Date:   Tue, 25 Jan 2022 11:46:15 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH v5 3/9] rethook: Add a generic return hook

On Tue, 25 Jan 2022 21:12:09 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Add a return hook framework which hooks the function
> return. Most of the idea came from the kretprobe, but
> this is independent from kretprobe.
> Note that this is expected to be used with other
> function entry hooking feature, like ftrace, fprobe,
> adn kprobes. Eventually this will replace the
> kretprobe (e.g. kprobe + rethook = kretprobe), but
> at this moment, this is just a additional hook.

BTW, 74 characters is the recommended width of a change log (I usually use
76). Not sure why your width is set to 55 characters.

> 
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>  Changes in v4:
>   - Fix rethook_trampoline_handler() loops as same as
>     what currently kretprobe does.  This will fix some
>     stacktrace issue in the rethook handler.
> ---
>  include/linux/rethook.h |   99 +++++++++++++++
>  include/linux/sched.h   |    3 
>  kernel/exit.c           |    2 
>  kernel/fork.c           |    3 
>  kernel/trace/Kconfig    |   11 ++
>  kernel/trace/Makefile   |    1 
>  kernel/trace/rethook.c  |  311 +++++++++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 430 insertions(+)
>  create mode 100644 include/linux/rethook.h
>  create mode 100644 kernel/trace/rethook.c
> 
> diff --git a/include/linux/rethook.h b/include/linux/rethook.h
> new file mode 100644
> index 000000000000..39cfbff1a03c
> --- /dev/null
> +++ b/include/linux/rethook.h
> @@ -0,0 +1,99 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Return hooking with list-based shadow stack.
> + */
> +#ifndef _LINUX_RETHOOK_H
> +#define _LINUX_RETHOOK_H
> +
> +#include <linux/compiler.h>
> +#include <linux/freelist.h>
> +#include <linux/llist.h>
> +#include <linux/rcupdate.h>
> +#include <linux/refcount.h>
> +
> +struct rethook_node;
> +
> +typedef void (*rethook_handler_t) (struct rethook_node *, void *, struct pt_regs *);
> +
> +/**
> + * struct rethook - The rethook management data structure.
> + * @data: The user-defined data storage.
> + * @handler: The user-defined return hook handler.
> + * @pool: The pool of struct rethook_node.
> + * @ref: The reference counter.
> + * @rcu: The rcu_head for deferred freeing.
> + *
> + * Don't embed to another data structure, because this is a self-destructive
> + * data structure when all rethook_node are freed.
> + */
> +struct rethook {
> +	void			*data;
> +	rethook_handler_t	handler;
> +	struct freelist_head	pool;
> +	refcount_t		ref;
> +	struct rcu_head		rcu;
> +};
> +
> +/**
> + * struct rethook_node - The rethook shadow-stack entry node.
> + * @freelist: The freelist, linked to struct rethook::pool.
> + * @rcu: The rcu_head for deferred freeing.
> + * @llist: The llist, linked to a struct task_struct::rethooks.
> + * @rethook: The pointer to the struct rethook.
> + * @ret_addr: The storage for the real return address.
> + * @frame: The stroage for the frame pointer.

		"storage"

> + *
> + * You can embed this with your extended data structure to store any data
> + * on the entry of shadow stack.

	"the shadow stack"?

> + */
> +struct rethook_node {
> +	union {
> +		struct freelist_node freelist;
> +		struct rcu_head      rcu;
> +	};
> +	struct llist_node	llist;
> +	struct rethook		*rethook;
> +	unsigned long		ret_addr;
> +	unsigned long		frame;
> +};
> +
> +struct rethook *rethook_alloc(void *data, rethook_handler_t handler);
> +void rethook_free(struct rethook *rh);
> +void rethook_add_node(struct rethook *rh, struct rethook_node *node);
> +struct rethook_node *rethook_try_get(struct rethook *rh);
> +void rethook_recycle(struct rethook_node *node);
> +void rethook_hook(struct rethook_node *node, struct pt_regs *regs);
> +unsigned long rethook_find_ret_addr(struct task_struct *tsk, unsigned long frame,
> +				    struct llist_node **cur);
> +
> +/* Arch dependent code must implement arch_* and trampoline code */
> +void arch_rethook_prepare(struct rethook_node *node, struct pt_regs *regs);
> +void arch_rethook_trampoline(void);
> +
> +/**
> + * is_rethook_trampoline() - Check whether the address is rethook trampoline
> + * @addr: The address to be checked
> + *
> + * Return true if the @addr is rethook trampoline address.

