Re: [PATCH -tip v2 2/3] ftrace, kprobes: Support IPMODIFY flag to find IP modify conflict

[Steven Rostedt]

On Tue, 17 Jun 2014 11:04:49 +0000
Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:

> Introduce FTRACE_OPS_FL_IPMODIFY to avoid conflict among
> ftrace users who may modify regs->ip to change the execution
> path. This also adds the flag to kprobe_ftrace_ops, since
> ftrace-based kprobes already modifies regs->ip. Thus, if
> another user modifies the regs->ip on the same function entry,
> one of them will be broken. So both should add IPMODIFY flag
> and make sure that ftrace_set_filter_ip() succeeds.
> 
> Note that currently conflicts of IPMODIFY are detected on the
> filter hash. It does NOT care about the notrace hash. This means
> that if you set filter hash all functions and notrace(mask)
> some of them, the IPMODIFY flag will be applied to all
> functions.

Hmm, this worries me. I'm not sure I care about ignoring the notrace
hash.

> 
> Changes in v2:
>  - Add a description how __ftrace_hash_update_ipmodify() will
>    handle the given hashes (NULL and EMPTY_HASH cases).
>  - Clear FTRACE_OPS_FL_ENABLED after calling
>    __unregister_ftrace_function() in error path.
> 
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Cc: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  Documentation/trace/ftrace.txt |    5 ++
>  include/linux/ftrace.h         |   10 ++-
>  kernel/kprobes.c               |    2 -
>  kernel/trace/ftrace.c          |  133 +++++++++++++++++++++++++++++++++++++++-
>  4 files changed, 145 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
> index 2479b2a..0fcad7d 100644
> --- a/Documentation/trace/ftrace.txt
> +++ b/Documentation/trace/ftrace.txt
> @@ -234,6 +234,11 @@ of ftrace. Here is a list of some of the key files:
>  	will be displayed on the same line as the function that
>  	is returning registers.
>  
> +	If the callback registered to be traced by a function with
> +	the "ip modify" attribute (thus the regs->ip can be changed),
> +	a 'I' will be displayed on the same line as the function that
> +	can be overridden.
> +
>    function_profile_enabled:
>  
>  	When set it will enable all functions with either the function
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 404a686..5a43f8f 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -90,6 +90,9 @@ typedef void (*ftrace_func_t)(unsigned long ip, unsigned long parent_ip,
>   * INITIALIZED - The ftrace_ops has already been initialized (first use time
>   *            register_ftrace_function() is called, it will initialized the ops)
>   * DELETED - The ops are being deleted, do not let them be registered again.
> + * IPMODIFY - The ops can modify IP register. This must be set with SAVE_REGS
> + *            and if the other ops has been set this on same function, filter
> + *            update must be failed.
>   */
>  enum {
>  	FTRACE_OPS_FL_ENABLED			= 1 << 0,
> @@ -101,6 +104,7 @@ enum {
>  	FTRACE_OPS_FL_STUB			= 1 << 6,
>  	FTRACE_OPS_FL_INITIALIZED		= 1 << 7,
>  	FTRACE_OPS_FL_DELETED			= 1 << 8,
> +	FTRACE_OPS_FL_IPMODIFY			= 1 << 9,
>  };
>  
>  /*
> @@ -306,6 +310,7 @@ extern int ftrace_nr_registered_ops(void);
>   * the dyn_ftrace descriptor represents.
>   *
>   * The second part is a mask:
> + *  IPMODIFY - the record wants to change IP address.
>   *  ENABLED - the function is being traced
>   *  REGS    - the record wants the function to save regs
>   *  REGS_EN - the function is set up to save regs.
> @@ -317,13 +322,14 @@ extern int ftrace_nr_registered_ops(void);
>   * from tracing that function.
>   */
>  enum {
> +	FTRACE_FL_IPMODIFY	= (1UL << 28),
>  	FTRACE_FL_ENABLED	= (1UL << 29),
>  	FTRACE_FL_REGS		= (1UL << 30),
>  	FTRACE_FL_REGS_EN	= (1UL << 31)
>  };
>  
> -#define FTRACE_FL_MASK		(0x7UL << 29)
> -#define FTRACE_REF_MAX		((1UL << 29) - 1)
> +#define FTRACE_FL_MASK		(0xfUL << 28)
> +#define FTRACE_REF_MAX		((1UL << 28) - 1)

Note, this is going to conflict with my queue for 3.17, as I'm starting
to write individual trampolines.

