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Date: Tue, 29 Apr 2014 21:09:33 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Denys Vlasenko <dvlasenk@...hat.com>
Cc: linux-kernel@...r.kernel.org, Jim Keniston <jkenisto@...ibm.com>,
Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3] uprobes: simplify rip-relative handling
This version looks fine to me.
Masami, Jim, Srikar, can you ack the change in riprel_analyze() ?
On 04/28, Denys Vlasenko wrote:
>
> It is possible to replace rip-relative addressing mode
> with addressing mode of the same length: (reg+disp32).
> This eliminates the need to fix up immediate
> and correct for changing instruction length.
>
> v2: Rebased on top of Oleg's latest changes and run-tested.
> v3: Removed unnecessary cast.
> Improved comments based on Oleg's feedback.
>
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Jim Keniston <jkenisto@...ibm.com>
> CC: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> CC: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Oleg Nesterov <oleg@...hat.com>
> ---
> arch/x86/include/asm/uprobes.h | 3 ---
> arch/x86/kernel/uprobes.c | 61 ++++++++++++++++++------------------------
> 2 files changed, 26 insertions(+), 38 deletions(-)
>
> diff --git a/arch/x86/include/asm/uprobes.h b/arch/x86/include/asm/uprobes.h
> index a040d49..7be3c07 100644
> --- a/arch/x86/include/asm/uprobes.h
> +++ b/arch/x86/include/asm/uprobes.h
> @@ -50,9 +50,6 @@ struct arch_uprobe {
> u8 opc1;
> } branch;
> struct {
> -#ifdef CONFIG_X86_64
> - long riprel_target;
> -#endif
> u8 fixups;
> u8 ilen;
> } def;
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index c229b5f..ae6df8e 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -251,9 +251,9 @@ static inline bool is_64bit_mm(struct mm_struct *mm)
> * If arch_uprobe->insn doesn't use rip-relative addressing, return
> * immediately. Otherwise, rewrite the instruction so that it accesses
> * its memory operand indirectly through a scratch register. Set
> - * def->fixups and def->riprel_target accordingly. (The contents of the
> - * scratch register will be saved before we single-step the modified
> - * instruction, and restored afterward).
> + * def->fixups accordingly. (The contents of the scratch register
> + * will be saved before we single-step the modified instruction,
> + * and restored afterward).
> *
> * We do this because a rip-relative instruction can access only a
> * relatively small area (+/- 2 GB from the instruction), and the XOL
> @@ -264,9 +264,12 @@ static inline bool is_64bit_mm(struct mm_struct *mm)
> *
> * Some useful facts about rip-relative instructions:
> *
> - * - There's always a modrm byte.
> + * - There's always a modrm byte with bit layout "00 reg 101".
> * - There's never a SIB byte.
> * - The displacement is always 4 bytes.
> + * - REX.B=1 bit in REX prefix, which normally extends r/m field,
> + * has no effect on rip-relative mode. It doesn't make modrm byte
> + * with r/m=101 refer to register 1101 = R13.
> */
> static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
> {
> @@ -293,9 +296,8 @@ static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
> */
> cursor = auprobe->insn + insn_offset_modrm(insn);
> /*
> - * Convert from rip-relative addressing to indirect addressing
> - * via a scratch register. Change the r/m field from 0x5 (%rip)
> - * to 0x0 (%rax) or 0x1 (%rcx), and squeeze out the offset field.
> + * Convert from rip-relative addressing to register-relative addressing
> + * via a scratch register.
> */
> reg = MODRM_REG(insn);
> if (reg == 0) {
> @@ -307,22 +309,21 @@ static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
> * #1) for the scratch register.
> */
> auprobe->def.fixups |= UPROBE_FIX_RIP_CX;
> - /* Change modrm from 00 000 101 to 00 000 001. */
> - *cursor = 0x1;
> + /*
> + * Change modrm from "00 000 101" to "10 000 001". Example:
> + * 89 05 disp32 mov %eax,disp32(%rip) becomes
> + * 89 81 disp32 mov %eax,disp32(%rcx)
> + */
> + *cursor = 0x81;
> } else {
> /* Use %rax (register #0) for the scratch register. */
> auprobe->def.fixups |= UPROBE_FIX_RIP_AX;
> - /* Change modrm from 00 xxx 101 to 00 xxx 000 */
> - *cursor = (reg << 3);
> - }
> -
> - /* Target address = address of next instruction + (signed) offset */
> - auprobe->def.riprel_target = (long)insn->length + insn->displacement.value;
> -
> - /* Displacement field is gone; slide immediate field (if any) over. */
> - if (insn->immediate.nbytes) {
> - cursor++;
> - memmove(cursor, cursor + insn->displacement.nbytes, insn->immediate.nbytes);
> + /*
> + * Change modrm from "00 reg 101" to "10 reg 000". Example:
> + * 89 1d disp32 mov %edx,disp32(%rip) becomes
> + * 89 98 disp32 mov %edx,disp32(%rax)
> + */
> + *cursor = (reg << 3) | 0x80;
> }
> }
>
> @@ -343,26 +344,17 @@ static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> unsigned long *sr = scratch_reg(auprobe, regs);
>
> utask->autask.saved_scratch_register = *sr;
> - *sr = utask->vaddr + auprobe->def.riprel_target;
> + *sr = utask->vaddr + auprobe->def.ilen;
> }
> }
>
> -static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs,
> - long *correction)
> +static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> if (auprobe->def.fixups & (UPROBE_FIX_RIP_AX | UPROBE_FIX_RIP_CX)) {
> struct uprobe_task *utask = current->utask;
> unsigned long *sr = scratch_reg(auprobe, regs);
>
> *sr = utask->autask.saved_scratch_register;
> - /*
> - * The original instruction includes a displacement, and so
> - * is 4 bytes longer than what we've just single-stepped.
> - * Caller may need to apply other fixups to handle stuff
> - * like "jmpq *...(%rip)" and "callq *...(%rip)".
> - */
> - if (correction)
> - *correction += 4;
> }
> }
> #else /* 32-bit: */
> @@ -379,8 +371,7 @@ static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
> static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> }
> -static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs,
> - long *correction)
> +static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> }
> #endif /* CONFIG_X86_64 */
> @@ -419,7 +410,7 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
> struct uprobe_task *utask = current->utask;
> long correction = (long)(utask->vaddr - utask->xol_vaddr);
>
> - riprel_post_xol(auprobe, regs, &correction);
> + riprel_post_xol(auprobe, regs);
> if (auprobe->def.fixups & UPROBE_FIX_IP) {
> regs->ip += correction;
> } else if (auprobe->def.fixups & UPROBE_FIX_CALL) {
> @@ -436,7 +427,7 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
>
> static void default_abort_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> - riprel_post_xol(auprobe, regs, NULL);
> + riprel_post_xol(auprobe, regs);
> }
>
> static struct uprobe_xol_ops default_xol_ops = {
> --
> 1.8.1.4
>
--
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