s390/ftrace,kprobes: allow to patch first instruction

If the function tracer is enabled, allow to set kprobes on the first
instruction of a function (which is the function trace caller):

If no kprobe is set handling of enabling and disabling function tracing
of a function simply patches the first instruction. Either it is a nop
(right now it's an unconditional branch, which skips the mcount block),
or it's a branch to the ftrace_caller() function.

If a kprobe is being placed on a function tracer calling instruction
we encode if we actually have a nop or branch in the remaining bytes
after the breakpoint instruction (illegal opcode).
This is possible, since the size of the instruction used for the nop
and branch is six bytes, while the size of the breakpoint is only
two bytes.
Therefore the first two bytes contain the illegal opcode and the last
four bytes contain either "0" for nop or "1" for branch. The kprobes
code will then execute/simulate the correct instruction.

Instruction patching for kprobes and function tracer is always done
with stop_machine(). Therefore we don't have any races where an
instruction is patched concurrently on a different cpu.
Besides that also the program check handler which executes the function
trace caller instruction won't be executed concurrently to any
stop_machine() execution.

This allows to keep full fault based kprobes handling which generates
correct pt_regs contents automatically.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 65 insertions, 27 deletions

diff --git a/arch/s390/kernel/kprobes.c b/arch/s390/kernel/kprobes.c
index 014d4729b134..d6716c29b7f8 100644
--- a/arch/s390/kernel/kprobes.c
+++ b/arch/s390/kernel/kprobes.c
@@ -29,6 +29,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/hardirq.h>
+#include <linux/ftrace.h>
 #include <asm/cacheflush.h>
 #include <asm/sections.h>
 #include <asm/dis.h>
@@ -60,10 +61,21 @@ struct kprobe_insn_cache kprobe_dmainsn_slots = {
 
 static void __kprobes copy_instruction(struct kprobe *p)
 {
+	unsigned long ip = (unsigned long) p->addr;
 	s64 disp, new_disp;
 	u64 addr, new_addr;
 
-	memcpy(p->ainsn.insn, p->addr, insn_length(p->opcode >> 8));
+	if (ftrace_location(ip) == ip) {
+		/*
+		 * If kprobes patches the instruction that is morphed by
+		 * ftrace make sure that kprobes always sees the branch
+		 * "jg .+24" that skips the mcount block
+		 */
+		ftrace_generate_nop_insn((struct ftrace_insn *)p->ainsn.insn);
+		p->ainsn.is_ftrace_insn = 1;
+	} else
+		memcpy(p->ainsn.insn, p->addr, insn_length(p->opcode >> 8));
+	p->opcode = p->ainsn.insn[0];
 	if (!probe_is_insn_relative_long(p->ainsn.insn))
 		return;
 	/*
@@ -85,18 +97,6 @@ static inline int is_kernel_addr(void *addr)
 	return addr < (void *)_end;
 }
 
-static inline int is_module_addr(void *addr)
-{
-#ifdef CONFIG_64BIT
-	BUILD_BUG_ON(MODULES_LEN > (1UL << 31));
-	if (addr < (void *)MODULES_VADDR)
-		return 0;
-	if (addr > (void *)MODULES_END)
-		return 0;
-#endif
-	return 1;
-}
-
 static int __kprobes s390_get_insn_slot(struct kprobe *p)
 {
 	/*
@@ -132,43 +132,63 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 		return -EINVAL;
 	if (s390_get_insn_slot(p))
 		return -ENOMEM;
-	p->opcode = *p->addr;
 	copy_instruction(p);
 	return 0;
 }
 
-struct ins_replace_args {
-	kprobe_opcode_t *ptr;
-	kprobe_opcode_t opcode;
+int arch_check_ftrace_location(struct kprobe *p)
+{
+	return 0;
+}
+
+struct swap_insn_args {
+	struct kprobe *p;
+	unsigned int arm_kprobe : 1;
 };
 
-static int __kprobes swap_instruction(void *aref)
+static int __kprobes swap_instruction(void *data)
 {
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 	unsigned long status = kcb->kprobe_status;
-	struct ins_replace_args *args = aref;
-
+	struct swap_insn_args *args = data;
+	struct ftrace_insn new_insn, *insn;
+	struct kprobe *p = args->p;
+	size_t len;
+
+	new_insn.opc = args->arm_kprobe ? BREAKPOINT_INSTRUCTION : p->opcode;
+	len = sizeof(new_insn.opc);
+	if (!p->ainsn.is_ftrace_insn)
+		goto skip_ftrace;
+	len = sizeof(new_insn);
+	insn = (struct ftrace_insn *) p->addr;
+	if (args->arm_kprobe) {
+		if (is_ftrace_nop(insn))
+			new_insn.disp = KPROBE_ON_FTRACE_NOP;
+		else
+			new_insn.disp = KPROBE_ON_FTRACE_CALL;
+	} else {
+		ftrace_generate_call_insn(&new_insn, (unsigned long)p->addr);
+		if (insn->disp == KPROBE_ON_FTRACE_NOP)
+			ftrace_generate_nop_insn(&new_insn);
+	}
+skip_ftrace:
 	kcb->kprobe_status = KPROBE_SWAP_INST;
-	probe_kernel_write(args->ptr, &args->opcode, sizeof(args->opcode));
+	probe_kernel_write(p->addr, &new_insn, len);
 	kcb->kprobe_status = status;
 	return 0;
 }
 
 void __kprobes arch_arm_kprobe(struct kprobe *p)
 {
-	struct ins_replace_args args;
+	struct swap_insn_args args = {.p = p, .arm_kprobe = 1};
 
-	args.ptr = p->addr;
-	args.opcode = BREAKPOINT_INSTRUCTION;
 	stop_machine(swap_instruction, &args, NULL);
 }
 
 void __kprobes arch_disarm_kprobe(struct kprobe *p)
 {
-	struct ins_replace_args args;
+	struct swap_insn_args args = {.p = p, .arm_kprobe = 0};
 
-	args.ptr = p->addr;
-	args.opcode = p->opcode;
 	stop_machine(swap_instruction, &args, NULL);
 }
 
@@ -459,6 +479,24 @@ static void __kprobes resume_execution(struct kprobe *p, struct pt_regs *regs)
 	unsigned long ip = regs->psw.addr & PSW_ADDR_INSN;
 	int fixup = probe_get_fixup_type(p->ainsn.insn);
 
+	/* Check if the kprobes location is an enabled ftrace caller */
+	if (p->ainsn.is_ftrace_insn) {
+		struct ftrace_insn *insn = (struct ftrace_insn *) p->addr;
+		struct ftrace_insn call_insn;
+
+		ftrace_generate_call_insn(&call_insn, (unsigned long) p->addr);
+		/*
+		 * A kprobe on an enabled ftrace call site actually single
+		 * stepped an unconditional branch (ftrace nop equivalent).
+		 * Now we need to fixup things and pretend that a brasl r0,...
+		 * was executed instead.
+		 */
+		if (insn->disp == KPROBE_ON_FTRACE_CALL) {
+			ip += call_insn.disp * 2 - MCOUNT_INSN_SIZE;
+			regs->gprs[0] = (unsigned long)p->addr + sizeof(*insn);
+		}
+	}
+
 	if (fixup & FIXUP_PSW_NORMAL)
 		ip += (unsigned long) p->addr - (unsigned long) p->ainsn.insn;