linux-kernel - Re: [RFC v5 00/57] objtool: Add support for arm64

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Date:   Tue, 21 Jan 2020 10:31:01 +0000
From:   Will Deacon <will@...nel.org>
To:     Julien Thierry <jthierry@...hat.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        jpoimboe@...hat.com, peterz@...radead.org, raphael.gault@....com,
        catalin.marinas@....com, clang-built-linux@...glegroups.com
Subject: Re: [RFC v5 00/57] objtool: Add support for arm64

On Mon, Jan 13, 2020 at 07:57:48AM +0000, Julien Thierry wrote:
> On 1/12/20 8:42 AM, Nathan Chancellor wrote:
> > The 0day bot reported a couple of issues with clang with this series;
> > the full report is available here (clang reports are only sent to our
> > mailing lists for manual triage for the time being):
> > 
> > https://groups.google.com/d/msg/clang-built-linux/MJbl_xPxawg/mWjgDgZgBwAJ
> > 
> 
> Thanks, I'll have a look at those.
> 
> > The first obvious issue is that this series appears to depend on a GCC
> > plugin? I'll be quite honest, objtool and everything it does is rather
> > over my head but I see this warning during configuration (allyesconfig):
> > 
> > WARNING: unmet direct dependencies detected for GCC_PLUGIN_SWITCH_TABLES
> >    Depends on [n]: GCC_PLUGINS [=n] && ARM64 [=y]
> >      Selected by [y]:
> >        - ARM64 [=y] && STACK_VALIDATION [=y]
> > 
> > Followed by the actual error:
> > 
> > error: unable to load plugin
> > './scripts/gcc-plugins/arm64_switch_table_detection_plugin.so':
> > './scripts/gcc-plugins/arm64_switch_table_detection_plugin.so: cannot
> > open shared object file: No such file or directory'
> > 
> > If this plugin is absolutely necessary and can't be implemented in
> > another way so that clang can be used, seems like STACK_VALIDATION
> > should only be selected on ARM64 when CONFIG_CC_IS_GCC is not zero.
> > 
> 
> So currently the plugin is necessary for proper validation. One option can
> be to just let objtool output false positives on files containing jump
> tables when the plugin cannot be used. But overall I guess it makes more
> sense to disable stack validation for non-gcc builds, for now.

Alternatively, could we add '-fno-jump-tables' to the KBUILD_CFLAGS if
STACK_VALIDATION is selected but we're not using GCC? Is that sufficient
to prevent generation of these things?

Will