linux-kernel - Re: [PATCH v10 08/15] x86/vsyscall: Reorganize the page fault emulation code

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Message-ID: <009a98b0999891d59e30b8d818e60adb35958fc7.camel@intel.com>
Date: Wed, 8 Oct 2025 17:42:49 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Mehta, Sohil" <sohil.mehta@...el.com>, "tglx@...utronix.de"
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Subject: Re: [PATCH v10 08/15] x86/vsyscall: Reorganize the page fault
 emulation code

On Tue, 2025-10-07 at 15:52 -0700, Sohil Mehta wrote:
> > 
> > If indeed we should add a check, it should probably go in one of the later
> > patches and not this one.
> 
> We already check CS before calling emulate_vsyscall_gp().
> 
> if (user_mode(regs)) {
> 
> ...
> 	if (emulate_vsyscall_gp(regs))
> 		goto exit;
> 
> ...
> }

Ah, right, I missed that. But in the new code, the way to get there is by taking
a GP with an RIP in the vsyscall range. Does this seem a bit stale though?

	/*
	 * No point in checking CS -- the only way to get here is a user mode
	 * trap to a high address, which means that we're in 64-bit user code.
	 */

For one, "No point in checking CS", while true kind of implies that CS wasn't
already checked. The second half I guess is still true if you call the fetch #GP
a trap, and actually maybe more accurate for LASS then it was for the older
paradigm with the "high address" verbiage.

I'm fine either way.