From: tip-bot for Andy Lutomirski <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, luto@amacapital.net,
oleg@redhat.com, bp@alien8.de, peterz@infradead.org,
stsp@list.ru, gorcunov@gmail.com, hpa@zytor.com, luto@kernel.org,
dvlasenk@redhat.com, brgerst@gmail.com, viro@zeniv.linux.org.uk,
xemul@parallels.com, mingo@kernel.org,
torvalds@linux-foundation.org, tglx@linutronix.de
Subject: [tip:x86/asm] x86/signal/64: Add a comment about sigcontext-> fs and gs
Date: Wed, 17 Feb 2016 04:10:27 -0800 [thread overview]
Message-ID: <tip-e54fdcca70a33a7e447e526b305db85e978c0563@git.kernel.org> (raw)
In-Reply-To: <baa78f3c84106fa5acbc319377b1850602f5deec.1455664054.git.luto@kernel.org>
Commit-ID: e54fdcca70a33a7e447e526b305db85e978c0563
Gitweb: http://git.kernel.org/tip/e54fdcca70a33a7e447e526b305db85e978c0563
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 16 Feb 2016 15:09:01 -0800
Committer: Ingo Molnar <mingo@kernel.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@kernel.org>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stas Sergeev <stsp@list.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/baa78f3c84106fa5acbc319377b1850602f5deec.1455664054.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.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;
next prev parent reply other threads:[~2016-02-17 12:11 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-16 23:09 [PATCH v5 0/4] x86: sigcontext fixes, again Andy Lutomirski
2016-02-16 23:09 ` [PATCH v5 1/4] x86/signal/64: Add a comment about sigcontext->fs and gs Andy Lutomirski
2016-02-17 7:21 ` Ingo Molnar
2016-02-17 10:03 ` Stas Sergeev
2016-02-17 11:05 ` Ingo Molnar
2016-02-17 12:10 ` tip-bot for Andy Lutomirski [this message]
2016-02-16 23:09 ` [PATCH v5 2/4] x86/signal/64: Fix SS if needed when delivering a 64-bit signal Andy Lutomirski
2016-02-17 12:10 ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-02-16 23:09 ` [PATCH v5 3/4] x86/signal/64: Re-add support for SS in the 64-bit signal context Andy Lutomirski
2016-02-17 12:11 ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-02-16 23:09 ` [PATCH v5 4/4] selftests/x86: Add tests for UC_SIGCONTEXT_SS and UC_STRICT_RESTORE_SS Andy Lutomirski
2016-02-17 12:11 ` [tip:x86/asm] " tip-bot for Andy Lutomirski
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