x86/asm/irq: Stop relying on magic JMP behavior for early_idt_handlers

The early_idt_handlers asm code generates an array of entry
points spaced nine bytes apart.  It's not really clear from that
code or from the places that reference it what's going on, and
the code only works in the first place because GAS never
generates two-byte JMP instructions when jumping to global
labels.

Clean up the code to generate the correct array stride (member size)
explicitly. This should be considerably more robust against
screw-ups, as GAS will warn if a .fill directive has a negative
count.  Using '. =' to advance would have been even more robust
(it would generate an actual error if it tried to move
backwards), but it would pad with nulls, confusing anyone who
tries to disassemble the code.  The new scheme should be much
clearer to future readers.

While we're at it, improve the comments and rename the array and
common code.

Binutils may start relaxing jumps to non-weak labels.  If so,
this change will fix our build, and we may need to backport this
change.

Before, on x86_64:

  0000000000000000 <early_idt_handlers>:
     0:   6a 00                   pushq  $0x0
     2:   6a 00                   pushq  $0x0
     4:   e9 00 00 00 00          jmpq   9 <early_idt_handlers+0x9>
                          5: R_X86_64_PC32        early_idt_handler-0x4
  ...
    48:   66 90                   xchg   %ax,%ax
    4a:   6a 08                   pushq  $0x8
    4c:   e9 00 00 00 00          jmpq   51 <early_idt_handlers+0x51>
                          4d: R_X86_64_PC32       early_idt_handler-0x4
  ...
   117:   6a 00                   pushq  $0x0
   119:   6a 1f                   pushq  $0x1f
   11b:   e9 00 00 00 00          jmpq   120 <early_idt_handler>
                          11c: R_X86_64_PC32      early_idt_handler-0x4

After:

  0000000000000000 <early_idt_handler_array>:
     0:   6a 00                   pushq  $0x0
     2:   6a 00                   pushq  $0x0
     4:   e9 14 01 00 00          jmpq   11d <early_idt_handler_common>
  ...
    48:   6a 08                   pushq  $0x8
    4a:   e9 d1 00 00 00          jmpq   120 <early_idt_handler_common>
    4f:   cc                      int3
    50:   cc                      int3
  ...
   117:   6a 00                   pushq  $0x0
   119:   6a 1f                   pushq  $0x1f
   11b:   eb 03                   jmp    120 <early_idt_handler_common>
   11d:   cc                      int3
   11e:   cc                      int3
   11f:   cc                      int3

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Binutils <binutils@sourceware.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H.J. Lu <hjl.tools@gmail.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/ac027962af343b0c599cbfcf50b945ad2ef3d7a8.1432336324.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 11 insertions, 9 deletions

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 43eafc8afb69..e5c27f729a38 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -321,26 +321,28 @@ bad_address:
 	jmp bad_address
 
 	__INIT
-	.globl early_idt_handlers
-early_idt_handlers:
+ENTRY(early_idt_handler_array)
 	# 104(%rsp) %rflags
 	#  96(%rsp) %cs
 	#  88(%rsp) %rip
 	#  80(%rsp) error code
 	i = 0
 	.rept NUM_EXCEPTION_VECTORS
-	.if (EXCEPTION_ERRCODE_MASK >> i) & 1
-	ASM_NOP2
-	.else
+	.ifeq (EXCEPTION_ERRCODE_MASK >> i) & 1
 	pushq $0		# Dummy error code, to make stack frame uniform
 	.endif
 	pushq $i		# 72(%rsp) Vector number
-	jmp early_idt_handler
+	jmp early_idt_handler_common
 	i = i + 1
+	.fill early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
 	.endr
+ENDPROC(early_idt_handler_array)
 
-/* This is global to keep gas from relaxing the jumps */
-ENTRY(early_idt_handler)
+early_idt_handler_common:
+	/*
+	 * The stack is the hardware frame, an error code or zero, and the
+	 * vector number.
+	 */
 	cld
 
 	cmpl $2,(%rsp)		# X86_TRAP_NMI
@@ -412,7 +414,7 @@ ENTRY(early_idt_handler)
 .Lis_nmi:
 	addq $16,%rsp		# drop vector number and error code
 	INTERRUPT_RETURN
-ENDPROC(early_idt_handler)
+ENDPROC(early_idt_handler_common)
 
 	__INITDATA