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Date:   Tue, 22 Mar 2022 09:08:22 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Stephen Rothwell <sfr@canb.auug.org.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        rostedt@goodmis.org, ast@kernel.org, hjl.tools@gmail.com,
        rick.p.edgecombe@intel.com, rppt@kernel.org,
        linux-toolchains@vger.kernel.org, Andrew.Cooper3@citrix.com,
        ndesaulniers@google.com
Subject: Re: linux-next: build warnings after merge of the tip tree
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On Tue, Mar 22, 2022 at 02:31:36PM +0900, Masami Hiramatsu wrote:

> > Also, I think both should fix regs->ss.
> 
> I'm not sure this part. Since the return trampoline should run in the same
> context of the called function, isn't ss same there too?

It creates pt_regs on the stack, so the trampolines do:

	push $arch_rethook_trampoline
	push %rsp
	pushf
	sub $24, %rsp /* cs, ip, orig_ax */
	push %rdi
	...
	push %r15

That means that if anybody looks at regs->ss, it'll find
$arch_rethook_trampoline, which isn't a valid segment descriptor, or am
I just really bad at counting today?

I'm thinking you want a copy of __KERNEL_DS in that stack slot, not a
function pointer.