linux-kernel - Re: [PATCH v3] modules: Ensure natural alignment for .altinstructions and __bug

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Date:   Mon, 11 Jul 2022 09:07:45 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Helge Deller <deller@....de>
Cc:     jeyu@...nel.org, linux-modules@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH v3] modules: Ensure natural alignment for
 .altinstructions and __bug_table sections

On Fri, Jul 08, 2022 at 11:44:54AM +0200, Helge Deller wrote:
> In the kernel image vmlinux.lds.S linker scripts the .altinstructions
> and __bug_table sections are 4- or 8-byte aligned because they hold 32-
> and/or 64-bit values.
> 
> Most architectures use altinstructions and BUG() or WARN() in modules as
> well, but in the module linker script (module.lds.S) those sections are
> currently missing. As consequence the linker will store their content
> byte-aligned by default, which then can lead to unnecessary unaligned
> memory accesses by the CPU when those tables are processed at runtime.
> 
> Usually unaligned memory accesses are unnoticed, because either the
> hardware (as on x86 CPUs) or in-kernel exception handlers (e.g. on
> parisc or sparc) emulate and fix them up at runtime. Nevertheless, such
> unaligned accesses introduce a performance penalty and can even crash
> the kernel if there is a bug in the unalignment exception handlers
> (which happened once to me on the parisc architecture and which is why I
> noticed that issue at all).
> 
> This patch fixes a non-critical issue and might be backported at any time.
> It's trivial and shouldn't introduce any regression because it simply
> tells the linker to use a different (8-byte alignment) for those
> sections by default.
> 
> Signed-off-by: Helge Deller <deller@....de>
> Link: https://lore.kernel.org/all/Yr8%2Fgr8e8I7tVX4d@p100/

Thanks! Queued onto modules-next.

  Luis