Date:   Tue, 19 Jul 2022 14:28:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Song Liu <song@...nel.org>
Cc:     <bpf@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <live-patching@...r.kernel.org>, <daniel@...earbox.net>,
        <kernel-team@...com>, <jolsa@...nel.org>
Subject: Re: [PATCH v4 bpf-next 2/4] ftrace: allow IPMODIFY and DIRECT ops
 on the same function

On Sun, 17 Jul 2022 22:54:47 -0700
Song Liu <song@...nel.org> wrote:

Again, make the subject:

  ftrace: Allow IPMODIFY and DIRECT ops on the same function


> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index acb35243ce5d..306bf08acda6 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -208,6 +208,43 @@ enum {
>  	FTRACE_OPS_FL_DIRECT			= BIT(17),
>  };
>  
> +/*
> + * FTRACE_OPS_CMD_* commands allow the ftrace core logic to request changes
> + * to a ftrace_ops. Note, the requests may fail.
> + *
> + * ENABLE_SHARE_IPMODIFY_SELF - enable a DIRECT ops to work on the same
> + *                              function as an ops with IPMODIFY. Called
> + *                              when the DIRECT ops is being registered.
> + *                              This is called with both direct_mutex and
> + *                              ftrace_lock are locked.
> + *
> + * ENABLE_SHARE_IPMODIFY_PEER - enable a DIRECT ops to work on the same
> + *                              function as an ops with IPMODIFY. Called
> + *                              when the other ops (the one with IPMODIFY)
> + *                              is being registered.
> + *                              This is called with direct_mutex locked.
> + *
> + * DISABLE_SHARE_IPMODIFY_PEER - disable a DIRECT ops to work on the same
> + *                               function as an ops with IPMODIFY. Called
> + *                               when the other ops (the one with IPMODIFY)
> + *                               is being unregistered.
> + *                               This is called with direct_mutex locked.
> + */
> +enum ftrace_ops_cmd {
> +	FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_SELF,
> +	FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_PEER,
> +	FTRACE_OPS_CMD_DISABLE_SHARE_IPMODIFY_PEER,
> +};
> +
> +/*
> + * For most ftrace_ops_cmd,
> + * Returns:
> + *        0 - Success.
> + *        -EBUSY - The operation cannot process
> + *        -EAGAIN - The operation cannot process tempoorarily.

Just state:

	Returns:
		0 - Success
		Negative on failure. The return value is dependent
		on the callback.

Let's not bind policy of the callback with ftrace.

> + */
> +typedef int (*ftrace_ops_func_t)(struct ftrace_ops *op, enum ftrace_ops_cmd cmd);
> +
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  /* The hash used to know what functions callbacks trace */
>  struct ftrace_ops_hash {
> @@ -250,6 +287,7 @@ struct ftrace_ops {
>  	unsigned long			trampoline;
>  	unsigned long			trampoline_size;
>  	struct list_head		list;
> +	ftrace_ops_func_t		ops_func;
>  #endif
>  };
>  
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 0c15ec997c13..f52efbd13e51 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1861,6 +1861,8 @@ static void ftrace_hash_rec_enable_modify(struct ftrace_ops *ops,
>  	ftrace_hash_rec_update_modify(ops, filter_hash, 1);
>  }
>  
> +static bool ops_references_ip(struct ftrace_ops *ops, unsigned long ip);
> +
>  /*
>   * 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
> @@ -1869,6 +1871,13 @@ static void ftrace_hash_rec_enable_modify(struct ftrace_ops *ops,
>   *  - If the hash is NULL, it hits all recs (if IPMODIFY is set, this is rejected)
>   *  - If the hash is EMPTY_HASH, it hits nothing
>   *  - Anything else hits the recs which match the hash entries.
> + *
> + * DIRECT ops does not have IPMODIFY flag, but we still need to check it
> + * against functions with FTRACE_FL_IPMODIFY. If there is any overlap, call
> + * ops_func(SHARE_IPMODIFY_SELF) to make sure current ops can share with
> + * IPMODIFY. If ops_func(SHARE_IPMODIFY_SELF) returns non-zero, propagate
> + * the return value to the caller and eventually to the owner of the DIRECT
> + * ops.
>   */
>  static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
>  					 struct ftrace_hash *old_hash,
> @@ -1877,17 +1886,23 @@ static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
>  	struct ftrace_page *pg;
>  	struct dyn_ftrace *rec, *end = NULL;
>  	int in_old, in_new;
> +	bool is_ipmodify, is_direct;
>  
>  	/* Only update if the ops has been registered */
>  	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
>  		return 0;
>  
> -	if (!(ops->flags & FTRACE_OPS_FL_IPMODIFY))
> +	is_ipmodify = ops->flags & FTRACE_OPS_FL_IPMODIFY;
> +	is_direct = ops->flags & FTRACE_OPS_FL_DIRECT;
> +
> +	/* either IPMODIFY nor DIRECT, skip */
> +	if (!is_ipmodify && !is_direct)
>  		return 0;

