linux-kernel - Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system

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Date:   Mon, 24 Aug 2020 16:52:15 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...nel.org>,
        "Robert O'Callahan" <robert@...llahan.org>
Cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Kyle Huey <me@...ehuey.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 2020-08-24 14:10, Andy Lutomirski wrote:
> 
> PTRACE_READ_SEGMENT_DESCRIPTOR to read a segment descriptor.
> 
> PTRACE_SET_FS / PTRACE_SET_GS: Sets FS or GS and updates the base accordingly.
> 
> PTRACE_READ_SEGMENT_BASE: pass in a segment selector, get a base out.
> You would use this to populate the base fields.
> 
> or perhaps a ptrace SETREGS variant that tries to preserve the old
> base semantics and magically sets the bases to match the selectors if
> the selectors are nonzero.
> 
> Do any of these choices sound preferable to any of you?
> 

My suggestion would be to export the GDT and LDT as a (readonly or mostly
readonly) regset(s) rather than adding entirely new operations. We could allow
the LDT and the per-thread GDT entries to be written, subject to the same
limitations as the corresponding system calls.

	-hpa