Re: [PATCH v5 2/9] fprobe: Add ftrace based probe APIs

[Steven Rostedt]

On Tue, 25 Jan 2022 21:11:57 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> The fprobe is a wrapper API for ftrace function tracer.
> Unlike kprobes, this probes only supports the function entry, but
> it can probe multiple functions by one fprobe. The usage is almost
> same as the kprobe, user will specify the function names by
> fprobe::syms, the number of syms by fprobe::nentry,
> and the user handler by fprobe::entry_handler.
> 
> struct fprobe fp = { 0 };
> const char *targets[] = { "func1", "func2", "func3"};
> 
> fp.handler = user_handler;
> fp.nentry = ARRAY_SIZE(targets);
> fp.syms = targets;
> 
> ret = register_fprobe(&fp);
> 
> CAUTION: if user entry handler changes registers including
> ip address, it will be applied when returns from the
> entry handler. So user handler must recover it.

Can you rephrase the above, I'm not sure what you mean by it.

> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  Changes in v4:
>   - Fix a memory leak when symbol lookup failed.
>   - Use ftrace location address instead of symbol address.
>   - Convert the given symbol address to ftrace location automatically.
>   - Rename fprobe::ftrace to fprobe::ops.
>   - Update the Kconfig description.
> ---
>  include/linux/fprobe.h |   80 ++++++++++++++++++++++++++++
>  kernel/trace/Kconfig   |   12 ++++
>  kernel/trace/Makefile  |    1 
>  kernel/trace/fprobe.c  |  135 ++++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 228 insertions(+)
>  create mode 100644 include/linux/fprobe.h
>  create mode 100644 kernel/trace/fprobe.c
> 
> diff --git a/include/linux/fprobe.h b/include/linux/fprobe.h
> new file mode 100644
> index 000000000000..f7de332b08c2
> --- /dev/null
> +++ b/include/linux/fprobe.h
> @@ -0,0 +1,80 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Simple ftrace probe wrapper */
> +#ifndef _LINUX_FPROBE_H
> +#define _LINUX_FPROBE_H
> +
> +#include <linux/compiler.h>
> +#include <linux/ftrace.h>
> +
> +/**
> + * struct fprobe - ftrace based probe.
> + * @syms: The array of symbols to probe.
> + * @addrs: The array of ftrace address of the symbols.
> + * @nentry: The number of entries of @syms or @addrs.
> + * @ops: The ftrace_ops.
> + * @nmissed: The counter for missing events.
> + * @flags: The status flag.
> + * @entry_handler: The callback function for function entry.
> + *
> + * User must set either @syms or @addrs, but not both. If user sets
> + * only @syms, the @addrs are generated when registering the fprobe.
> + * That auto-generated @addrs will be freed when unregistering.
> + */
> +struct fprobe {
> +	const char		**syms;
> +	unsigned long		*addrs;
> +	unsigned int		nentry;
> +
> +	struct ftrace_ops	ops;
> +	unsigned long		nmissed;
> +	unsigned int		flags;
> +	void (*entry_handler)(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs);
> +};
> +
> +#define FPROBE_FL_DISABLED	1
> +
> +static inline bool fprobe_disabled(struct fprobe *fp)
> +{
> +	return (fp) ? fp->flags & FPROBE_FL_DISABLED : false;
> +}
> +
> +#ifdef CONFIG_FPROBE
> +int register_fprobe(struct fprobe *fp);
> +int unregister_fprobe(struct fprobe *fp);
> +#else
> +static inline int register_fprobe(struct fprobe *fp)
> +{
> +	return -EOPNOTSUPP;
> +}
> +static inline int unregister_fprobe(struct fprobe *fp)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif
> +
> +/**
> + * disable_fprobe() - Disable fprobe
> + * @fp: The fprobe to be disabled.
> + *
> + * This will soft-disable @fp. Note that this doesn't remove the ftrace
> + * hooks from the function entry.
> + */
> +static inline void disable_fprobe(struct fprobe *fp)
> +{
> +	if (fp)
> +		fp->flags |= FPROBE_FL_DISABLED;
> +}
> +
> +/**
> + * enable_fprobe() - Enable fprobe
> + * @fp: The fprobe to be enabled.
> + *
> + * This will soft-enable @fp.
> + */
> +static inline void enable_fprobe(struct fprobe *fp)
> +{
> +	if (fp)
> +		fp->flags &= ~FPROBE_FL_DISABLED;
> +}
> +
> +#endif
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 420ff4bc67fd..23483dd474b0 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -223,6 +223,18 @@ config DYNAMIC_FTRACE_WITH_ARGS
>  	depends on DYNAMIC_FTRACE
>  	depends on HAVE_DYNAMIC_FTRACE_WITH_ARGS
>  
> +config FPROBE
> +	bool "Kernel Function Probe (fprobe)"
> +	depends on FUNCTION_TRACER
> +	depends on DYNAMIC_FTRACE_WITH_REGS
> +	default n
> +	help
> +	  This option enables kernel function probe (fprobe) based on ftrace,
> +	  which is similar to kprobes, but probes only for kernel function
> +	  entries and it can probe multiple functions by one fprobe.
> +
> +	  If unsure, say N.
> +
>  config FUNCTION_PROFILER
>  	bool "Kernel function profiler"
>  	depends on FUNCTION_TRACER
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index bedc5caceec7..79255f9de9a4 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -97,6 +97,7 @@ obj-$(CONFIG_PROBE_EVENTS) += trace_probe.o
>  obj-$(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_TRACEPOINT_BENCHMARK) += trace_benchmark.o
>  
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> new file mode 100644
> index 000000000000..748cc34765c1
> --- /dev/null
> +++ b/kernel/trace/fprobe.c
> @@ -0,0 +1,135 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * fprobe - Simple ftrace probe wrapper for function entry.
> + */
> +#define pr_fmt(fmt) "fprobe: " fmt
> +
> +#include <linux/fprobe.h>
> +#include <linux/kallsyms.h>
> +#include <linux/kprobes.h>
> +#include <linux/slab.h>
> +#include <linux/sort.h>
> +
> +static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
> +			   struct ftrace_ops *ops, struct ftrace_regs *fregs)
> +{
> +	struct fprobe *fp;
> +	int bit;
> +
> +	fp = container_of(ops, struct fprobe, ops);
> +	if (fprobe_disabled(fp))
> +		return;
> +
> +	bit = ftrace_test_recursion_trylock(ip, parent_ip);
> +	if (bit < 0) {
> +		fp->nmissed++;
> +		return;
> +	}
> +
> +	if (fp->entry_handler)
> +		fp->entry_handler(fp, ip, ftrace_get_regs(fregs));
> +
> +	ftrace_test_recursion_unlock(bit);
> +}
> +NOKPROBE_SYMBOL(fprobe_handler);
> +
> +/* Convert ftrace location address from symbols */
> +static int convert_func_addresses(struct fprobe *fp)
> +{
> +	unsigned long addr, size;
> +	unsigned int i;
> +
> +	/* Convert symbols to symbol address */
> +	if (fp->syms) {
> +		fp->addrs = kcalloc(fp->nentry, sizeof(*fp->addrs), GFP_KERNEL);
> +		if (!fp->addrs)
> +			return -ENOMEM;
> +
> +		for (i = 0; i < fp->nentry; i++) {
> +			fp->addrs[i] = kallsyms_lookup_name(fp->syms[i]);
> +			if (!fp->addrs[i])	/* Maybe wrong symbol */
> +				goto error;
> +		}
> +	}

