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

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

This borrows from 9a036b93a3 ("x86/signal/64: Remove 'fs' and 'gs'
from sigcontext"), which was reverted by ed596cde94 ("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>
This commit is contained in:
Andy Lutomirski 2016-02-16 15:09:01 -08:00 committed by Ingo Molnar
parent f2cc8e0791
commit e54fdcca70
1 changed files with 25 additions and 0 deletions
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25
arch/x86/include/uapi/asm/sigcontext.h
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@ -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;