	"is a rethook"

> + */
> +static inline bool is_rethook_trampoline(unsigned long addr)
> +{
> +	return addr == (unsigned long)arch_rethook_trampoline;

Will this work on architectures like PPC that have strange ways of holding
the function addresses? Or is that what the below fixup handles?

> +}
> +
> +/* If the architecture needs a fixup the return address, implement it. */

	"needs to fixup the"

> +void arch_rethook_fixup_return(struct pt_regs *regs,
> +			       unsigned long correct_ret_addr);
> +
> +/* Generic trampoline handler, arch code must prepare asm stub */
> +unsigned long rethook_trampoline_handler(struct pt_regs *regs,
> +					 unsigned long frame);
> +
> +#ifdef CONFIG_RETHOOK
> +void rethook_flush_task(struct task_struct *tk);
> +#else
> +#define rethook_flush_task(tsk)	do { } while (0)
> +#endif
> +
> +#endif
> +
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 78c351e35fec..2bfabf5355b7 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1473,6 +1473,9 @@ struct task_struct {
>  #ifdef CONFIG_KRETPROBES
>  	struct llist_head               kretprobe_instances;
>  #endif
> +#ifdef CONFIG_RETHOOK
> +	struct llist_head               rethooks;
> +#endif
>  
>  #ifdef CONFIG_ARCH_HAS_PARANOID_L1D_FLUSH
>  	/*
> diff --git a/kernel/exit.c b/kernel/exit.c
> index f702a6a63686..a39a321c1f37 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -64,6 +64,7 @@
>  #include <linux/compat.h>
>  #include <linux/io_uring.h>
>  #include <linux/kprobes.h>
> +#include <linux/rethook.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/unistd.h>
> @@ -169,6 +170,7 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
>  	struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);
>  
>  	kprobe_flush_task(tsk);
> +	rethook_flush_task(tsk);
>  	perf_event_delayed_put(tsk);
>  	trace_sched_process_free(tsk);
>  	put_task_struct(tsk);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 3244cc56b697..ffae38be64c4 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2282,6 +2282,9 @@ static __latent_entropy struct task_struct *copy_process(
>  #ifdef CONFIG_KRETPROBES
>  	p->kretprobe_instances.first = NULL;
>  #endif
> +#ifdef CONFIG_RETHOOK
> +	p->rethooks.first = NULL;
> +#endif
>  
>  	/*
>  	 * Ensure that the cgroup subsystem policies allow the new process to be
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 23483dd474b0..4d27e56c6e76 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -10,6 +10,17 @@ config USER_STACKTRACE_SUPPORT
>  config NOP_TRACER
>  	bool
>  
> +config HAVE_RETHOOK
> +	bool
> +
> +config RETHOOK
> +	bool
> +	depends on HAVE_RETHOOK
> +	help
> +	  Enable generic return hooking feature. This is an internal
> +	  API, which will be used by other function-entry hooking
> +	  feature like fprobe and kprobes.

