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

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Date: Thu, 18 Apr 2024 22:31:16 +0300
From: Nadav Amit <nadav.amit@...il.com>
To: Mike Rapoport <rppt@...nel.org>
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 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?

Anyhow, sorry for the noise.