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[PATCH v3] x86: Stop relying on magic jmp behavior for early_idt_handlers
- From: Andy Lutomirski <luto at kernel dot org>
- To: x86 at kernel dot org, "H. Peter Anvin" <hpa at zytor dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Borislav Petkov <bp at alien8 dot de>, Jan Beulich <JBeulich at suse dot com>, Binutils <binutils at sourceware dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, Andy Lutomirski <luto at kernel dot org>
- Date: Fri, 22 May 2015 16:15:47 -0700
- Subject: [PATCH v3] x86: Stop relying on magic jmp behavior for early_idt_handlers
- Authentication-results: sourceware.org; auth=none
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 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_handlers>:
0: 6a 00 pushq $0x0
2: 6a 00 pushq $0x0
4: e9 14 01 00 00 jmpq 11d <early_idt_handler>
...
48: 6a 08 pushq $0x8
4a: e9 d1 00 00 00 jmpq 120 <early_idt_handler>
4f: cc int3
50: cc int3
...
117: 6a 00 pushq $0x0
119: 6a 1f pushq $0x1f
11b: eb 03 jmp 120 <early_idt_handler>
11d: cc int3
11e: cc int3
11f: cc int3
Acked-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
Changes from v2:
- Further improve comments.
- Rename early_idt_handlers to early_idt_handler_array and
early_idt_handler to early_idt_handler_common.
- Combine the .fill directives in the array loop. (I'm not sure why
I missed this simplification the first time around.)
Changes from v1:
- Changed .globl to ENTRY.
- Removed superfluous endif and ifdef
arch/x86/include/asm/segment.h | 14 ++++++++++++--
arch/x86/kernel/head64.c | 2 +-
arch/x86/kernel/head_32.S | 33 ++++++++++++++++++---------------
arch/x86/kernel/head_64.S | 20 +++++++++++---------
4 files changed, 42 insertions(+), 27 deletions(-)
diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 5a9856eb12ba..3a0dcb7b114f 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -231,11 +231,21 @@
#define TLS_SIZE (GDT_ENTRY_TLS_ENTRIES* 8)
#ifdef __KERNEL__
+
+/*
+ * early_idt_handler_array is an array of entry points referenced in the
+ * early IDT. For simplicity, it's a real array with one entry point
+ * every nine bytes. That leaves room for an optional 'push $0' if the
+ * vector has no error code (two bytes), a 'push $vector_number' (two
+ * bytes), and a jump to the common entry code (up to five bytes).
+ */
+#define EARLY_IDT_HANDLER_STRIDE 9
+
#ifndef __ASSEMBLY__
-extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][2+2+5];
+extern const char early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_STRIDE];
#ifdef CONFIG_TRACING
-# define trace_early_idt_handlers early_idt_handlers
+# define trace_early_idt_handler_array early_idt_handler_array
#endif
/*
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 2b55ee6db053..5a4668136e98 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -167,7 +167,7 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
clear_bss();
for (i = 0; i < NUM_EXCEPTION_VECTORS; i++)
- set_intr_gate(i, early_idt_handlers[i]);
+ set_intr_gate(i, early_idt_handler_array[i]);
load_idt((const struct desc_ptr *)&idt_descr);
copy_bootdata(__va(real_mode_data));
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index d031bad9e07e..0c7ca05a8855 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -478,21 +478,22 @@ is486:
__INIT
setup_once:
/*
- * Set up a idt with 256 entries pointing to ignore_int,
- * interrupt gates. It doesn't actually load idt - that needs
- * to be done on each CPU. Interrupts are enabled elsewhere,
- * when we can be relatively sure everything is ok.
+ * Set up a idt with 256 interrupt gates that push zero if there
+ * is no error code and then jump to early_idt_handler_common.
+ * It doesn't actually load the idt - that needs to be done on
+ * each CPU. Interrupts are enabled elsewhere, when we can be
+ * relatively sure everything is ok.
*/
movl $idt_table,%edi
- movl $early_idt_handlers,%eax
+ movl $early_idt_handler_array,%eax
movl $NUM_EXCEPTION_VECTORS,%ecx
1:
movl %eax,(%edi)
movl %eax,4(%edi)
/* interrupt gate, dpl=0, present */
movl $(0x8E000000 + __KERNEL_CS),2(%edi)
- addl $9,%eax
+ addl $EARLY_IDT_HANDLER_STRIDE,%eax
addl $8,%edi
loop 1b
@@ -524,26 +525,28 @@ setup_once:
andl $0,setup_once_ref /* Once is enough, thanks */
ret
-ENTRY(early_idt_handlers)
+ENTRY(early_idt_handler_array)
# 36(%esp) %eflags
# 32(%esp) %cs
# 28(%esp) %eip
# 24(%rsp) error code
i = 0
.rept NUM_EXCEPTION_VECTORS
- .if (EXCEPTION_ERRCODE_MASK >> i) & 1
- ASM_NOP2
- .else
+ .ifeq (EXCEPTION_ERRCODE_MASK >> i) & 1
pushl $0 # Dummy error code, to make stack frame uniform
.endif
pushl $i # 20(%esp) Vector number
- jmp early_idt_handler
+ jmp early_idt_handler_common
i = i + 1
+ .fill early_idt_handler_array + i*EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
.endr
-ENDPROC(early_idt_handlers)
+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,(%esp) # X86_TRAP_NMI
@@ -603,7 +606,7 @@ ex_entry:
is_nmi:
addl $8,%esp /* drop vector number and error code */
iret
-ENDPROC(early_idt_handler)
+ENDPROC(early_idt_handler_common)
/* This is the default interrupt "handler" :-) */
ALIGN
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index ae6588b301c2..b60e7253560d 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_STRIDE - ., 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)
is_nmi:
addq $16,%rsp # drop vector number and error code
INTERRUPT_RETURN
-ENDPROC(early_idt_handler)
+ENDPROC(early_idt_handler_common)
__INITDATA
--
2.3.0