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Commit e54fdcca authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar
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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 9a036b93 ("x86/signal/64: Remove 'fs' and 'gs'
from sigcontext"), which was reverted by ed596cde ("Revert x86
sigcontext cleanups").

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Acked-by: default avatarBorislav 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: default avatarIngo Molnar <mingo@kernel.org>
parent f2cc8e07

arch/x86/include/uapi/asm/sigcontext.h

+25 −0
Original line number Diff line number Diff line
@@ -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;