ` Andy Lutomirski
2021-08-26 21:47                       ` David Hildenbrand
2021-08-26 22:13                         ` Eric W. Biederman
2021-08-27  8:22                           ` David Laight
2021-08-27 15:58                             ` Eric W. Biederman
2021-09-01  8:28                           ` David Hildenbrand
2021-08-27 10:18                         ` Christian Brauner
2021-08-14  3:04                   ` Matthew Wilcox
2021-08-17 16:48                     ` Removing Mandatory Locks Eric W. Biederman
2021-08-17 16:50                       ` David Hildenbrand
2021-08-18  9:34                       ` Rodrigo Campos
2021-08-19 19:18                         ` Jeff Layton
2021-08-19 20:03                           ` Willy Tarreau
2021-08-19 18:39                       ` Jeff Layton
2021-08-19 19:15                         ` Linus Torvalds
2021-08-19 19:55                           ` Eric Biggers
2021-08-19 20:18                           ` Jeff Layton
2021-08-19 20:31                             ` Linus Torvalds
2021-08-19 21:43                               ` Jeff Layton
2021-08-19 22:32                                 ` Linus Torvalds
2021-08-20  8:30                                   ` David Laight
2021-08-23  7:55                                     ` Geert Uytterhoeven
2021-08-23  8:14                                       ` David Laight
2021-08-20 13:43                                   ` Steven Rostedt
2021-08-20 16:06                                     ` Linus Torvalds
2021-08-20  2:10                               ` Matthew Wilcox
2021-08-20  6:36                               ` Amir Goldstein
2021-08-20  7:14                                 ` Amir Goldstein
2021-08-20 12:27                                   ` Jeff Layton
2021-08-20 12:38                                     ` Willy Tarreau
2021-08-20 13:03                                       ` Jeff Layton
2021-08-20 13:11                                         ` Willy Tarreau
2021-08-20 16:30                           ` Kees Cook
2021-08-20 19:17                             ` H. Peter Anvin
2021-08-20 21:29                               ` Jeff Layton
2021-08-20 21:29                                 ` Jeff Layton
2021-08-21 12:45                                 ` Jeff Layton
2021-08-21 12:45                                   ` Jeff Layton
2021-08-23 22:15                                   ` J. Bruce Fields
2021-08-20 22:31                               ` Matthew Wilcox
2021-08-18  7:51                     ` [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE Christian Brauner
2021-08-18 15:42                   ` J. Bruce Fields
2021-08-19 13:56                     ` Eric W. Biederman
2021-08-19 14:33                       ` J. Bruce Fields
2021-08-20 12:54                         ` Jeff Layton
2021-08-20  3:44                     ` NeilBrown [this message]
2021-08-20  8:25                       ` David Laight
2021-08-12 19:24         ` David Hildenbrand
2021-08-12 18:15       ` Florian Weimer
2021-08-12 18:21         ` Linus Torvalds

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