linux-kernel - Re: [RFC PATCH 3/7] module: [

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Date: Thu, 18 Apr 2024 22:44:31 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Nadav Amit <nadav.amit@...il.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Christoph Hellwig <hch@...radead.org>, Helge Deller <deller@....de>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Palmer Dabbelt <palmer@...belt.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <linux@...linux.org.uk>, Song Liu <song@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uladzislau Rezki <urezki@...il.com>, Will Deacon <will@...nel.org>,
	bpf <bpf@...r.kernel.org>, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	linux-modules@...r.kernel.org, linux-parisc@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-trace-kernel@...r.kernel.org,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC PATCH 3/7] module: [

On Thu, Apr 18, 2024 at 10:31:16PM +0300, Nadav Amit wrote:
> 
> > On 18 Apr 2024, at 13:20, Mike Rapoport <rppt@...nel.org> wrote:
> > 
> > On Tue, Apr 16, 2024 at 12:36:08PM +0300, Nadav Amit wrote:
> >> 
> >> 
> >> 
> >> I might be missing something, but it seems a bit racy.
> >> 
> >> IIUC, module_finalize() calls alternatives_smp_module_add(). At this
> >> point, since you don’t hold the text_mutex, some might do text_poke(),
> >> e.g., by enabling/disabling static-key, and the update would be
> >> overwritten. No?
> > 
> > Right :(
> > Even worse, for UP case alternatives_smp_unlock() will "patch" still empty
> > area.
> > 
> > So I'm thinking about calling alternatives_smp_module_add() from an
> > additional callback after the execmem_update_copy().
> > 
> > Does it make sense to you?
> 
> Going over the code again - I might have just been wrong: I confused the
> alternatives and the jump-label mechanisms (as they do share a lot of
> code and characteristics).
> 
> The jump-labels are updated when prepare_coming_module() is called, which
> happens after post_relocation() [which means they would be updated using
> text_poke() “inefficiently” but should be safe].
> 
> The “alternatives” appear only to use text_poke() (in contrast for
> text_poke_early()) from very specific few flows, e.g., 
> common_cpu_up() -> alternatives_enable_smp().
> 
> Are those flows pose a problem after boot?

Yes, common_cpu_up is called  on CPU hotplug, so it's possible to have a
race between alternatives_smp_module_add() and common_cpu_up() ->
alternatives_enable_smp().

And in UP case alternatives_smp_module_add() will call
alternatives_smp_unlock() that will patch module text before it is updated.

> Anyhow, sorry for the noise.

On the contrary, I would have missed it.

-- 
Sincerely yours,
Mike.