Re: [for-next][PATCH 1/4] ftrace/x86: Add dynamic allocated trampoline for ftrace_ops

[Borislav Petkov]

On Mon, Oct 27, 2014 at 02:27:03PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
> 
> The current method of handling multiple function callbacks is to register
> a list function callback that calls all the other callbacks based on
> their hash tables and compare it to the function that the callback was
> called on. But this is very inefficient.
> 
> For example, if you are tracing all functions in the kernel and then
> add a kprobe to a function such that the kprobe uses ftrace, the
> mcount trampoline will switch from calling the function trace callback
> to calling the list callback that will iterate over all registered
> ftrace_ops (in this case, the function tracer and the kprobes callback).
> That means for every function being traced it checks the hash of the
> ftrace_ops for function tracing and kprobes, even though the kprobes
> is only set at a single function. The kprobes ftrace_ops is checked
> for every function being traced!
> 
> Instead of calling the list function for functions that are only being
> traced by a single callback, we can call a dynamically allocated
> trampoline that calls the callback directly. The function graph tracer
> already uses a direct call trampoline when it is being traced by itself
> but it is not dynamically allocated. It's trampoline is static in the
> kernel core. The infrastructure that called the function graph trampoline
> can also be used to call a dynamically allocated one.
> 
> For now, only ftrace_ops that are not dynamically allocated can have
> a trampoline. That is, users such as function tracer or stack tracer.
> kprobes and perf allocate their ftrace_ops, and until there's a safe
> way to free the trampoline, it can not be used. The dynamically allocated
> ftrace_ops may, although, use the trampoline if the kernel is not
> compiled with CONFIG_PREEMPT. But that will come later.
> 
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
>  arch/x86/kernel/ftrace.c    | 195 ++++++++++++++++++++++++++++++++++++++++++--
>  arch/x86/kernel/mcount_64.S |  25 +++++-
>  include/linux/ftrace.h      |   8 ++
>  kernel/trace/ftrace.c       |  40 ++++++++-
>  4 files changed, 254 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 3386dc9aa333..e4d48f6cad86 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -17,6 +17,7 @@
>  #include <linux/ftrace.h>
>  #include <linux/percpu.h>
>  #include <linux/sched.h>
> +#include <linux/slab.h>
>  #include <linux/init.h>
>  #include <linux/list.h>
>  #include <linux/module.h>
> @@ -644,13 +645,8 @@ int __init ftrace_dyn_arch_init(void)
>  {
>  	return 0;
>  }
> -#endif
> -
> -#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> -
> -#ifdef CONFIG_DYNAMIC_FTRACE
> -extern void ftrace_graph_call(void);
>  
> +#if defined(CONFIG_X86_64) || defined(CONFIG_FUNCTION_GRAPH_TRACER)
>  static unsigned char *ftrace_jmp_replace(unsigned long ip, unsigned long addr)
>  {
>  	static union ftrace_code_union calc;
> @@ -664,6 +660,193 @@ static unsigned char *ftrace_jmp_replace(unsigned long ip, unsigned long addr)
>  	 */
>  	return calc.code;
>  }
> +#endif
> +
> +/* Currently only x86_64 supports dynamic trampolines */
> +#ifdef CONFIG_X86_64
> +
> +#ifdef CONFIG_MODULES
> +#include <linux/moduleloader.h>
> +/* Module allocation simplifies allocating memory for code */

... because it makes it EXEC too?

> +static inline void *alloc_tramp(unsigned long size)

Yeah, naming is, hmm, rated R. But we talked about it :-)

> +{
> +	return module_alloc(size);
> +}
> +static inline void tramp_free(void *tramp)
> +{
> +	module_free(NULL, tramp);
> +}
> +#else
> +/* Trampolines can only be created if modules are supported */

Just for my own understanding: why? You're not loading additional .ko's
right?

