linux-kernel - Re: [PATCH 2/2] module: add support to avoid duplicates early on load

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Message-ID: <CAHk-=whu8Wh4JP1hrc80ZvGgVW4GV6hw1vwzSiwOo9-1=Y1dWw@mail.gmail.com>
Date:   Thu, 25 May 2023 14:12:49 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Lucas De Marchi <lucas.demarchi@...el.com>
Cc:     Petr Pavlu <petr.pavlu@...e.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        gregkh@...uxfoundation.org, rafael@...nel.org, song@...nel.org,
        lucas.de.marchi@...il.com, christophe.leroy@...roup.eu,
        peterz@...radead.org, rppt@...nel.org, dave@...olabs.net,
        willy@...radead.org, vbabka@...e.cz, mhocko@...e.com,
        dave.hansen@...ux.intel.com, colin.i.king@...il.com,
        jim.cromie@...il.com, catalin.marinas@....com, jbaron@...mai.com,
        rick.p.edgecombe@...el.com, yujie.liu@...el.com, david@...hat.com,
        tglx@...utronix.de, hch@....de, patches@...ts.linux.dev,
        linux-modules@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pmladek@...e.com, prarit@...hat.com,
        lennart@...ttering.net
Subject: Re: [PATCH 2/2] module: add support to avoid duplicates early on load

On Thu, May 25, 2023 at 11:45 AM Lucas De Marchi
<lucas.demarchi@...el.com> wrote:
>
> Are you willig to merge (a possibly improved version of) your patch
> or the userspace change is still something that would be desired?

I think a user space change should still be something that people
should look at, particularly as the kernel side patch I'm willing to
accept doesn't catch the "completely serial" cases, only the "trying
to load at the same time that the same module is literally busy being
loaded".

But I've cleaned up my patch a bit, and while the cleaned-up version
is rather larger as a patch (mainly because of just also re-organizing
the finit_module() code to do all the 'struct file' prep), I'm
actually pretty happy with this attached patch conceptually.

In this form, it actually "makes sense" to me, rather than being just
clearly a workaround.  Also, unlike the previous patch, this doesn't
actually make any changes to the basic kernel_read_file() set of
functions, it's all done by the module loading code itself.

Luis, would you mind testing this version on your load? It still won't
actually handle the purely serial case, so there *will* be those
spurious double module reads from different CPU's just doing the
things serially, but the exclusive file access region has been
extended to not just cover the actual file content reading, but to
cover the whole "turn it into a a real module" part too.

Also, this does *not* update some of the comments in the module
loading. I changed finit_module to use "kernel_read_file()" instead of
"kernel_read_file_from_fd()", since it actually now has to look up the
file descriptor anyway. But the comments still talk about that
"from_fd" thing.

Anyway, this is back to "ENTIRELY UNTESTED" territory, in that I've
compiled this, but haven't booted it. The changes look obvious, but
hey, mistakes happen.

And the commit message is just a place-holder. Obviously. I won't sign
off on this or write more of a commit message until it has had some
real testing.

                  Linus

View attachment "0001-Attempt-at-workaround-for-horrible-udev-module-loadi.patch" of type "text/x-patch" (3708 bytes)