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

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Message-ID: <20200122144741.GA16622@willie-the-truck>
Date:   Wed, 22 Jan 2020 14:47:42 +0000
From:   Will Deacon <will@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Julien Thierry <jthierry@...hat.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Peter Zijlstra <peterz@...radead.org>, raphael.gault@....com,
        Catalin Marinas <catalin.marinas@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [RFC v5 00/57] objtool: Add support for arm64

On Tue, Jan 21, 2020 at 12:30:09PM -0600, Josh Poimboeuf wrote:
> On Tue, Jan 21, 2020 at 06:06:34PM +0000, Will Deacon wrote:
> > On Tue, Jan 21, 2020 at 09:08:29AM -0800, Nick Desaulniers wrote:
> > > On Tue, Jan 21, 2020 at 2:31 AM Will Deacon <will@...nel.org> wrote:
> > > >
> > > > 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?
> > > 
> > > Surely we wouldn't want to replace jump tables with long chains of
> > > comparisons just because objtool couldn't validate jump tables without
> > > a GCC plugin for aarch64 for some reason, right?  objtool validation
> > > is valuable, but tying runtime performance to a GCC plugin used for
> > > validation seems bad.
> > 
> > I'm only suggesting it if STACK_VALIDATION is selected. It's off by default,
> > and lives in Kconfig.debug. I'd prefer that to "cross your fingers are do
> > nothing differently", which is what the other option seems to be.
> 
> I don't know what the right answer is here, but keep in mind that
> objtool is on by default for x86, so don't be surprised if that
> eventually happens to arch64 too.
> 
> Short term it might be ok to disable jump tables with objtool enabled,
> or to disable objtool when clang is in use, but long term we'll need to
> figure out a better solution.

Oh, absolutely. No objection from me fixing this properly in the long
term. I just don't want to be in a situation where STACK_VALIDATION is
silently ignored in the meantime.

Will