x86/elf/64: Clear more registers in elf_common_init()

Before we start calling execve in contexts that honor the full
pt_regs, we need to teach it to initialize all registers.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/65a38a9edee61a1158cfd230800c61dbd963dac5.1444091584.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/elf.h

@@ -171,11 +171,11 @@ do {						\
 static inline void elf_common_init(struct thread_struct *t,
 				   struct pt_regs *regs, const u16 ds)
 {
-	/* Commented-out registers are cleared in stub_execve */
-	/*regs->ax = regs->bx =*/ regs->cx = regs->dx = 0;
-	regs->si = regs->di /*= regs->bp*/ = 0;
+	/* ax gets execve's return value. */
+	/*regs->ax = */ regs->bx = regs->cx = regs->dx = 0;
+	regs->si = regs->di = regs->bp = 0;
 	regs->r8 = regs->r9 = regs->r10 = regs->r11 = 0;
-	/*regs->r12 = regs->r13 = regs->r14 = regs->r15 = 0;*/
+	regs->r12 = regs->r13 = regs->r14 = regs->r15 = 0;
 	t->fs = t->gs = 0;
 	t->fsindex = t->gsindex = 0;
 	t->ds = t->es = ds;