I wonder if we should also add:

	if (WARN_ON_ONCE(is_ipmodify && is_direct))
		return 0;

As a direct should never have an ipmodify.

>  
>  	/*
> -	 * Since the IPMODIFY is a very address sensitive action, we do not
> -	 * allow ftrace_ops to set all functions to new hash.
> +	 * Since the IPMODIFY and DIRECT are very address sensitive
> +	 * actions, we do not allow ftrace_ops to set all functions to new
> +	 * hash.
>  	 */
>  	if (!new_hash || !old_hash)
>  		return -EINVAL;
> @@ -1905,12 +1920,30 @@ static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
>  			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 */
> +			if (rec->flags & FTRACE_FL_IPMODIFY) {
> +				int ret;
> +
> +				/* Cannot have two ipmodify on same rec */
> +				if (is_ipmodify)
> +					goto rollback;
> +

I might add a

				FTRACE_WARN_ON(rec->flags &
				FTRACE_FL_DIRECT);

Just to be safe.

That is, if this is true, we are adding a new direct function to a record
that already has one.

> +				/*
> +				 * Another ops with IPMODIFY is already
> +				 * attached. We are now attaching a direct
> +				 * ops. Run SHARE_IPMODIFY_SELF, to check
> +				 * whether sharing is supported.
> +				 */
> +				if (!ops->ops_func)
> +					return -EBUSY;
> +				ret = ops->ops_func(ops, FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_SELF);
> +				if (ret)
> +					return ret;
> +			} else if (is_ipmodify) {
> +				rec->flags |= FTRACE_FL_IPMODIFY;
> +			}
> +		} else if (is_ipmodify) {
>  			rec->flags &= ~FTRACE_FL_IPMODIFY;
> +		}
>  	} while_for_each_ftrace_rec();
>  
>  	return 0;
> @@ -2455,7 +2488,7 @@ static void call_direct_funcs(unsigned long ip, unsigned long pip,
>  struct ftrace_ops direct_ops = {
>  	.func		= call_direct_funcs,
>  	.flags		= FTRACE_OPS_FL_IPMODIFY
> -			  | FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS
> +			  | FTRACE_OPS_FL_SAVE_REGS
>  			  | FTRACE_OPS_FL_PERMANENT,
>  	/*
>  	 * By declaring the main trampoline as this trampoline
> @@ -3072,14 +3105,14 @@ static inline int ops_traces_mod(struct ftrace_ops *ops)
>  }
>  
>  /*
> - * Check if the current ops references the record.
> + * Check if the current ops references the given ip.
>   *
>   * If the ops traces all functions, then it was already accounted for.
>   * If the ops does not trace the current record function, skip it.
>   * If the ops ignores the function via notrace filter, skip it.
>   */
> -static inline bool
> -ops_references_rec(struct ftrace_ops *ops, struct dyn_ftrace *rec)
> +static bool
> +ops_references_ip(struct ftrace_ops *ops, unsigned long ip)
>  {
>  	/* If ops isn't enabled, ignore it */
>  	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
> @@ -3091,16 +3124,29 @@ ops_references_rec(struct ftrace_ops *ops, struct dyn_ftrace *rec)
>  
>  	/* The function must be in the filter */
>  	if (!ftrace_hash_empty(ops->func_hash->filter_hash) &&
> -	    !__ftrace_lookup_ip(ops->func_hash->filter_hash, rec->ip))
> +	    !__ftrace_lookup_ip(ops->func_hash->filter_hash, ip))
>  		return false;
>  
>  	/* If in notrace hash, we ignore it too */
> -	if (ftrace_lookup_ip(ops->func_hash->notrace_hash, rec->ip))
> +	if (ftrace_lookup_ip(ops->func_hash->notrace_hash, ip))
>  		return false;
>  
>  	return true;
>  }
>  
> +/*
> + * Check if the current ops references the record.
> + *
> + * If the ops traces all functions, then it was already accounted for.
> + * If the ops does not trace the current record function, skip it.
> + * If the ops ignores the function via notrace filter, skip it.
> + */
> +static bool
> +ops_references_rec(struct ftrace_ops *ops, struct dyn_ftrace *rec)
> +{
> +	return ops_references_ip(ops, rec->ip);
> +}
> +
>  static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
>  {
>  	bool init_nop = ftrace_need_init_nop();
> @@ -5545,8 +5591,7 @@ int modify_ftrace_direct(unsigned long ip,
>  }
>  EXPORT_SYMBOL_GPL(modify_ftrace_direct);
>  
> -#define MULTI_FLAGS (FTRACE_OPS_FL_IPMODIFY | FTRACE_OPS_FL_DIRECT | \
> -		     FTRACE_OPS_FL_SAVE_REGS)
> +#define MULTI_FLAGS (FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS)
>  
>  static int check_direct_multi(struct ftrace_ops *ops)
>  {
> @@ -8004,6 +8049,137 @@ int ftrace_is_dead(void)
>  	return ftrace_disabled;
>  }
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> +/*
> + * When registering ftrace_ops with IPMODIFY, it is necessary to make sure
> + * it doesn't conflict with any direct ftrace_ops. If there is existing
> + * direct ftrace_ops on a kernel function being patched, call
> + * FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_PEER on it to enable sharing.
> + *
> + * @ops:     ftrace_ops being registered.
> + *
> + * Returns:
> + *         0 - @ops does not have IPMODIFY or @ops itself is DIRECT, no
> + *             change needed;
> + *         1 - @ops has IPMODIFY, hold direct_mutex;

