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Subject[PATCH][RFC] ftrace/x86: Have arch x86_64 use breakpoints instead of stop machine
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Date
This is an RFC on the ftrace/x86 breakpoint version of converting nops
to ftrace calls and back, instead of using stop-machine. I've sent a
version to Masami off list, and he gave me some feedback (which I
included).

I want to post this as an RFC before I push this for inclusion in 3.5.

-- Steve



Steven Rostedt (1):
ftrace/x86: Have arch x86_64 use breakpoints instead of stop machine

----
arch/x86/include/asm/ftrace.h | 4 +
arch/x86/kernel/ftrace.c | 343 +++++++++++++++++++++++++++++++++++++++++
arch/x86/kernel/traps.c | 9 +-
include/linux/ftrace.h | 6 +
4 files changed, 361 insertions(+), 1 deletion(-)
---------------------------
commit fbf665f4209d086415bbddd17cae76758314e42c
Author: Steven Rostedt <srostedt@redhat.com>
Date: Tue Aug 16 09:57:10 2011 -0400

ftrace/x86: Have arch x86_64 use breakpoints instead of stop machine

This method changes x86 to add a breakpoint to the mcount locations
instead of calling stop machine.

Now that iret can be handled by NMIs, we perform the following to
update code:

1) Add a breakpoint to all locations that will be modified

2) Sync all cores

3) Update all locations to be either a nop or call (except breakpoint
op)

4) Sync all cores

5) Remove the breakpoint with the new code.

6) Sync all cores

[
Added updates that Masami suggested:
Use unlikely(modifying_ftrace_code) in int3 trap to keep kprobes efficient.
Don't use NOTIFY_* in ftrace handler in int3 as it is not a notifier.
]

Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index 268c783..f866c10 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -34,6 +34,7 @@

#ifndef __ASSEMBLY__
extern void mcount(void);
+extern int modifying_ftrace_code;

static inline unsigned long ftrace_call_adjust(unsigned long addr)
{
@@ -50,6 +51,9 @@ struct dyn_arch_ftrace {
/* No extra data needed for x86 */
};

+int ftrace_int3_handler(int cmd, const char *str,
+ struct pt_regs *regs, long err, int trap, int sig);
+
#endif /* CONFIG_DYNAMIC_FTRACE */
#endif /* __ASSEMBLY__ */
#endif /* CONFIG_FUNCTION_TRACER */
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index c9a281f..9422fad 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -20,6 +20,7 @@
#include <linux/init.h>
#include <linux/list.h>
#include <linux/module.h>
+#include <linux/kprobes.h>

#include <trace/syscall.h>

@@ -334,6 +335,348 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
return ret;
}

