linux-kernel - [tip:x86/asm] x86/signal/64: Add a comment about sigcontext-> fs and gs

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Message-ID: <tip-e54fdcca70a33a7e447e526b305db85e978c0563@git.kernel.org>
Date:	Wed, 17 Feb 2016 04:10:27 -0800
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, luto@...capital.net, oleg@...hat.com,
	bp@...en8.de, peterz@...radead.org, stsp@...t.ru,
	gorcunov@...il.com, hpa@...or.com, luto@...nel.org,
	dvlasenk@...hat.com, brgerst@...il.com, viro@...iv.linux.org.uk,
	xemul@...allels.com, mingo@...nel.org,
	torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: [tip:x86/asm] x86/signal/64: Add a comment about sigcontext->
 fs and gs

Commit-ID:  e54fdcca70a33a7e447e526b305db85e978c0563
Gitweb:     http://git.kernel.org/tip/e54fdcca70a33a7e447e526b305db85e978c0563
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Tue, 16 Feb 2016 15:09:01 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 17 Feb 2016 08:32:11 +0100

x86/signal/64: Add a comment about sigcontext->fs and gs

These fields have a strange history.  This tries to document it.

This borrows from 9a036b93a344 ("x86/signal/64: Remove 'fs' and 'gs'
from sigcontext"), which was reverted by ed596cde9425 ("Revert x86
sigcontext cleanups").

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Acked-by: Borislav Petkov <bp@...en8.de>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Cyrill Gorcunov <gorcunov@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Pavel Emelyanov <xemul@...allels.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stas Sergeev <stsp@...t.ru>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/baa78f3c84106fa5acbc319377b1850602f5deec.1455664054.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/include/uapi/asm/sigcontext.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index d485232..702c404 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -341,6 +341,31 @@ struct sigcontext {
 	__u64				rip;
 	__u64				eflags;		/* RFLAGS */
 	__u16				cs;
+
+	/*
+	 * Prior to 2.5.64 ("[PATCH] x86-64 updates for 2.5.64-bk3"),
+	 * Linux saved and restored fs and gs in these slots.  This
+	 * was counterproductive, as fsbase and gsbase were never
+	 * saved, so arch_prctl was presumably unreliable.
+	 *
+	 * These slots should never be reused without extreme caution:
+	 *
+	 *  - Some DOSEMU versions stash fs and gs in these slots manually,
+	 *    thus overwriting anything the kernel expects to be preserved
+	 *    in these slots.
+	 *
+	 *  - If these slots are ever needed for any other purpose,
+	 *    there is some risk that very old 64-bit binaries could get
+	 *    confused.  I doubt that many such binaries still work,
+	 *    though, since the same patch in 2.5.64 also removed the
+	 *    64-bit set_thread_area syscall, so it appears that there
+	 *    is no TLS API beyond modify_ldt that works in both pre-
+	 *    and post-2.5.64 kernels.
+	 *
+	 * If the kernel ever adds explicit fs, gs, fsbase, and gsbase
+	 * save/restore, it will most likely need to be opt-in and use
+	 * different context slots.
+	 */
 	__u16				gs;
 	__u16				fs;
 	__u16				__pad0;