> +static inline void *alloc_tramp(unsigned long size)
> +{
> +	return NULL;
> +}
> +static inline void tramp_free(void *tramp) { }
> +#endif
> +
> +/* Defined as markers to the end of the ftrace default trampolines */
> +extern void ftrace_caller_end(void);
> +extern void ftrace_regs_caller_end(void);
> +extern void ftrace_return(void);
> +extern void ftrace_caller_op_ptr(void);
> +extern void ftrace_regs_caller_op_ptr(void);
> +
> +/* movq function_trace_op(%rip), %rdx */
> +/* 0x48 0x8b 0x15 <offset-to-ftrace_trace_op (4 bytes)> */
> +#define OP_REF_SIZE	7
> +
> +/*
> + * The ftrace_ops is passed to the function callback. Since the
> + * trampoline only services a single ftrace_ops, we can pass in
> + * that ops directly.
> + *
> + * The ftrace_op_code_union is used to create a pointer to the
> + * ftrace_ops that will be passed to the callback function.
> + */
> +union ftrace_op_code_union {
> +	char code[OP_REF_SIZE];
> +	struct {
> +		char op[3];
> +		int offset;
> +	} __attribute__((packed));
> +};
> +
> +static unsigned long create_trampoline(struct ftrace_ops *ops)
> +{
> +	unsigned const char *jmp;
> +	unsigned long start_offset;
> +	unsigned long end_offset;
> +	unsigned long op_offset;
> +	unsigned long offset;
> +	unsigned long size;
> +	unsigned long ip;
> +	unsigned long *ptr;
> +	void *trampoline;
> +	/* 48 8b 15 <offset> is movq <offset>(%rip), %rdx */
> +	unsigned const char op_ref[] = { 0x48, 0x8b, 0x15 };

Ok, this 0x15 is the ModRM which says that the destination register is
hammered to be %rdx. I guess this has something to do with the magic
third parameter thing where ftrace_ops go into...

> +	union ftrace_op_code_union op_ptr;
> +	int ret;
> +
> +	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
> +		start_offset = (unsigned long)ftrace_regs_caller;
> +		end_offset = (unsigned long)ftrace_regs_caller_end;
> +		op_offset = (unsigned long)ftrace_regs_caller_op_ptr;
> +	} else {
> +		start_offset = (unsigned long)ftrace_caller;
> +		end_offset = (unsigned long)ftrace_caller_end;
> +		op_offset = (unsigned long)ftrace_caller_op_ptr;
> +	}
> +
> +	size = end_offset - start_offset;
> +
> +	/*
> +	 * Allocate enough size to store the ftrace_caller code,
> +	 * the jmp to ftrace_return, as well as the address of
> +	 * the ftrace_ops this trampoline is used for.
> +	 */
> +	trampoline = alloc_tramp(size + MCOUNT_INSN_SIZE + sizeof(void *));
> +	if (!trampoline)
> +		return 0;
> +
> +	/* Copy ftrace_caller onto the trampoline memory */
> +	ret = probe_kernel_read(trampoline, (void *)start_offset, size);
> +	if (WARN_ON(ret < 0)) {
> +		tramp_free(trampoline);

A tramp-free trampoline? :-P

Btw, "trmp" could be a good, short and politically correct prefix to
this stuff:

trmp_alloc()
trmp_free()
...

:-)

> +		return 0;
> +	}
> +
> +	ip = (unsigned long)trampoline + size;
> +
> +	/* The trampoline ends with a jmp to ftrace_return */
> +	jmp = ftrace_jmp_replace(ip, (unsigned long)ftrace_return);
> +	memcpy(trampoline + size, jmp, MCOUNT_INSN_SIZE);
> +
> +	/*
> +	 * The address of the ftrace_ops that is used for this trampoline
> +	 * is stored at the end of the trampoline. This will be used to
> +	 * load the third parameter for the callback. Basically, that
> +	 * location at the end of the trampoline takes the place of
> +	 * the global function_trace_op variable.
> +	 */
> +

Spurious newline.

> +	ptr = (unsigned long *)(trampoline + size + MCOUNT_INSN_SIZE);

Maybe we should call this variable ftrace_ops_ptr to be more clear?

> +	*ptr = (unsigned long)ops;
> +
> +	op_offset -= start_offset;
> +	memcpy(&op_ptr, trampoline + op_offset, OP_REF_SIZE);

I guess we can copy only 3 bytes here and not OP_REF_SIZE as we
overwrite op_ptr.offset later anyway.

> +
> +	/* Are we pointing to the reference? */

"Are we pointing to the movq" is what we ask here, no?

> +	if (WARN_ON(memcmp(op_ptr.op, op_ref, 3) != 0)) {
> +		tramp_free(trampoline);
> +		return 0;
> +	}
> +
> +	/* Load the contents of ptr into the callback parameter */
> +	offset = (unsigned long)ptr;
> +	offset -= (unsigned long)trampoline + op_offset + OP_REF_SIZE;
> +
> +	op_ptr.offset = offset;
> +
> +	/* put in the new offset to the ftrace_ops */
> +	memcpy(trampoline + op_offset, &op_ptr, OP_REF_SIZE);
> +
> +	/* ALLOC_TRAMP flags lets us know we created it */
> +	ops->flags |= FTRACE_OPS_FL_ALLOC_TRAMP;
> +
> +	return (unsigned long)trampoline;
> +}
> +

Yeah, it looks ok as far as I can tell by looking around and trying to
piece it together from staring at ftrace-design.txt and the rest of the
fun.

But don't take my word for it :-)

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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