x86/signal/64: Remove 'fs' and 'gs' from sigcontext

As far as I can tell, these fields have been set to zero on save
and ignored on restore since Linux was imported into git.
Rename them '__pad1' and '__pad2' to avoid confusion.  This may
also allow us to recycle them some day.

This also adds a comment clarifying the history of those fields.

I'm intentionally avoiding calling either of them '__pad0': the
field formerly known as '__pad0' is now 'ss'.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/844f8490e938780c03355be4c9b69eb4c494bf4e.1426193719.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/sigcontext.h

@@ -57,8 +57,8 @@ struct sigcontext {
 	unsigned long ip;
 	unsigned long flags;
 	unsigned short cs;
-	unsigned short gs;
-	unsigned short fs;
+	unsigned short __pad2;	/* Was called gs, but was always zero. */
+	unsigned short __pad1;	/* Was called fs, but was always zero. */
 	unsigned short ss;
 	unsigned long err;
 	unsigned long trapno;

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

@@ -177,8 +177,23 @@ struct sigcontext {
 	__u64 rip;
 	__u64 eflags;		/* RFLAGS */
 	__u16 cs;
-	__u16 gs;
-	__u16 fs;
+
+	/*
+	 * 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 that works in both pre- and post-2.5.64 kernels.
+	 */
+	__u16 __pad2;		/* Was gs. */
+	__u16 __pad1;		/* Was fs. */
+
 	__u16 ss;
 	__u64 err;
 	__u64 trapno;

arch/x86/kernel/signal.c

@@ -155,8 +155,8 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
 #else /* !CONFIG_X86_32 */
 		put_user_ex(regs->flags, &sc->flags);
 		put_user_ex(regs->cs, &sc->cs);
-		put_user_ex(0, &sc->gs);
-		put_user_ex(0, &sc->fs);
+		put_user_ex(0, &sc->__pad2);
+		put_user_ex(0, &sc->__pad1);
 		put_user_ex(regs->ss, &sc->ss);
 #endif /* CONFIG_X86_32 */