From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757634AbbEVXPy (ORCPT ); Fri, 22 May 2015 19:15:54 -0400 Received: from mail.kernel.org ([198.145.29.136]:33599 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756963AbbEVXPw (ORCPT ); Fri, 22 May 2015 19:15:52 -0400 From: Andy Lutomirski To: x86@kernel.org, "H. Peter Anvin" , "H.J. Lu" Cc: Borislav Petkov , Jan Beulich , Binutils , "linux-kernel@vger.kernel.org" , Andy Lutomirski Subject: [PATCH v3] x86: Stop relying on magic jmp behavior for early_idt_handlers Date: Fri, 22 May 2015 16:15:47 -0700 Message-Id: X-Mailer: git-send-email 2.3.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 : 0: 6a 00 pushq $0x0 2: 6a 00 pushq $0x0 4: e9 00 00 00 00 jmpq 9 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 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 11c: R_X86_64_PC32 early_idt_handler-0x4 After: 0000000000000000 : 0: 6a 00 pushq $0x0 2: 6a 00 pushq $0x0 4: e9 14 01 00 00 jmpq 11d ... 48: 6a 08 pushq $0x8 4a: e9 d1 00 00 00 jmpq 120 4f: cc int3 50: cc int3 ... 117: 6a 00 pushq $0x0 119: 6a 1f pushq $0x1f 11b: eb 03 jmp 120 11d: cc int3 11e: cc int3 11f: cc int3 Acked-by: H. Peter Anvin Signed-off-by: Andy Lutomirski --- 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