You can take a look at my ftrace/next branch, but be warned, it will
rebase.

>  
>  struct dyn_ftrace {
>  	unsigned long		ip; /* address of mcount call-site */
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 3214289..e52d86f 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -915,7 +915,7 @@ static struct kprobe *alloc_aggr_kprobe(struct kprobe *p)
>  #ifdef CONFIG_KPROBES_ON_FTRACE
>  static struct ftrace_ops kprobe_ftrace_ops __read_mostly = {
>  	.func = kprobe_ftrace_handler,
> -	.flags = FTRACE_OPS_FL_SAVE_REGS,
> +	.flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_IPMODIFY,

We probably should comment somewhere that once you set the IPMODIFY
flag (or do not set it), it should never change. An ops either has it
or it doesn't, it can't change its mind. Otherwise it could play havoc
with the update code below.

>  };
>  static int kprobe_ftrace_enabled;
>  
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index d65719d..5e09f39 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1293,6 +1293,9 @@ ftrace_hash_rec_disable(struct ftrace_ops *ops, int filter_hash);
>  static void
>  ftrace_hash_rec_enable(struct ftrace_ops *ops, int filter_hash);
>  
> +static int ftrace_hash_ipmodify_update(struct ftrace_ops *ops,
> +				       struct ftrace_hash *new_hash);
> +
>  static int
>  ftrace_hash_move(struct ftrace_ops *ops, int enable,
>  		 struct ftrace_hash **dst, struct ftrace_hash *src)
> @@ -1304,6 +1307,7 @@ ftrace_hash_move(struct ftrace_ops *ops, int enable,
>  	struct ftrace_hash *new_hash;
>  	int size = src->count;
>  	int bits = 0;
> +	int ret;
>  	int i;
>  
>  	/*
> @@ -1339,6 +1343,16 @@ ftrace_hash_move(struct ftrace_ops *ops, int enable,
>  	}
>  
>  update:
> +	/* Before everything, make sure this can be applied */
> +	if (enable) {
> +		/* IPMODIFY should be updated only when filter_hash updating */
> +		ret = ftrace_hash_ipmodify_update(ops, new_hash);
> +		if (ret < 0) {
> +			free_ftrace_hash(new_hash);
> +			return ret;
> +		}
> +	}
> +
>  	/*
>  	 * Remove the current set, update the hash and add
>  	 * them back.
> @@ -1593,6 +1607,109 @@ static void ftrace_hash_rec_enable(struct ftrace_ops *ops,
>  	__ftrace_hash_rec_update(ops, filter_hash, 1);
>  }
>  
> +/*
> + * Try to update IPMODIFY flag on each ftrace_rec. Return 0 if it is OK
> + * or no-needed to update, -EBUSY if it detects a conflict of the flag
> + * on a ftrace_rec.
> + * Note that old_hash and new_hash has below meanings
> + *  - If the hash is NULL, it hits all recs
> + *  - If the hash is EMPTY_HASH, it hits nothing
> + *  - Others hits the recs which match the hash entries.

 - Anything else hits the recs ...

> + */
> +static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
> +					 struct ftrace_hash *old_hash,
> +					 struct ftrace_hash *new_hash)
> +{
> +	struct ftrace_page *pg;
> +	struct dyn_ftrace *rec, *end = NULL;
> +	int in_old, in_new;
> +
> +	/* Only update if the ops has been registered */
> +	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
> +		return 0;
> +
> +	if (!(ops->flags & FTRACE_OPS_FL_SAVE_REGS) ||
> +	    !(ops->flags & FTRACE_OPS_FL_IPMODIFY))

Only check the IPMODIFY flag. In the future, I may allow this without
SAVE_REGS. That is, the function will get a regs pointer that has a
limited number of regs set. Maybe just ip.


> +		return 0;
> +
> +	/* Update rec->flags */
> +	do_for_each_ftrace_rec(pg, rec) {
> +		/* We need to update only differences of filter_hash */
> +		in_old = !old_hash || ftrace_lookup_ip(old_hash, rec->ip);
> +		in_new = !new_hash || ftrace_lookup_ip(new_hash, rec->ip);
> +		if (in_old == in_new)
> +			continue;
> +
> +		if (in_new) {
> +			/* New entries must ensure no others are using it */
> +			if (rec->flags & FTRACE_FL_IPMODIFY)
> +				goto rollback;
> +			rec->flags |= FTRACE_FL_IPMODIFY;
> +		} else /* Removed entry */
> +			rec->flags &= ~FTRACE_FL_IPMODIFY;

Like I mentioned before. This code depends on oldhash also having
IPMODIFY set. I wonder if we can detect if it changed.

