[PATCH v5 7/9] fprobe: Add exit_handler support

Masami Hiramatsu
2022-01-25T12:15:17Z

Add exit_handler to fprobe. fprobe + rethook allows us
to hook the kernel function return without fgraph tracer.
Eventually, the fgraph tracer will be generic array based
return hooking and fprobe may use it if user requests.
Since both array-based approach and list-based approach
have Pros and Cons, (e.g. memory consumption v.s. less
missing events) it is better to keep both but fprobe
will provide the same exit-handler interface.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
Changes in v5:
- Add dependency for HAVE_RETHOOK.
Changes in v4:
- Check fprobe is disabled in the exit handler.
Changes in v3:
- Make sure to clear rethook->data before free.
- Handler checks the data is not NULL.
- Free rethook only if the rethook is using.
---
include/linux/fprobe.h | 6 ++++
kernel/trace/Kconfig | 2 +
kernel/trace/fprobe.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/include/linux/fprobe.h b/include/linux/fprobe.h
index f7de332b08c2..4692089b9118 100644
--- a/include/linux/fprobe.h
+++ b/include/linux/fprobe.h
@@ -5,6 +5,7 @@

#include <linux/compiler.h>
#include <linux/ftrace.h>
+#include <linux/rethook.h>

/**
* struct fprobe - ftrace based probe.
@@ -14,7 +15,9 @@
* @ops: The ftrace_ops.
* @nmissed: The counter for missing events.
* @flags: The status flag.
+ * @rethook: The rethook data structure. (internal data)
* @entry_handler: The callback function for function entry.
+ * @exit_handler: The callback function for function exit.
*
* User must set either @syms or @addrs, but not both. If user sets
* only @syms, the @addrs are generated when registering the fprobe.
@@ -28,7 +31,10 @@ struct fprobe {
struct ftrace_ops ops;
unsigned long nmissed;
unsigned int flags;
+ struct rethook *rethook;
+
void (*entry_handler)(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs);
+ void (*exit_handler)(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs);
};

#define FPROBE_FL_DISABLED 1
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 4d27e56c6e76..f47213140499 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -238,6 +238,8 @@ config FPROBE
bool "Kernel Function Probe (fprobe)"
depends on FUNCTION_TRACER
depends on DYNAMIC_FTRACE_WITH_REGS
+ depends on HAVE_RETHOOK
+ select RETHOOK
default n
help
This option enables kernel function probe (fprobe) based on ftrace,
diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 748cc34765c1..4d089dda89c2 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -7,12 +7,20 @@
#include <linux/fprobe.h>
#include <linux/kallsyms.h>
#include <linux/kprobes.h>
+#include <linux/rethook.h>
#include <linux/slab.h>
#include <linux/sort.h>

+struct fprobe_rethook_node {
+ struct rethook_node node;
+ unsigned long entry_ip;
+};
+
static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
struct ftrace_ops *ops, struct ftrace_regs *fregs)
{
+ struct fprobe_rethook_node *fpr;
+ struct rethook_node *rh;
struct fprobe *fp;
int bit;

@@ -29,10 +37,37 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
if (fp->entry_handler)
fp->entry_handler(fp, ip, ftrace_get_regs(fregs));

+ if (fp->exit_handler) {
+ rh = rethook_try_get(fp->rethook);
+ if (!rh) {
+ fp->nmissed++;
+ goto out;
+ }
+ fpr = container_of(rh, struct fprobe_rethook_node, node);
+ fpr->entry_ip = ip;
+ rethook_hook(rh, ftrace_get_regs(fregs));
+ }
+
+out:
ftrace_test_recursion_unlock(bit);
}
NOKPROBE_SYMBOL(fprobe_handler);

+static void fprobe_exit_handler(struct rethook_node *rh, void *data,
+ struct pt_regs *regs)
+{
+ struct fprobe *fp = (struct fprobe *)data;
+ struct fprobe_rethook_node *fpr;
+
+ if (!fp || fprobe_disabled(fp))
+ return;
+
+ fpr = container_of(rh, struct fprobe_rethook_node, node);
+
+ fp->exit_handler(fp, fpr->entry_ip, regs);
+}
+NOKPROBE_SYMBOL(fprobe_exit_handler);
+
/* Convert ftrace location address from symbols */
static int convert_func_addresses(struct fprobe *fp)
{
@@ -82,6 +117,7 @@ static int convert_func_addresses(struct fprobe *fp)
*/
int register_fprobe(struct fprobe *fp)
{
+ unsigned int i, size;
int ret;

if (!fp || !fp->nentry || (!fp->syms && !fp->addrs) ||
@@ -96,10 +132,29 @@ int register_fprobe(struct fprobe *fp)
fp->ops.func = fprobe_handler;
fp->ops.flags = FTRACE_OPS_FL_SAVE_REGS;

+ /* Initialize rethook if needed */
+ if (fp->exit_handler) {
+ size = fp->nentry * num_possible_cpus() * 2;
+ fp->rethook = rethook_alloc((void *)fp, fprobe_exit_handler);
+ for (i = 0; i < size; i++) {
+ struct rethook_node *node;
+
+ node = kzalloc(sizeof(struct fprobe_rethook_node), GFP_KERNEL);
+ if (!node) {
+ rethook_free(fp->rethook);
+ ret = -ENOMEM;
+ goto out;
+ }
+ rethook_add_node(fp->rethook, node);
+ }
+ } else
+ fp->rethook = NULL;
+
ret = ftrace_set_filter_ips(&fp->ops, fp->addrs, fp->nentry, 0, 0);
if (!ret)
ret = register_ftrace_function(&fp->ops);

+out:
if (ret < 0 && fp->syms) {
kfree(fp->addrs);
fp->addrs = NULL;
@@ -125,8 +180,16 @@ int unregister_fprobe(struct fprobe *fp)
return -EINVAL;

ret = unregister_ftrace_function(&fp->ops);
+ if (ret < 0)
+ return ret;

- if (!ret && fp->syms) {
+ if (fp->rethook) {
+ /* Make sure to clear rethook->data before freeing. */
+ WRITE_ONCE(fp->rethook->data, NULL);
+ barrier();
+ rethook_free(fp->rethook);
+ }
+ if (fp->syms) {
kfree(fp->addrs);
fp->addrs = NULL;
}