Retpoline Support Backport Lands In GCC 7
Written by Michael Larabel in GNU on 16 January 2018 at 07:35 AM EST. Add A Comment
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The backporting of -mindirect-branch, -mindirect-return and -mindirect-branch-register, a.k.a. the GCC "Retpoline" patches, have been back-ported and merged into the GCC 7 branch.

Given the severity of the Spectre vulnerability, these features for Retpoline support are being back-ported to GCC branches normally only reserved for bug/regression/documentation fixes.

A few days ago the support was added to GCC 8 in time for the GCC 8.1 stable release due out in the next few months.

Hitting the GCC 7 branch this morning are the back-ported patches. Thus with the GCC 7.3 "bug-fix" release coming in the near future there will be these new GCC options that can be used for converting indirect call and jumps to call and return thunks. The Linux kernel Retpoline patches make use of the new GCC switches when available for full support.

There's been talk of these functions being back-ported all the way to GCC 4.x releases, but as of writing only the back-port has happened for GCC 7.x with no work merged in the GCC 4/5/6 branches, but if that happens we'll of course update. I currently have some GCC 8.0.1 benchmarks coming looking at the impact of -mindirect-branch=thunk for user-space applications rather than just the kernel Retpoline benchmarks.

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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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