Messages in this thread | | | Date | Wed, 22 Jan 2020 14:47:42 +0000 | From | Will Deacon <> | Subject | Re: [RFC v5 00/57] objtool: Add support for arm64 |
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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@kernel.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
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