+int modifying_ftrace_code __read_mostly;
+
+/*
+ * A breakpoint was added to the code address we are about to
+ * modify, and this is the handle that will just skip over it.
+ * We are either changing a nop into a trace call, or a trace
+ * call to a nop. While the change is taking place, we treat
+ * it just like it was a nop.
+ */
+int ftrace_int3_handler(int cmd, const char *str,
+ struct pt_regs *regs, long err, int trap, int sig)
+{
+ if (!modifying_ftrace_code || cmd != DIE_INT3 || !regs)
+ return 0;
+
+ if (!ftrace_location(regs->ip - 1))
+ return 0;
+
+ regs->ip += MCOUNT_INSN_SIZE - 1;
+
+ return 1;
+}
+
+static int ftrace_write(unsigned long ip, const char *val, int size)
+{
+ /*
+ * On x86_64, kernel text mappings are mapped read-only with
+ * CONFIG_DEBUG_RODATA. So we use the kernel identity mapping instead
+ * of the kernel text mapping to modify the kernel text.
+ *
+ * For 32bit kernels, these mappings are same and we can use
+ * kernel identity mapping to modify code.
+ */
+ if (within(ip, (unsigned long)_text, (unsigned long)_etext))
+ ip = (unsigned long)__va(__pa(ip));
+
+ return probe_kernel_write((void *)ip, val, size);
+}
+
+static int add_break(unsigned long ip, const char *old)
+{
+ unsigned char replaced[MCOUNT_INSN_SIZE];
+ unsigned char brk = BREAKPOINT_INSTRUCTION;
+
+ if (probe_kernel_read(replaced, (void *)ip, MCOUNT_INSN_SIZE))
+ return -EFAULT;
+
+ /* Make sure it is what we expect it to be */
+ if (memcmp(replaced, old, MCOUNT_INSN_SIZE) != 0)
+ return -EINVAL;
+
+ if (ftrace_write(ip, &brk, 1))
+ return -EPERM;
+
+ return 0;
+}
+
+static int add_brk_on_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+ unsigned const char *old;
+ unsigned long ip = rec->ip;
+
+ old = ftrace_call_replace(ip, addr);
+
+ return add_break(rec->ip, old);
+}
+
+
+static int add_brk_on_nop(struct dyn_ftrace *rec)
+{
+ unsigned const char *old;
+
+ old = ftrace_nop_replace();
+
+ return add_break(rec->ip, old);
+}
+
+static int add_breakpoints(struct dyn_ftrace *rec, int enable)
+{
+ unsigned long ftrace_addr;
+ int ret;
+
+ ret = ftrace_test_record(rec, enable);
+
+ ftrace_addr = (unsigned long)FTRACE_ADDR;
+
+ switch (ret) {
+ case FTRACE_UPDATE_IGNORE:
+ return 0;
+
+ case FTRACE_UPDATE_MAKE_CALL:
+ /* converting nop to call */
+ return add_brk_on_nop(rec);
+
+ case FTRACE_UPDATE_MAKE_NOP:
+ /* converting a call to a nop */
+ return add_brk_on_call(rec, ftrace_addr);
+ }
+ return 0;
+}
+
+/*
+ * On error, we need to remove breakpoints. This needs to
+ * be done caefully. If the address does not currently have a
+ * breakpoint, we know we are done. Otherwise, we look at the
+ * remaining 4 bytes of the instruction. If it matches a nop
+ * we replace the breakpoint with the nop. Otherwise we replace
+ * it with the call instruction.
+ */
+static int remove_breakpoint(struct dyn_ftrace *rec)
+{
+ unsigned char ins[MCOUNT_INSN_SIZE];
+ unsigned char brk = BREAKPOINT_INSTRUCTION;
+ const unsigned char *nop;
+ unsigned long ftrace_addr;
+ unsigned long ip = rec->ip;
+
+ /* If we fail the read, just give up */
+ if (probe_kernel_read(ins, (void *)ip, MCOUNT_INSN_SIZE))
+ return -EFAULT;
+
+ /* If this does not have a breakpoint, we are done */
+ if (ins[0] != brk)
+ return -1;
+
+ nop = ftrace_nop_replace();
+
+ /*
+ * If the last 4 bytes of the instruction do not match
+ * a nop, then we assume that this is a call to ftrace_addr.
+ */
+ if (memcmp(&ins[1], &nop[1], MCOUNT_INSN_SIZE - 1) != 0) {
+ /*
+ * For extra paranoidism, we check if the breakpoint is on
+ * a call that would actually jump to the ftrace_addr.
+ * If not, don't touch the breakpoint, we make just create
+ * a disaster.
+ */
+ ftrace_addr = (unsigned long)FTRACE_ADDR;
+ nop = ftrace_call_replace(ip, ftrace_addr);
+
+ if (memcmp(&ins[1], &nop[1], MCOUNT_INSN_SIZE - 1) != 0)
+ return -EINVAL;
+ }
+
+ return probe_kernel_write((void *)ip, &nop[0], 1);
+}
+
+static int add_update_code(unsigned long ip, unsigned const char *new)
+{
+ /* skip breakpoint */
+ ip++;
+ new++;
+ if (ftrace_write(ip, new, MCOUNT_INSN_SIZE - 1))
+ return -EPERM;
+ return 0;
+}
+
+static int add_update_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+ unsigned long ip = rec->ip;
+ unsigned const char *new;
+
+ new = ftrace_call_replace(ip, addr);
+ return add_update_code(ip, new);
+}
+
+static int add_update_nop(struct dyn_ftrace *rec)
+{
+ unsigned long ip = rec->ip;
+ unsigned const char *new;
+
+ new = ftrace_nop_replace();
+ return add_update_code(ip, new);
+}
+
+static int add_update(struct dyn_ftrace *rec, int enable)
+{
+ unsigned long ftrace_addr;
+ int ret;
+
+ ret = ftrace_test_record(rec, enable);
+
+ ftrace_addr = (unsigned long)FTRACE_ADDR;
+
+ switch (ret) {
+ case FTRACE_UPDATE_IGNORE:
+ return 0;
+
+ case FTRACE_UPDATE_MAKE_CALL:
+ /* converting nop to call */
+ return add_update_call(rec, ftrace_addr);
+
+ case FTRACE_UPDATE_MAKE_NOP:
+ /* converting a call to a nop */
+ return add_update_nop(rec);
+ }
+
+ return 0;
+}
+
+static int finish_update_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+ unsigned long ip = rec->ip;
+ unsigned const char *new;
+
+ new = ftrace_call_replace(ip, addr);
+
+ if (ftrace_write(ip, new, 1))
+ return -EPERM;
+
+ return 0;
+}
+
+static int finish_update_nop(struct dyn_ftrace *rec)
+{
+ unsigned long ip = rec->ip;
+ unsigned const char *new;
+
+ new = ftrace_nop_replace();
+
+ if (ftrace_write(ip, new, 1))
+ return -EPERM;
+ return 0;
+}
+
+static int finish_update(struct dyn_ftrace *rec, int enable)
+{
+ unsigned long ftrace_addr;
+ int ret;
+
+ ret = ftrace_update_record(rec, enable);
+
+ ftrace_addr = (unsigned long)FTRACE_ADDR;
+
+ switch (ret) {
+ case FTRACE_UPDATE_IGNORE:
+ return 0;
+
+ case FTRACE_UPDATE_MAKE_CALL:
+ /* converting nop to call */
+ return finish_update_call(rec, ftrace_addr);
+
+ case FTRACE_UPDATE_MAKE_NOP:
+ /* converting a call to a nop */
+ return finish_update_nop(rec);
+ }
+
+ return 0;
+}
+
+static void do_sync_core(void *data)
+{
+ sync_core();
+}
+
+static void run_sync(void)
+{
+ int enable_irqs = irqs_disabled();
+
+ /* We may be called with interrupts disbled (on bootup). */
+ if (enable_irqs)
+ local_irq_enable();
+ on_each_cpu(do_sync_core, NULL, 1);
+ if (enable_irqs)
+ local_irq_disable();
+}
+
+static void ftrace_replace_code(int enable)
+{
+ struct ftrace_rec_iter *iter;
+ struct dyn_ftrace *rec;
+ const char *report = "adding breakpoints";
+ int count = 0;
+ int ret;
+
+ for_ftrace_rec_iter(iter) {
+ rec = ftrace_rec_iter_record(iter);
+
+ ret = add_breakpoints(rec, enable);
+ if (ret)
+ goto remove_breakpoints;
+ count++;
+ }
+
+ run_sync();
+
+ report = "updating code";
+
+ for_ftrace_rec_iter(iter) {
+ rec = ftrace_rec_iter_record(iter);
+
+ ret = add_update(rec, enable);
+ if (ret)
+ goto remove_breakpoints;
+ }
+
+ run_sync();
+
+ report = "removing breakpoints";
+
+ for_ftrace_rec_iter(iter) {
+ rec = ftrace_rec_iter_record(iter);
+
+ ret = finish_update(rec, enable);
+ if (ret)
+ goto remove_breakpoints;
+ }
+
+ run_sync();
+
+ return;
+
+ remove_breakpoints:
+ ftrace_bug(ret, rec ? rec->ip : 0);
+ printk(KERN_WARNING "Failed on %s (%d) (%d):\n", report, count, modifying_code);
+ for_ftrace_rec_iter(iter) {
+ rec = ftrace_rec_iter_record(iter);
+ remove_breakpoint(rec);
+ }
+}
+
+void arch_ftrace_update_code(int command)
+{
+ modifying_ftrace_code++;
+
+ if (command & FTRACE_UPDATE_CALLS)
+ ftrace_replace_code(1);
+ else if (command & FTRACE_DISABLE_CALLS)
+ ftrace_replace_code(0);
+
+ if (command & FTRACE_UPDATE_TRACE_FUNC)
+ ftrace_update_ftrace_func(ftrace_trace_function);
+
+ if (command & FTRACE_START_FUNC_RET)
+ ftrace_enable_ftrace_graph_caller();
+ else if (command & FTRACE_STOP_FUNC_RET)
+ ftrace_disable_ftrace_graph_caller();
+
+ modifying_ftrace_code--;
+}
+
int __init ftrace_dyn_arch_init(void *data)
{
/* The return code is retured via data */
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index ff9281f1..1712485 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -50,6 +50,7 @@
#include <asm/processor.h>
#include <asm/debugreg.h>
#include <linux/atomic.h>
+#include <asm/ftrace.h>
#include <asm/traps.h>
#include <asm/desc.h>
#include <asm/i387.h>
@@ -303,8 +304,14 @@ gp_in_kernel:
}

