Re: [PATCH v2 11/20] objtool: Detect code coverage instrumentation in noinstr code

[Peter Zijlstra]

On Thu, Nov 12, 2026 at 08:09:06AM -0500, Sasha Levin wrote:
> On Wed, Nov 11, 2026 at 10:23:41PM -0800, Josh Poimboeuf wrote:
> >What exactly is the problem with noinstr code incrementing counters
> >inline?
> 
> The counter is a memory write, and a memory write can trap. In noinstr
> code that can happen before we're ready to take another exception.
> 
> With GCOV_PROFILE_ALL today, exc_debug_kernel() bumps a counter before
> it clears DR7. Set a watchpoint on that counter and the next kernel #DB
> never returns, it just keeps re-entering the handler. I tried it on
> v7.4-rc1 in a VM and it hangs.
> 
> noinstr is already supposed to keep these out, it has had __no_profile
> for years. But that doesn't cover what gets inlined into it, and nothing
> checks. This patch adds the check.

Right, strictly speaking this is a toolchain bug, but since we need to
build with existing tools we need to bodge it somewhat :/