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>

diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index d485232f1e9f9a1b0f0ba4d44bf8382c39175757..702c404688598684d21889614c8c2ac7f9a47433 100644 (file)
--- 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;