Re: [RFC v5 00/57] objtool: Add support for arm64
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2020-01-21 17:08:42
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On Tue, Jan 21, 2020 at 2:31 AM Will Deacon [off-list ref] wrote:
On Mon, Jan 13, 2020 at 07:57:48AM +0000, Julien Thierry wrote:quoted
On 1/12/20 8:42 AM, Nathan Chancellor wrote:quoted
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/mWjgDgZgBwAJThanks, I'll have a look at those.quoted
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. -- Thanks, ~Nick Desaulniers _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel