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Date: Tue, 25 Aug 2020 09:32:08 -0700 From: Kyle Huey <me@...ehuey.com> To: Andy Lutomirski <luto@...capital.net> Cc: Andy Lutomirski <luto@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, "Robert O'Callahan" <robert@...llahan.org>, "Bae, Chang Seok" <chang.seok.bae@...el.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, Andi Kleen <ak@...ux.intel.com>, "Shankar, Ravi V" <ravi.v.shankar@...el.com>, LKML <linux-kernel@...r.kernel.org>, "Hansen, Dave" <dave.hansen@...el.com> Subject: Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <luto@...capital.net> wrote: > I don’t like this at all. Your behavior really shouldn’t depend on > whether the new instructions are available. Also, some day I would > like to change Linux to have the new behavior even if FSGSBASE > instructions are not available, and this will break rr again. (The > current !FSGSBASE behavior is an ugly optimization of dubious value. > I would not go so far as to describe it as correct.) Ok. > I would suggest you do one of the following things: > > 1. Use int $0x80 directly to load 32-bit regs into a child. This > might dramatically simplify your code and should just do the right > thing. I don't know what that means. > 2. Something like your patch but make it unconditional. > > 3. Ask for, and receive, real kernel support for setting FS and GS in > the way that 32-bit code expects. I think the easiest way forward for us would be a PTRACE_GET/SETREGSET like operation that operates on the regsets according to the *tracee*'s bitness (rather than the tracer, as it works currently). Does that sound workable? - Kyle
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