[PATCHv4] uprobes: simplify rip-relative handling
From: Denys Vlasenko
Date: Thu May 01 2014 - 10:53:06 EST
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.
v4: Changed arch_uprobe_xol_was_trapped() comment to reflect new logic.
Rebased to Oleg's uprobes/core,
Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
CC: Jim Keniston <jkenisto@xxxxxxxxxx>
CC: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
CC: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
---
arch/x86/include/asm/uprobes.h | 3 --
arch/x86/kernel/uprobes.c | 72 ++++++++++++++++++------------------------
2 files changed, 31 insertions(+), 44 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 2ebadb2..c41bb4b 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 */
@@ -417,10 +408,11 @@ static int push_ret_address(struct pt_regs *regs, unsigned long ip)
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) {
+ long correction = (long)(utask->vaddr - utask->xol_vaddr);
+
regs->ip += correction;
} else if (auprobe->def.fixups & UPROBE_FIX_CALL) {
regs->sp += sizeof_long();
@@ -436,7 +428,7 @@ 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 = {
@@ -732,11 +724,9 @@ bool arch_uprobe_xol_was_trapped(struct task_struct *t)
*
* If the original instruction was a rip-relative instruction such as
* "movl %edx,0xnnnn(%rip)", we have instead executed an equivalent
- * instruction using a scratch register -- e.g., "movl %edx,(%rax)".
- * We need to restore the contents of the scratch register and adjust
- * the ip, keeping in mind that the instruction we executed is 4 bytes
- * shorter than the original instruction (since we squeezed out the offset
- * field). (FIX_RIP_AX or FIX_RIP_CX)
+ * instruction using a scratch register -- e.g., "movl %edx,0xnnnn(%rax)".
+ * We need to restore the contents of the scratch register
+ * (FIX_RIP_AX or FIX_RIP_CX).
*/
int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
--
1.8.1.4
--
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