[PATCH v7 14/36] function_graph: Use a simple LRU for fgraph_array index number

Masami Hiramatsu (Google)
2024-02-06T15:10:10Z

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Since the fgraph_array index is used for the bitmap on the shadow
stack, it may leave some entries after a function_graph instance is
removed. Thus if another instance reuses the fgraph_array index soon
after releasing it, the fgraph may confuse to call the newer callback
for the entries which are pushed by the older instance.
To avoid reusing the fgraph_array index soon after releasing, introduce
a simple LRU table for managing the index number. This will reduce the
possibility of this confusion.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v5:
- Fix the underflow bug in fgraph_lru_release_index() and return 0
if the release is succeded.
Changes in v4:
- Newly added.
---
kernel/trace/fgraph.c | 67 ++++++++++++++++++++++++++++++++++---------------
1 file changed, 47 insertions(+), 20 deletions(-)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index ae42de909845..323a74623543 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -99,10 +99,44 @@ enum {
DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);
int ftrace_graph_active;

-static int fgraph_array_cnt;
-
static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE];

+/* LRU index table for fgraph_array */
+static int fgraph_lru_table[FGRAPH_ARRAY_SIZE];
+static int fgraph_lru_next;
+static int fgraph_lru_last;
+
+static void fgraph_lru_init(void)
+{
+ int i;
+
+ for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
+ fgraph_lru_table[i] = i;
+}
+
+static int fgraph_lru_release_index(int idx)
+{
+ if (idx < 0 || idx >= FGRAPH_ARRAY_SIZE ||
+ fgraph_lru_table[fgraph_lru_last] != -1)
+ return -1;
+
+ fgraph_lru_table[fgraph_lru_last] = idx;
+ fgraph_lru_last = (fgraph_lru_last + 1) % FGRAPH_ARRAY_SIZE;
+ return 0;
+}
+
+static int fgraph_lru_alloc_index(void)
+{
+ int idx = fgraph_lru_table[fgraph_lru_next];
+
+ if (idx == -1)
+ return -1;
+
+ fgraph_lru_table[fgraph_lru_next] = -1;
+ fgraph_lru_next = (fgraph_lru_next + 1) % FGRAPH_ARRAY_SIZE;
+ return idx;
+}
+
static inline int get_ret_stack_index(struct task_struct *t, int offset)
{
return t->ret_stack[offset] & FGRAPH_RET_INDEX_MASK;
@@ -367,7 +401,7 @@ int function_graph_enter(unsigned long ret, unsigned long func,
if (index < 0)
goto out;

- for (i = 0; i < fgraph_array_cnt; i++) {
+ for (i = 0; i < FGRAPH_ARRAY_SIZE; i++) {
struct fgraph_ops *gops = fgraph_array[i];

if (gops == &fgraph_stub)
@@ -935,21 +969,17 @@ int register_ftrace_graph(struct fgraph_ops *gops)
/* The array must always have real data on it */
for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
fgraph_array[i] = &fgraph_stub;
+ fgraph_lru_init();
}

- /* Look for an available spot */
- for (i = 0; i < FGRAPH_ARRAY_SIZE; i++) {
- if (fgraph_array[i] == &fgraph_stub)
- break;
- }
- if (i >= FGRAPH_ARRAY_SIZE) {
+ i = fgraph_lru_alloc_index();
+ if (i < 0 ||
+ WARN_ON_ONCE(fgraph_array[i] != &fgraph_stub)) {
ret = -EBUSY;
goto out;
}

fgraph_array[i] = gops;
- if (i + 1 > fgraph_array_cnt)
- fgraph_array_cnt = i + 1;
gops->idx = i;

ftrace_graph_active++;
@@ -979,25 +1009,22 @@ int register_ftrace_graph(struct fgraph_ops *gops)
void unregister_ftrace_graph(struct fgraph_ops *gops)
{
int command = 0;
- int i;

mutex_lock(&ftrace_lock);

if (unlikely(!ftrace_graph_active))
goto out;

- if (unlikely(gops->idx < 0 || gops->idx >= fgraph_array_cnt))
+ if (unlikely(gops->idx < 0 || gops->idx >= FGRAPH_ARRAY_SIZE))
+ goto out;
+
+ if (WARN_ON_ONCE(fgraph_array[gops->idx] != gops))
goto out;

- WARN_ON_ONCE(fgraph_array[gops->idx] != gops);
+ if (fgraph_lru_release_index(gops->idx) < 0)
+ goto out;

fgraph_array[gops->idx] = &fgraph_stub;
- if (gops->idx + 1 == fgraph_array_cnt) {
- i = gops->idx;
- while (i >= 0 && fgraph_array[i] == &fgraph_stub)
- i--;
- fgraph_array_cnt = i + 1;
- }

ftrace_graph_active--;