x86/asm/32: Make pt_regs's segment registers be 16 bits

Many 32-bit x86 CPUs do 16-bit writes when storing segment registers to
memory.  This can cause the high word of regs->[cdefgs]s to
occasionally contain garbage.

Rather than making the entry code more complicated to fix up the
garbage, just change pt_regs to reflect reality.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/ptrace.h

@@ -9,23 +9,43 @@
 #ifdef __i386__
 
 struct pt_regs {
+	/*
+	 * NB: 32-bit x86 CPUs are inconsistent as what happens in the
+	 * following cases (where %seg represents a segment register):
+	 *
+	 * - pushl %seg: some do a 16-bit write and leave the high
+	 *   bits alone
+	 * - movl %seg, [mem]: some do a 16-bit write despite the movl
+	 * - IDT entry: some (e.g. 486) will leave the high bits of CS
+	 *   and (if applicable) SS undefined.
+	 *
+	 * Fortunately, x86-32 doesn't read the high bits on POP or IRET,
+	 * so we can just treat all of the segment registers as 16-bit
+	 * values.
+	 */
 	unsigned long bx;
 	unsigned long cx;
 	unsigned long dx;
 	unsigned long si;
 	unsigned long di;
 	unsigned long bp;
 	unsigned long ax;
-	unsigned long ds;
-	unsigned long es;
-	unsigned long fs;
-	unsigned long gs;
+	unsigned short ds;
+	unsigned short __dsh;
+	unsigned short es;
+	unsigned short __esh;
+	unsigned short fs;
+	unsigned short __fsh;
+	unsigned short gs;
+	unsigned short __gsh;
 	unsigned long orig_ax;
 	unsigned long ip;
-	unsigned long cs;
+	unsigned short cs;
+	unsigned short __csh;
 	unsigned long flags;
 	unsigned long sp;
-	unsigned long ss;
+	unsigned short ss;
+	unsigned short __ssh;
 };
 
 #else /* __i386__ */