linux-kernel - Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble memory as ROX

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Message-ID: <20230605081143.GA3460@kernel.org>
Date:   Mon, 5 Jun 2023 11:11:43 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "kent.overstreet@...ux.dev" <kent.overstreet@...ux.dev>,
        Thomas Gleixner <tglx@...utronix.de>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "deller@....de" <deller@....de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
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        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "hca@...ux.ibm.com" <hca@...ux.ibm.com>,
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        Will Deacon <will@...nel.org>,
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        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble
 memory as ROX

On Sun, Jun 04, 2023 at 10:52:44PM -0400, Steven Rostedt wrote:
> On Thu, 1 Jun 2023 16:54:36 -0700
> Nadav Amit <nadav.amit@...il.com> wrote:
> 
> > > The way text_poke() is used here, it is creating a new writable alias
> > > and flushing it for *each* write to the module (like for each write of
> > > an individual relocation, etc). I was just thinking it might warrant
> > > some batching or something.  

> > I am not advocating to do so, but if you want to have many efficient
> > writes, perhaps you can just disable CR0.WP. Just saying that if you
> > are about to write all over the memory, text_poke() does not provide
> > too much security for the poking thread.

Heh, this is definitely and easier hack to implement :)

> Batching does exist, which is what the text_poke_queue() thing does.

For module loading text_poke_queue() will still be much slower than a bunch
of memset()s for no good reason because we don't need all the complexity of
text_poke_bp_batch() for module initialization because we are sure we are
not patching live code.

What we'd need here is a new batching mode that will create a writable
alias mapping at the beginning of apply_relocate_*() and module_finalize(),
then it will use memcpy() to that writable alias and will tear the mapping
down in the end.

Another option is to teach alternatives to update a writable copy rather
than do in place changes like Song suggested. My feeling is that it will be
more intrusive change though.

> -- Steve
> 

-- 
Sincerely yours,
Mike.