	"features"

> +
>  config HAVE_FUNCTION_TRACER
>  	bool
>  	help
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index 79255f9de9a4..c6f11a139eac 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -98,6 +98,7 @@ obj-$(CONFIG_UPROBE_EVENTS) += trace_uprobe.o
>  obj-$(CONFIG_BOOTTIME_TRACING) += trace_boot.o
>  obj-$(CONFIG_FTRACE_RECORD_RECURSION) += trace_recursion_record.o
>  obj-$(CONFIG_FPROBE) += fprobe.o
> +obj-$(CONFIG_RETHOOK) += rethook.o
>  
>  obj-$(CONFIG_TRACEPOINT_BENCHMARK) += trace_benchmark.o
>  
> diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> new file mode 100644
> index 000000000000..76c9848b44a9
> --- /dev/null
> +++ b/kernel/trace/rethook.c
> @@ -0,0 +1,311 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#define pr_fmt(fmt) "rethook: " fmt
> +
> +#include <linux/bug.h>
> +#include <linux/kallsyms.h>
> +#include <linux/kprobes.h>
> +#include <linux/preempt.h>
> +#include <linux/rethook.h>
> +#include <linux/slab.h>
> +#include <linux/sort.h>
> +
> +/* Return hook list (shadow stack by list) */
> +
> +/*
> + * This function is called from delayed_put_task_struct() when a task is
> + * dead and cleaned up to recycle any kretprobe instances associated with
> + * this task. These left over instances represent probed functions that
> + * have been called but will never return.
> + */
> +void rethook_flush_task(struct task_struct *tk)
> +{
> +	struct rethook_node *rhn;
> +	struct llist_node *node;
> +
> +	preempt_disable();
> +
> +	node = __llist_del_all(&tk->rethooks);

Hmm, this keeps preemption disabled for the entire walk of the list.
Can we enable it here, and then just disable it when calling
rethook_recycle()?

> +	while (node) {
> +		rhn = container_of(node, struct rethook_node, llist);
> +		node = node->next;

		preempt_disable();
> +		rethook_recycle(rhn);
		preempt_enable();

? I'm concerned about the latency that this can add on RT tasks.

