linux-kernel - [PATCH v4 1/4] x86/signal/64: Add a comment about sigcontext->fs and gs

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Message-Id: <aa2558ac1f98963f5c73158b17aaeebcfb76f270.1455589976.git.luto@kernel.org>
Date:	Mon, 15 Feb 2016 18:38:13 -0800
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	Denys Vlasenko <dvlasenk@...hat.com>, Stas Sergeev <stsp@...t.ru>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Brian Gerst <brgerst@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v4 1/4] 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>
---
 arch/x86/include/uapi/asm/sigcontext.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index d485232f1e9f..47dae8150520 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -341,6 +341,25 @@ 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.
+	 *
+	 * 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.
+	 *
+	 * There is at least one additional concern if these slots are
+	 * recycled for another purpose: some DOSEMU versions stash fs
+	 * and gs in these slots manually.
+	 */
 	__u16				gs;
 	__u16				fs;
 	__u16				__pad0;
-- 
2.5.0