From 6457a396bbc20656009eaf950ca165912a943520 Mon Sep 17 00:00:00 2001 From: Maneesh Soni Date: Tue, 8 Nov 2011 17:08:26 +0530 Subject: [PATCH] MIPS Kprobes: Support branch instructions probing This patch provides support for kprobes on branch instructions. The branch instruction at the probed address is actually emulated and not executed out-of-line like other normal instructions. Instead the delay-slot instruction is copied and single stepped out of line. At the time of probe hit, the original branch instruction is evaluated and the target cp0_epc is computed similar to compute_retrun_epc(). It is also checked if the delay slot instruction can be skipped, which is true if there is a NOP in delay slot or branch is taken in case of branch likely instructions. Once the delay slot instruction is single stepped the normal execution resume with the cp0_epc updated the earlier computed cp0_epc as per the branch instructions. Signed-off-by: Maneesh Soni Signed-off-by: Victor Kamensky Cc: David Daney Cc: ananth@in.ibm.com Cc: linux-kernel@vger.kernel.org Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/2914/ Signed-off-by: Ralf Baechle --- arch/mips/include/asm/kprobes.h | 5 ++ arch/mips/kernel/kprobes.c | 145 ++++++++++++++++++++++++-------- 2 files changed, 117 insertions(+), 33 deletions(-) diff --git a/arch/mips/include/asm/kprobes.h b/arch/mips/include/asm/kprobes.h index e6ea4d4d7205..1fbbca01e681 100644 --- a/arch/mips/include/asm/kprobes.h +++ b/arch/mips/include/asm/kprobes.h @@ -74,6 +74,8 @@ struct prev_kprobe { : MAX_JPROBES_STACK_SIZE) +#define SKIP_DELAYSLOT 0x0001 + /* per-cpu kprobe control block */ struct kprobe_ctlblk { unsigned long kprobe_status; @@ -82,6 +84,9 @@ struct kprobe_ctlblk { unsigned long kprobe_saved_epc; unsigned long jprobe_saved_sp; struct pt_regs jprobe_saved_regs; + /* Per-thread fields, used while emulating branches */ + unsigned long flags; + unsigned long target_epc; u8 jprobes_stack[MAX_JPROBES_STACK_SIZE]; struct prev_kprobe prev_kprobe; }; diff --git a/arch/mips/kernel/kprobes.c b/arch/mips/kernel/kprobes.c index 0ab1a5ff1049..158467da9bc1 100644 --- a/arch/mips/kernel/kprobes.c +++ b/arch/mips/kernel/kprobes.c @@ -30,6 +30,7 @@ #include #include +#include #include #include @@ -152,13 +153,6 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p) goto out; } - if (insn_has_delayslot(insn)) { - pr_notice("Kprobes for branch and jump instructions are not" - "supported\n"); - ret = -EINVAL; - goto out; - } - if ((probe_kernel_read(&prev_insn, p->addr - 1, sizeof(mips_instruction)) == 0) && insn_has_delayslot(prev_insn)) { @@ -178,9 +172,20 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p) * In the kprobe->ainsn.insn[] array we store the original * instruction at index zero and a break trap instruction at * index one. + * + * On MIPS arch if the instruction at probed address is a + * branch instruction, we need to execute the instruction at + * Branch Delayslot (BD) at the time of probe hit. As MIPS also + * doesn't have single stepping support, the BD instruction can + * not be executed in-line and it would be executed on SSOL slot + * using a normal breakpoint instruction in the next slot. + * So, read the instruction and save it for later execution. */ + if (insn_has_delayslot(insn)) + memcpy(&p->ainsn.insn[0], p->addr + 1, sizeof(kprobe_opcode_t)); + else + memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t)); - memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t)); p->ainsn.insn[1] = breakpoint2_insn; p->opcode = *p->addr; @@ -231,16 +236,96 @@ static void set_current_kprobe(struct kprobe *p, struct pt_regs *regs, kcb->kprobe_saved_epc = regs->cp0_epc; } -static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs) +/** + * evaluate_branch_instrucion - + * + * Evaluate the branch instruction at probed address during probe hit. The + * result of evaluation would be the updated epc. The insturction in delayslot + * would actually be single stepped using a normal breakpoint) on SSOL slot. + * + * The result is also saved in the kprobe control block for later use, + * in case we need to execute the