> + *         -EBUSY - currently registered DIRECT ftrace_ops cannot share the
> + *                  same function with IPMODIFY, abort the register.
> + *         -EAGAIN - cannot make changes to currently registered DIRECT
> + *                   ftrace_ops due to rare race conditions. Should retry
> + *                   later. This is needed to avoid potential deadlocks
> + *                   on the DIRECT ftrace_ops side.

Again, these are ops_func() specific and has nothing to do with the logic
in this file. Just state:

 * Returns:
 *         0 - @ops does not have IPMODIFY or @ops itself is DIRECT, no
 *             change needed;
 *         1 - @ops has IPMODIFY, hold direct_mutex;
 *         Negative on error.

And if we move the logic that this does not keep hold of the direct_mutex,
we could just let the callback return any non-zero on error.

> + */
> +static int prepare_direct_functions_for_ipmodify(struct ftrace_ops *ops)
> +	__acquires(&direct_mutex)
> +{
> +	struct ftrace_func_entry *entry;
> +	struct ftrace_hash *hash;
> +	struct ftrace_ops *op;
> +	int size, i, ret;
> +
> +	if (!(ops->flags & FTRACE_OPS_FL_IPMODIFY))
> +		return 0;
> +
> +	mutex_lock(&direct_mutex);
> +
> +	hash = ops->func_hash->filter_hash;
> +	size = 1 << hash->size_bits;
> +	for (i = 0; i < size; i++) {
> +		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> +			unsigned long ip = entry->ip;
> +			bool found_op = false;
> +
> +			mutex_lock(&ftrace_lock);
> +			do_for_each_ftrace_op(op, ftrace_ops_list) {
> +				if (!(op->flags & FTRACE_OPS_FL_DIRECT))
> +					continue;
> +				if (ops_references_ip(op, ip)) {
> +					found_op = true;
> +					break;

I think you want a goto here. The macros "do_for_each_ftrace_op() { .. }
while_for_each_ftrace_op()" is a double loop. The break just moves to the
next set of pages and does not break out of the outer loop.

					goto out_loop;

> +				}
> +			} while_for_each_ftrace_op(op);

 out_loop:

> +			mutex_unlock(&ftrace_lock);
> +
> +			if (found_op) {
> +				if (!op->ops_func) {
> +					ret = -EBUSY;
> +					goto err_out;
> +				}
> +				ret = op->ops_func(op, FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_PEER);
> +				if (ret)
> +					goto err_out;
> +			}
> +		}
> +	}
> +
> +	/*
> +	 * Didn't find any overlap with direct ftrace_ops, or the direct
> +	 * function can share with ipmodify. Hold direct_mutex to make sure
> +	 * this doesn't change until we are done.
> +	 */
> +	return 1;
> +
> +err_out:
> +	mutex_unlock(&direct_mutex);
> +	return ret;
> +}
> +
> +/*
> + * Similar to prepare_direct_functions_for_ipmodify, clean up after ops
> + * with IPMODIFY is unregistered. The cleanup is optional for most DIRECT
> + * ops.
> + */
> +static void cleanup_direct_functions_after_ipmodify(struct ftrace_ops *ops)
> +{
> +	struct ftrace_func_entry *entry;
> +	struct ftrace_hash *hash;
> +	struct ftrace_ops *op;
> +	int size, i;
> +
> +	if (!(ops->flags & FTRACE_OPS_FL_IPMODIFY))
> +		return;
> +
> +	mutex_lock(&direct_mutex);
> +
> +	hash = ops->func_hash->filter_hash;
> +	size = 1 << hash->size_bits;
> +	for (i = 0; i < size; i++) {
> +		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> +			unsigned long ip = entry->ip;
> +			bool found_op = false;
> +
> +			mutex_lock(&ftrace_lock);
> +			do_for_each_ftrace_op(op, ftrace_ops_list) {
> +				if (!(op->flags & FTRACE_OPS_FL_DIRECT))
> +					continue;
> +				if (ops_references_ip(op, ip)) {
> +					found_op = true;
> +					break;

					goto out_loop;

> +				}
> +			} while_for_each_ftrace_op(op);

 out_loop:

> +			mutex_unlock(&ftrace_lock);
> +
> +			/* The cleanup is optional, iggore any errors */

					"ignore"

> +			if (found_op && op->ops_func)
> +				op->ops_func(op, FTRACE_OPS_CMD_DISABLE_SHARE_IPMODIFY_PEER);
> +		}
> +	}
> +	mutex_unlock(&direct_mutex);
> +}
> +
> +#else  /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
> +
> +static int prepare_direct_functions_for_ipmodify(struct ftrace_ops *ops)
> +{
> +	return 0;
> +}
> +
> +static void cleanup_direct_functions_after_ipmodify(struct ftrace_ops *ops)
> +{
> +}
> +
> +#endif  /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
> +
>  /**
>   * register_ftrace_function - register a function for profiling
>   * @ops:	ops structure that holds the function for profiling.
> @@ -8016,17 +8192,29 @@ int ftrace_is_dead(void)
>   *       recursive loop.
>   */
>  int register_ftrace_function(struct ftrace_ops *ops)
> +	__releases(&direct_mutex)
>  {
> +	bool direct_mutex_locked = false;
>  	int ret;
>  
>  	ftrace_ops_init(ops);
>  

I agree with Petr.

Just grab the direct_mutex_lock here.

	mutex_lock(&direct_mutex);

> +	ret = prepare_direct_functions_for_ipmodify(ops);
> +	if (ret < 0)
> +		return ret;

		goto out_unlock;


> +	else if (ret == 1)
> +		direct_mutex_locked = true;
> +

Nuke the above;

>  	mutex_lock(&ftrace_lock);
>  
>  	ret = ftrace_startup(ops, 0);
>  
>  	mutex_unlock(&ftrace_lock);
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> +	if (direct_mutex_locked)
> +		mutex_unlock(&direct_mutex);
> +#endif

Change this to:

 out_unlock:
	mutex_unlock(&direct_mutex);

>  	return ret;
>  }

-- Steve

>  EXPORT_SYMBOL_GPL(register_ftrace_function);
> @@ -8045,6 +8233,7 @@ int unregister_ftrace_function(struct ftrace_ops *ops)
>  	ret = ftrace_shutdown(ops, 0);
>  	mutex_unlock(&ftrace_lock);
>  
> +	cleanup_direct_functions_after_ipmodify(ops);
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(unregister_ftrace_function);

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