I wonder if we should just copy the addrs when fp->syms is not set, and
not have to worry about not freeing addrs (see below). This will make
things easier to maintain. Or better yet, have the syms and addrs passed
in, and then we assign it.

static int convert_func_addresses(struct fprobe *fp, const char **syms,
				  unsigned long *addrs)
{
	unsigned int i;

	fp->addrs = kcalloc(fp->nentry, sizeof(*fp->addrs), GFP_KERNEL);
	if (!fp->addrs)
		return -ENOMEM;

	if (syms) {
		for (i = 0; i < fp->nentry; i++) {
			fp->addrs[i] = kallsyms_lookup_name(fp->syms[i]);
			if (!fp->addrs[i])	/* Maybe wrong symbol */
				goto error;
		}
	} else {
		memcpy(fp->addrs, addrs, fp->nentry * sizeof(*addrs));
	}

> +
> +	/* Convert symbol address to ftrace location. */
> +	for (i = 0; i < fp->nentry; i++) {
> +		if (!kallsyms_lookup_size_offset(fp->addrs[i], &size, NULL))
> +			size = MCOUNT_INSN_SIZE;
> +		addr = ftrace_location_range(fp->addrs[i], fp->addrs[i] + size);
> +		if (!addr) /* No dynamic ftrace there. */
> +			goto error;
> +		fp->addrs[i] = addr;
> +	}
> +
> +	return 0;
> +
> +error:
> +	kfree(fp->addrs);

The above doesn't check if fp->syms was set, so if it wasn't we just freed
the addrs that was passed in. Again, I think these should be passed into
the register function as separate parameters and not via the fp handle.

> +	fp->addrs = NULL;
> +	return -ENOENT;
> +}
> +
> +/**
> + * register_fprobe() - Register fprobe to ftrace
> + * @fp: A fprobe data structure to be registered.
> + *
> + * This expects the user set @fp::entry_handler, @fp::syms or @fp:addrs,
> + * and @fp::nentry. If @fp::addrs are set, that will be updated to point
> + * the ftrace location. If @fp::addrs are NULL, this will generate it
> + * from @fp::syms.
> + * Note that you do not set both of @fp::addrs and @fp::syms.

Again, I think this should pass in the syms and addrs as parameters.

-- Steve

> + */
> +int register_fprobe(struct fprobe *fp)
> +{
> +	int ret;
> +
> +	if (!fp || !fp->nentry || (!fp->syms && !fp->addrs) ||
> +	    (fp->syms && fp->addrs))
> +		return -EINVAL;
> +
> +	ret = convert_func_addresses(fp);
> +	if (ret < 0)
> +		return ret;
> +
> +	fp->nmissed = 0;
> +	fp->ops.func = fprobe_handler;
> +	fp->ops.flags = FTRACE_OPS_FL_SAVE_REGS;
> +
> +	ret = ftrace_set_filter_ips(&fp->ops, fp->addrs, fp->nentry, 0, 0);
> +	if (!ret)
> +		ret = register_ftrace_function(&fp->ops);
> +
> +	if (ret < 0 && fp->syms) {
> +		kfree(fp->addrs);
> +		fp->addrs = NULL;
> +	}
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(register_fprobe);
> +
> +/**
> + * unregister_fprobe() - Unregister fprobe from ftrace
> + * @fp: A fprobe data structure to be unregistered.
> + *
> + * Unregister fprobe (and remove ftrace hooks from the function entries).
> + * If the @fp::addrs are generated by register_fprobe(), it will be removed
> + * automatically.
> + */
> +int unregister_fprobe(struct fprobe *fp)
> +{
> +	int ret;
> +
> +	if (!fp || !fp->nentry || !fp->addrs)
> +		return -EINVAL;
> +
> +	ret = unregister_ftrace_function(&fp->ops);
> +
> +	if (!ret && fp->syms) {
> +		kfree(fp->addrs);
> +		fp->addrs = NULL;
> +	}
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(unregister_fprobe);