> +	}
> +
> +	preempt_enable();
> +}
> +
> +static void rethook_free_rcu(struct rcu_head *head)
> +{
> +	struct rethook *rh = container_of(head, struct rethook, rcu);
> +	struct rethook_node *rhn;
> +	struct freelist_node *node;
> +	int count = 1;
> +
> +	node = rh->pool.head;
> +	while (node) {
> +		rhn = container_of(node, struct rethook_node, freelist);
> +		node = node->next;
> +		kfree(rhn);
> +		count++;
> +	}
> +
> +	/* The rh->ref is the number of pooled node + 1 */
> +	if (refcount_sub_and_test(count, &rh->ref))
> +		kfree(rh);
> +}
> +
> +/**
> + * rethook_free() - Free struct rethook.
> + * @rh: the struct rethook to be freed.
> + *
> + * Free the rethook. Before calling this function, user must ensure the
> + * @rh::data is cleaned if needed (or, the handler can access it after
> + * calling this function.) This function will set the @rh to be freed
> + * after all rethook_node are freed (not soon). And the caller must
> + * not touch @rh after calling this.
> + */
> +void rethook_free(struct rethook *rh)
> +{
> +	rcu_assign_pointer(rh->handler, NULL);
> +
> +	call_rcu(&rh->rcu, rethook_free_rcu);
> +}
> +
> +/**
> + * rethook_alloc() - Allocate struct rethook.
> + * @data: a data to pass the @handler when hooking the return.
> + * @handler: the return hook callback function.
> + *
> + * Allocate and initialize a new rethook with @data and @handler.
> + * Return NULL if memory allocation fails or @handler is NULL.
> + * Note that @handler == NULL means this rethook is going to be freed.
> + */
> +struct rethook *rethook_alloc(void *data, rethook_handler_t handler)
> +{
> +	struct rethook *rh = kzalloc(sizeof(struct rethook), GFP_KERNEL);
> +
> +	if (!rh || !handler)
> +		return NULL;
> +
> +	rh->data = data;
> +	rh->handler = handler;
> +	rh->pool.head = NULL;
> +	refcount_set(&rh->ref, 1);
> +
> +	return rh;
> +}
> +
> +/**
> + * rethook_add_node() - Add a new node to the rethook.
> + * @rh: the struct rethook.
> + * @node: the struct rethook_node to be added.
> + *
> + * Add @node to @rh. User must allocate @node (as a part of user's
> + * data structure.) The @node fields are initialized in this function.
> + */
> +void rethook_add_node(struct rethook *rh, struct rethook_node *node)
> +{
> +	node->rethook = rh;
> +	freelist_add(&node->freelist, &rh->pool);
> +	refcount_inc(&rh->ref);
> +}
> +
> +static void free_rethook_node_rcu(struct rcu_head *head)
> +{
> +	struct rethook_node *node = container_of(head, struct rethook_node, rcu);
> +
> +	if (refcount_dec_and_test(&node->rethook->ref))
> +		kfree(node->rethook);
> +	kfree(node);
> +}
> +
> +/**
> + * rethook_recycle() - return the node to rethook.
> + * @node: The struct rethook_node to be returned.
> + *
> + * Return back the @node to @node::rethook. If the @node::rethook is already
> + * marked as freed, this will free the @node.
> + */
> +void rethook_recycle(struct rethook_node *node)
> +{
> +	lockdep_assert_preemption_disabled();
> +
> +	if (likely(READ_ONCE(node->rethook->handler)))
> +		freelist_add(&node->freelist, &node->rethook->pool);
> +	else
> +		call_rcu(&node->rcu, free_rethook_node_rcu);
> +}
> +NOKPROBE_SYMBOL(rethook_recycle);
> +
> +/**
> + * rethook_try_get() - get an unused rethook node.
> + * @rh: The struct rethook which pools the nodes.
> + *
> + * Get an unused rethook node from @rh. If the node pool is empty, this
> + * will return NULL. Caller must disable preemption.
> + */
> +struct rethook_node *rethook_try_get(struct rethook *rh)
> +{
> +	rethook_handler_t handler = READ_ONCE(rh->handler);
> +	struct freelist_node *fn;
> +
> +	lockdep_assert_preemption_disabled();
> +
> +	/* Check whether @rh is going to be freed. */
> +	if (unlikely(!handler))
> +		return NULL;
> +
> +	fn = freelist_try_get(&rh->pool);
> +	if (!fn)
> +		return NULL;
> +
> +	return container_of(fn, struct rethook_node, freelist);
> +}
> +NOKPROBE_SYMBOL(rethook_try_get);
> +
> +/**
> + * rethook_hook() - Hook the current function return.
> + * @node: The struct rethook node to hook the function return.
> + * @regs: The struct pt_regs for the function entry.
> + *
> + * Hook the current running function return. This must be called when the
> + * function entry (or at least @regs must be the registers of the function
> + * entry.)
> + */
> +void rethook_hook(struct rethook_node *node, struct pt_regs *regs)
> +{
> +	arch_rethook_prepare(node, regs);
> +	__llist_add(&node->llist, &current->rethooks);
> +}
> +NOKPROBE_SYMBOL(rethook_hook);
> +
> +/* This assumes the 'tsk' is the current task or the is not running. */

	"or the is not running" ?

> +static unsigned long __rethook_find_ret_addr(struct task_struct *tsk,
> +					     struct llist_node **cur)
> +{
> +	struct rethook_node *rh = NULL;
> +	struct llist_node *node = *cur;
> +
> +	if (!node)
> +		node = tsk->rethooks.first;
> +	else
> +		node = node->next;
> +
> +	while (node) {
> +		rh = container_of(node, struct rethook_node, llist);
> +		if (rh->ret_addr != (unsigned long)arch_rethook_trampoline) {
> +			*cur = node;
> +			return rh->ret_addr;
> +		}
> +		node = node->next;
> +	}
> +	return 0;
> +}
> +NOKPROBE_SYMBOL(__rethook_find_ret_addr);
> +
> +/**
> + * rethook_find_ret_addr -- Find correct return address modified by rethook
> + * @tsk: Target task
> + * @frame: A frame pointer
> + * @cur: a storage of the loop cursor llist_node pointer for next call
> + *
> + * Find the correct return address modified by a rethook on @tsk in unsigned
> + * long type. If it finds the return address, this returns that address value,
> + * or this returns 0.

space

> + * The @tsk must be 'current' or a task which is not running. @frame is a hint

How do you know a tsk is not running? How can that be guaranteed?

