linux-kernel - Re: [RFC PATCH] fs: elide the smp_rmb fence in fd

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAGudoHH3HFDgu61S4VW2H2DXj1GMJzFRstTWhDx=jjHcb-ArwQ@mail.gmail.com>
Date: Thu, 5 Dec 2024 15:43:41 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: paulmck@...nel.org, brauner@...nel.org, jack@...e.cz, 
	linux-fsdevel@...r.kernel.org, torvalds@...ux-foundation.org, 
	edumazet@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: elide the smp_rmb fence in fd_install()

On Thu, Dec 5, 2024 at 3:18 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Thu, Dec 05, 2024 at 01:03:32PM +0100, Mateusz Guzik wrote:
> >  void fd_install(unsigned int fd, struct file *file)
> >  {
> > -     struct files_struct *files = current->files;
> > +     struct files_struct *files;
> >       struct fdtable *fdt;
> >
> >       if (WARN_ON_ONCE(unlikely(file->f_mode & FMODE_BACKING)))
> >               return;
> >
> > +     /*
> > +      * Synchronized with expand_fdtable(), see that routine for an
> > +      * explanation.
> > +      */
> >       rcu_read_lock_sched();
> > +     files = READ_ONCE(current->files);
>
> What are you trying to do with that READ_ONCE()?  current->files
> itself is *not* changed by any of that code; current->files->fdtab is.

To my understanding this is the idiomatic way of spelling out the
non-existent in Linux smp_consume_load, for the resize_in_progress
flag.

Anyway to elaborate I'm gunning for a setup where the code is
semantically equivalent to having a lock around the work.
Pretend ->resize_lock exists, then:
fd_install:
files = current->files;
read_lock(files->resize_lock);
fdt = rcu_dereference_sched(files->fdt);
rcu_assign_pointer(fdt->fd[fd], file);
read_unlock(files->resize_lock);

expand_fdtable:
write_lock(files->resize_lock);
[snip]
rcu_assign_pointer(files->fdt, new_fdt);
write_unlock(files->resize_lock);

Except rcu_read_lock_sched + appropriately fenced resize_in_progress +
synchronize_rcu do it.

-- 
Mateusz Guzik <mjguzik gmail.com>