> +	} while_for_each_ftrace_rec();
> +
> +	return 0;
> +
> +rollback:
> +	end = rec;
> +
> +	/* Roll back what we did above */
> +	do_for_each_ftrace_rec(pg, rec) {
> +		if (rec == end)
> +			goto err_out;
> +
> +		in_old = !old_hash || ftrace_lookup_ip(old_hash, rec->ip);
> +		in_new = !new_hash || ftrace_lookup_ip(new_hash, rec->ip);
> +		if (in_old == in_new)
> +			continue;
> +
> +		if (in_new)
> +			rec->flags &= ~FTRACE_FL_IPMODIFY;
> +		else
> +			rec->flags |= FTRACE_FL_IPMODIFY;
> +	} while_for_each_ftrace_rec();
> +
> +err_out:
> +	return -EBUSY;
> +}
> +
> +static int ftrace_hash_ipmodify_enable(struct ftrace_ops *ops)
> +{
> +	struct ftrace_hash *hash = ops->filter_hash;
> +
> +	if (ftrace_hash_empty(hash))
> +		hash = NULL;
> +
> +	return __ftrace_hash_update_ipmodify(ops, EMPTY_HASH, hash);
> +}
> +
> +/* Disabling always succeeds */
> +static void ftrace_hash_ipmodify_disable(struct ftrace_ops *ops)
> +{
> +	struct ftrace_hash *hash = ops->filter_hash;
> +
> +	if (ftrace_hash_empty(hash))
> +		hash = NULL;
> +
> +	__ftrace_hash_update_ipmodify(ops, hash, EMPTY_HASH);
> +}
> +
> +static int ftrace_hash_ipmodify_update(struct ftrace_ops *ops,
> +				       struct ftrace_hash *new_hash)
> +{
> +	struct ftrace_hash *old_hash = ops->filter_hash;
> +
> +	if (ftrace_hash_empty(old_hash))
> +		old_hash = NULL;
> +
> +	if (ftrace_hash_empty(new_hash))
> +		new_hash = NULL;
> +
> +	return __ftrace_hash_update_ipmodify(ops, old_hash, new_hash);
> +}
> +
> +
>  static void print_ip_ins(const char *fmt, unsigned char *p)
>  {
>  	int i;
> @@ -2078,6 +2195,15 @@ static int ftrace_startup(struct ftrace_ops *ops, int command)
>  
>  	ops->flags |= FTRACE_OPS_FL_ENABLED;
>  
> +	ret = ftrace_hash_ipmodify_enable(ops);
> +	if (ret < 0) {
> +		/* Rollback registration process */
> +		__unregister_ftrace_function(ops);
> +		ftrace_start_up--;
> +		ops->flags &= ~FTRACE_OPS_FL_ENABLED;
> +		return ret;
> +	}
> +
>  	ftrace_hash_rec_enable(ops, 1);
>  
>  	ftrace_startup_enable(command);
> @@ -2104,6 +2230,8 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
>  	 */
>  	WARN_ON_ONCE(ftrace_start_up < 0);
>  
> +	/* Disabling ipmodify never be failed */

          "Disabling ipmodify never fails"

> +	ftrace_hash_ipmodify_disable(ops);
>  	ftrace_hash_rec_disable(ops, 1);
>  
>  	if (!global_start_up)
> @@ -2641,9 +2769,10 @@ static int t_show(struct seq_file *m, void *v)
>  
>  	seq_printf(m, "%ps", (void *)rec->ip);
>  	if (iter->flags & FTRACE_ITER_ENABLED)
> -		seq_printf(m, " (%ld)%s",
> +		seq_printf(m, " (%ld)%s%s",
>  			   rec->flags & ~FTRACE_FL_MASK,
> -			   rec->flags & FTRACE_FL_REGS ? " R" : "");
> +			   rec->flags & FTRACE_FL_REGS ? " R" : "",
> +			   rec->flags & FTRACE_FL_IPMODIFY ? " I" : "");

Perhaps even add 'I' as we may allow setting it without regs.

-- Steve

>  	seq_printf(m, "\n");
>  
>  	return 0;
> 

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