delayslot instruction. The latter will be + * false for NOP instruction in dealyslot and the branch-likely instructions + * when the branch is taken. And for those cases we set a flag as + * SKIP_DELAYSLOT in the kprobe control block + */ +static int evaluate_branch_instruction(struct kprobe *p, struct pt_regs *regs, + struct kprobe_ctlblk *kcb) { + union mips_instruction insn = p->opcode; + long epc; + int ret = 0; + + epc = regs->cp0_epc; + if (epc & 3) + goto unaligned; + + if (p->ainsn.insn->word == 0) + kcb->flags |= SKIP_DELAYSLOT; + else + kcb->flags &= ~SKIP_DELAYSLOT; + + ret = __compute_return_epc_for_insn(regs, insn); + if (ret < 0) + return ret; + + if (ret == BRANCH_LIKELY_TAKEN) + kcb->flags |= SKIP_DELAYSLOT; + + kcb->target_epc = regs->cp0_epc; + + return 0; + +unaligned: + pr_notice("%s: unaligned epc - sending SIGBUS.\n", current->comm); + force_sig(SIGBUS, current); + return -EFAULT; + +} + +static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs, + struct kprobe_ctlblk *kcb) +{ + int ret = 0; + regs->cp0_status &= ~ST0_IE; /* single step inline if the instruction is a break */ if (p->opcode.word == breakpoint_insn.word || p->opcode.word == breakpoint2_insn.word) regs->cp0_epc = (unsigned long)p->addr; - else - regs->cp0_epc = (unsigned long)&p->ainsn.insn[0]; + else if (insn_has_delayslot(p->opcode)) { + ret = evaluate_branch_instruction(p, regs, kcb); + if (ret < 0) { + pr_notice("Kprobes: Error in evaluating branch\n"); + return; + } + } + regs->cp0_epc = (unsigned long)&p->ainsn.insn[0]; +} + +/* + * Called after single-stepping. p->addr is the address of the + * instruction whose first byte has been replaced by the "break 0" + * instruction. To avoid the SMP problems that can occur when we + * temporarily put back the original opcode to single-step, we + * single-stepped a copy of the instruction. The address of this + * copy is p->ainsn.insn. + * + * This function prepares to return from the post-single-step + * breakpoint trap. In case of branch instructions, the target + * epc to be restored. + */ +static void __kprobes resume_execution(struct kprobe *p, + struct pt_regs *regs, + struct kprobe_ctlblk *kcb) +{ + if (insn_has_delayslot(p->opcode)) + regs->cp0_epc = kcb->target_epc; + else { + unsigned long orig_epc = kcb->kprobe_saved_epc; + regs->cp0_epc = orig_epc + 4; + } } static int __kprobes kprobe_handler(struct pt_regs *regs) @@ -279,8 +364,13 @@ static int __kprobes kprobe_handler(struct pt_regs *regs) save_previous_kprobe(kcb); set_current_kprobe(p, regs, kcb); kprobes_inc_nmissed_count(p); - prepare_singlestep(p, regs); + prepare_singlestep(p, regs, kcb); kcb->kprobe_status = KPROBE_REENTER; + if (kcb->flags & SKIP_DELAYSLOT) { + resume_execution(p, regs, kcb); + restore_previous_kprobe(kcb); + preempt_enable_no_resched(); + } return 1; } else { if (addr->word != breakpoint_insn.word) { @@ -324,8 +414,16 @@ static int __kprobes kprobe_handler(struct pt_regs *regs) } ss_probe: - prepare_singlestep(p, regs); - kcb->kprobe_status = KPROBE_HIT_SS; + prepare_singlestep(p, regs, kcb); + if (kcb->flags & SKIP_DELAYSLOT) { + kcb->kprobe_status = KPROBE_HIT_SSDONE; + if (p->post_handler) + p->post_handler(p, regs, 0); + resume_execution(p, regs, kcb); + preempt_enable_no_resched(); + } else + kcb->kprobe_status = KPROBE_HIT_SS; + return 1; no_kprobe: @@ -334,25 +432,6 @@ no_kprobe: } -/* - * Called after single-stepping. p->addr is the address of the - * instruction whose first byte has been replaced by the "break 0" - * instruction. To avoid the SMP problems that can occur when we - * temporarily put back the original opcode to single-step, we - * single-stepped a copy of the instruction. The address of this - * copy is p->ainsn.insn. - * - * This function prepares to return from the post-single-step - * breakpoint trap. - */ -static void __kprobes resume_execution(struct kprobe *p, - struct pt_regs *regs, - struct kprobe_ctlblk *kcb) -{ - unsigned long orig_epc = kcb->kprobe_saved_epc; - regs->cp0_epc = orig_epc + 4; -} - static inline int post_kprobe_handler(struct pt_regs *regs) { struct kprobe *cur = kprobe_running(); -- 2.39.5