linux-kernel - Re: [PATCH] rseq/selftests: ensure the rseq abi TLS is actually 1024 bytes

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Message-ID: <78518446-083c-4db1-8c96-61fd49eddd8e@sirena.org.uk>
Date: Tue, 18 Mar 2025 14:01:47 +0000
From: Mark Brown <broonie@...nel.org>
To: Michael Jeanson <mjeanson@...icios.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Shuah Khan <shuah@...nel.org>,
	linux-kselftest@...r.kernel.org, Aishwarya.TCV@....com,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] rseq/selftests: ensure the rseq abi TLS is actually 1024
 bytes

On Tue, Mar 11, 2025 at 03:21:45PM -0400, Michael Jeanson wrote:

> Adding the aligned(1024) attribute to the definition of __rseq_abi did
> not increase its size to 1024, for this attribute to impact the size of
> __rseq_abi it would need to be added to the declaration of 'struct
> rseq_abi'. We only want to increase the size of the TLS allocation to
> ensure registration will succeed with future extended ABI. Use a union
> with a dummy member to ensure we allocate 1024 bytes.

This is in today's -next and breaks the build of the KVM selftests:

In file included from rseq_test.c:24:
/home/broonie/git/bisect/usr/include/linux/rseq.h:62:1: error: use of 'rseq' with tag type that does not match previous declaration
   62 | struct rseq {
      | ^
./../rseq/rseq.c:78:7: note: previous use is here
   78 | union rseq {
      |       ^
In file included from rseq_test.c:24:
/home/broonie/git/bisect/usr/include/linux/rseq.h:62:8: error: redefinition of 'rseq'
   62 | struct rseq {
      |        ^
./../rseq/rseq.c:78:7: note: previous definition is here
   78 | union rseq {
      |       ^

since unlike the rseq tests the KVM rseq test includes the UAPI header
for rseq which the padded union conflicts with.

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