/* May run on IST stack. */
-dotraplinkage void __kprobes do_int3(struct pt_regs *regs, long error_code)
+dotraplinkage void __kprobes notrace do_int3(struct pt_regs *regs, long error_code)
{
+#ifdef CONFIG_FUNCTION_TRACER
+ /* ftrace must be first, everything else may cause a recursive crash */
+ if (unlikely(modifying_ftrace_code) &&
+ ftrace_int3_handler(DIE_INT3, "int3", regs, error_code, 3, SIGTRAP))
+ return;
+#endif
#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
SIGTRAP) == NOTIFY_STOP)
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 72a6cab..0b55903 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -286,6 +286,12 @@ struct ftrace_rec_iter *ftrace_rec_iter_start(void);
struct ftrace_rec_iter *ftrace_rec_iter_next(struct ftrace_rec_iter *iter);
struct dyn_ftrace *ftrace_rec_iter_record(struct ftrace_rec_iter *iter);

+#define for_ftrace_rec_iter(iter) \
+ for (iter = ftrace_rec_iter_start(); \
+ iter; \
+ iter = ftrace_rec_iter_next(iter))
+
+
int ftrace_update_record(struct dyn_ftrace *rec, int enable);
int ftrace_test_record(struct dyn_ftrace *rec, int enable);
void ftrace_run_stop_machine(int command);



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