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    Subject[PATCH v8 13/35] function_graph: Use a simple LRU for fgraph_array index number
    Date
    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 v8:
    - Add a WARN_ON_ONCE() if fgraph_lru_table[] is broken when releasing
    index, and remove WARN_ON_ONCE() from unregister_ftrace_graph()
    - Fix to release allocated index if register_ftrace_graph() fails.
    - Add comments and code cleanup.
    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 | 72 +++++++++++++++++++++++++++++++++++--------------
    1 file changed, 51 insertions(+), 21 deletions(-)

    diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
    index ae42de909845..da5f269ce718 100644
    --- a/kernel/trace/fgraph.c
    +++ b/kernel/trace/fgraph.c
    @@ -99,10 +99,48 @@ 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;
    +
    +/* Initialize fgraph_lru_table with unused index */
    +static void fgraph_lru_init(void)
    +{
    + int i;
    +
    + for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
    + fgraph_lru_table[i] = i;
    +}
    +
    +/* Release the used index to the LRU table */
    +static int fgraph_lru_release_index(int idx)
    +{
    + if (idx < 0 || idx >= FGRAPH_ARRAY_SIZE ||
    + WARN_ON_ONCE(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;
    +}
    +
    +/* Allocate a new index from LRU table */
    +static int fgraph_lru_alloc_index(void)
    +{
    + int idx = fgraph_lru_table[fgraph_lru_next];
    +
    + /* No id is available */
    + 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 +405,7 @@ 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)
    @@ -919,7 +957,7 @@ int register_ftrace_graph(struct fgraph_ops *gops)
    {
    int command = 0;
    int ret = 0;
    - int i;
    + int i = -1;

    mutex_lock(&ftrace_lock);

    @@ -935,21 +973,16 @@ 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++;
    @@ -972,6 +1005,8 @@ int register_ftrace_graph(struct fgraph_ops *gops)

    ret = ftrace_startup(&gops->ops, command);
    out:
    + if (ret && i >= 0)
    + fgraph_lru_release_index(i);
    mutex_unlock(&ftrace_lock);
    return ret;
    }
    @@ -979,25 +1014,20 @@ 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 ||
    + 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--;


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