linux-kernel - Re: [PATCH v2] module: Make sure relocations are applied to the per-CPU section

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Message-ID: <20250610145502.pA_kA7GU@linutronix.de>
Date: Tue, 10 Jun 2025 16:55:02 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Petr Pavlu <petr.pavlu@...e.com>
Cc: linux-modules@...r.kernel.org, oe-lkp@...ts.linux.dev, lkp@...el.com,
	linux-kernel@...r.kernel.org,
	kernel test robot <oliver.sang@...el.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Allison Henderson <allison.henderson@...cle.com>,
	netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
	rds-devel@....oracle.com, Luis Chamberlain <mcgrof@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Daniel Gomez <da.gomez@...sung.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] module: Make sure relocations are applied to the
 per-CPU section

On 2025-06-05 18:50:27 [+0200], Petr Pavlu wrote:
> On 6/5/25 5:54 PM, Sebastian Andrzej Siewior wrote:
> > On 2025-06-05 15:44:23 [+0200], Petr Pavlu wrote:
> >> Isn't this broken earlier by "Don't relocate non-allocated regions in modules."
> >> (pre-Git, [1])?
> > 
> > Looking further back into the history, we have
> > 	21af2f0289dea ("[PATCH] per-cpu support inside modules (minimal)")
> > 
> > which does
> > 
> > +       if (pcpuindex) {
> > +               /* We have a special allocation for this section. */
> > +               mod->percpu = percpu_modalloc(sechdrs[pcpuindex].sh_size,
> > +                                             sechdrs[pcpuindex].sh_addralign);
> > +               if (!mod->percpu) {
> > +                       err = -ENOMEM;
> > +                       goto free_mod;
> > +               }
> > +               sechdrs[pcpuindex].sh_flags &= ~(unsigned long)SHF_ALLOC;
> > +       }
> > 
> > so this looks like the origin.
> 
> This patch added the initial per-cpu support for modules. The relocation
> handling at that point appears correct to me. I think it's the mentioned patch
> "Don't relocate non-allocated regions in modules" that broke it.

Ach, it ignores that bit. Okay then.

> It seems logical to me that the SHF_ALLOC flag is removed for the percpu section
> since it isn't directly allocated by the regular process. This is consistent
> with what the module loader does in other similar cases. I could also understand
> keeping the flag and explicitly skipping the layout and allocate process for the
> section. However, adjusting the flag back and forth to trigger the right code
> paths in between seems fragile to me and harder to maintain if we need to
> shuffle things around in the future.

Okay. Let me add this exception later on instead of adding the bit back.

Sebastian