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Date: Mon, 28 Apr 2014 19:34:32 +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] uprobes: use BX register for rip-relative fixups, not
AX
Thanks...
Again, the change in riprel_analyze() needs the review from someone
who understands the instruction decoding/encoding.
On 04/28, Denys Vlasenko wrote:
>
> Otherwise, instructions such as cmpxchg and div will be mishandled.
It seems that you are right. But it would be really great if you also
provide the test-case which proves the fix ;)
> 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/kernel/uprobes.c | 57 ++++++++++++++++++++++++++++++-----------------
> 1 file changed, 37 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index ae6df8e..bc70182 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -41,7 +41,7 @@
> /* Instruction will modify TF, don't change it */
> #define UPROBE_FIX_SETF 0x04
>
> -#define UPROBE_FIX_RIP_AX 0x08
> +#define UPROBE_FIX_RIP_BX 0x08
> #define UPROBE_FIX_RIP_CX 0x10
>
> #define UPROBE_TRAP_NR UINT_MAX
> @@ -282,7 +282,7 @@ static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
> /*
> * insn_rip_relative() would have decoded rex_prefix, modrm.
> * Clear REX.b bit (extension of MODRM.rm field):
> - * we want to encode rax/rcx, not r8/r9.
> + * we want to encode rbx/rcx, not r11/r9.
> */
> if (insn->rex_prefix.nbytes) {
> cursor = auprobe->insn + insn_offset_rex_prefix(insn);
> @@ -296,41 +296,58 @@ static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
> */
> cursor = auprobe->insn + insn_offset_modrm(insn);
> /*
> - * Convert from rip-relative addressing to register-relative addressing
> - * via a scratch register.
> + * Convert from rip-relative addressing
> + * to register-relative addressing via a scratch register.
> */
> reg = MODRM_REG(insn);
> - if (reg == 0) {
> + if (reg == 3) {
> /*
> - * The register operand (if any) is either the A register
> - * (%rax, %eax, etc.) or (if the 0x4 bit is set in the
> - * REX prefix) %r8. In any case, we know the C register
> + * The register operand (if any) is either the B register
> + * (%rbx, %ebx, etc.) or (if the 0x4 bit is set in the
> + * REX prefix) %r11. In any case, we know the C register
> * is NOT the register operand, so we use %rcx (register
> * #1) for the scratch register.
> */
> auprobe->def.fixups |= UPROBE_FIX_RIP_CX;
> /*
> - * 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)
> + * Change modrm from "00 011 101" to "10 011 001". Example:
> + * 89 1d disp32 mov %ebx,disp32(%rip) becomes
> + * 89 99 disp32 mov %ebx,disp32(%rcx)
> */
> - *cursor = 0x81;
> + *cursor = 0x99;
> } else {
> - /* Use %rax (register #0) for the scratch register. */
> - auprobe->def.fixups |= UPROBE_FIX_RIP_AX;
> + /* Use %rbx (register #3) for the scratch register. */
> + auprobe->def.fixups |= UPROBE_FIX_RIP_BX;
> /*
> - * Change modrm from "00 reg 101" to "10 reg 000". Example:
> + * Change modrm from "00 reg 101" to "10 reg 011". Example:
> * 89 1d disp32 mov %edx,disp32(%rip) becomes
> - * 89 98 disp32 mov %edx,disp32(%rax)
> + * 89 9b disp32 mov %edx,disp32(%rbx)
> */
> - *cursor = (reg << 3) | 0x80;
> + *cursor = (reg << 3) | 0x83;
> }
> + /*
> + * Note: we can't use rax or rdx registers as scratch!
> + * There are 3-operand insns which use rax or rdx:rax
> + * as an implicit operand, _and_ they use modrm byte
> + * whose reg field indicates third register or opcode extension.
> + * In particular, these insns:
> + * f7/6 r/m div r/m
> + * 0f b1 r/m cmpxchg r/m,reg
> + * 0f c7/1 mem cmpxchg{8b,16b} mem
> + * Looking at "reg" field won't allow to detect that rax or rdx
> + * are in use.
> + *
> + * FIXME: handle vex-encoded 3-operand insns too:
> + * c4 e2 60 f7 0d disp32 bextr %ebx,disp32(%rip),%ecx
> + * %ebx is encoded in the vex.vvvv field, which we don't check
> + * (in this example, it's in byte 60 bits 6..3).
> + */
> }
>
> static inline unsigned long *
> scratch_reg(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> - return (auprobe->def.fixups & UPROBE_FIX_RIP_AX) ? ®s->ax : ®s->cx;
> + return (auprobe->def.fixups & UPROBE_FIX_RIP_BX) ? ®s->bx : ®s->cx;
> }
>
> /*
> @@ -339,7 +356,7 @@ scratch_reg(struct arch_uprobe *auprobe, struct pt_regs *regs)
> */
> static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> - if (auprobe->def.fixups & (UPROBE_FIX_RIP_AX | UPROBE_FIX_RIP_CX)) {
> + if (auprobe->def.fixups & (UPROBE_FIX_RIP_BX | UPROBE_FIX_RIP_CX)) {
> struct uprobe_task *utask = current->utask;
> unsigned long *sr = scratch_reg(auprobe, regs);
>
> @@ -350,7 +367,7 @@ 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)
> {
> - if (auprobe->def.fixups & (UPROBE_FIX_RIP_AX | UPROBE_FIX_RIP_CX)) {
> + if (auprobe->def.fixups & (UPROBE_FIX_RIP_BX | UPROBE_FIX_RIP_CX)) {
> struct uprobe_task *utask = current->utask;
> unsigned long *sr = scratch_reg(auprobe, regs);
>
> --
> 1.8.1.4
>
--
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