> + * to get the currect return address - which is compared with the
> + * rethook::frame field. The @cur is a loop cursor for searching the
> + * kretprobe return addresses on the @tsk. The '*@...' should be NULL at the
> + * first call, but '@...' itself must NOT NULL.

I know you state what the return value is above, but it should be stated
(again) here. As kernel-doc should have a separate section for return
values:

 * Returns found address value or zero if not found.

> + */
> +unsigned long rethook_find_ret_addr(struct task_struct *tsk, unsigned long frame,
> +				    struct llist_node **cur)
> +{
> +	struct rethook_node *rhn = NULL;
> +	unsigned long ret;
> +
> +	if (WARN_ON_ONCE(!cur))
> +		return 0;
> +
> +	do {
> +		ret = __rethook_find_ret_addr(tsk, cur);
> +		if (!ret)
> +			break;
> +		rhn = container_of(*cur, struct rethook_node, llist);
> +	} while (rhn->frame != frame);
> +
> +	return ret;
> +}

-- Steve


> +NOKPROBE_SYMBOL(rethook_find_ret_addr);
> +
> +void __weak arch_rethook_fixup_return(struct pt_regs *regs,
> +				      unsigned long correct_ret_addr)
> +{
> +	/*
> +	 * Do nothing by default. If the architecture which uses a
> +	 * frame pointer to record real return address on the stack,
> +	 * it should fill this function to fixup the return address
> +	 * so that stacktrace works from the rethook handler.
> +	 */
> +}
> +
> +/* This function will be called from each arch-defined trampoline. */
> +unsigned long rethook_trampoline_handler(struct pt_regs *regs,
> +					 unsigned long frame)
> +{
> +	struct llist_node *first, *node = NULL;
> +	unsigned long correct_ret_addr;
> +	rethook_handler_t handler;
> +	struct rethook_node *rhn;
> +
> +	correct_ret_addr = __rethook_find_ret_addr(current, &node);
> +	if (!correct_ret_addr) {
> +		pr_err("rethook: Return address not found! Maybe there is a bug in the kernel\n");
> +		BUG_ON(1);
> +	}
> +
> +	instruction_pointer_set(regs, correct_ret_addr);
> +
> +	/*
> +	 * These loops must be protected from rethook_free_rcu() because those
> +	 * are accessing 'rhn->rethook'.
> +	 */
> +	preempt_disable();
> +
> +	/*
> +	 * Run the handler on the shadow stack. Do not unlink the list here because
> +	 * stackdump inside the handlers needs to decode it.
> +	 */
> +	first = current->rethooks.first;
> +	while (first) {
> +		rhn = container_of(first, struct rethook_node, llist);
> +		if (WARN_ON_ONCE(rhn->frame != frame))
> +			break;
> +		handler = READ_ONCE(rhn->rethook->handler);
> +		if (handler)
> +			handler(rhn, rhn->rethook->data, regs);
> +
> +		if (first == node)
> +			break;
> +		first = first->next;
> +	}
> +
> +	/* Fixup registers for returning to correct address. */
> +	arch_rethook_fixup_return(regs, correct_ret_addr);
> +
> +	/* Unlink used shadow stack */
> +	first = current->rethooks.first;
> +	current->rethooks.first = node->next;
> +	node->next = NULL;
> +
> +	while (first) {
> +		rhn = container_of(first, struct rethook_node, llist);
> +		first = first->next;
> +		rethook_recycle(rhn);
> +	}
> +	preempt_enable();
> +
> +	return correct_ret_addr;
> +}
> +NOKPROBE_SYMBOL(rethook_trampoline_handler);

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