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SubjectRe: [RFC][ PATCH -tip v2 2/7] kprobes: introducing generic insn_slot framework

Hi Masami,

I'm currently traveling so my responses are very slow this week.


On Mon, 22 Jun 2009, Masami Hiramatsu wrote:

> Make insn_slot framework support various size slots.
> Current insn_slot just supports one-size instruction buffer slot. However,
> kprobes jump optimization needs larger size buffers.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
> Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Jim Keniston <jkenisto@us.ibm.com>
> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Anders Kaseorg <andersk@ksplice.com>
> Cc: Tim Abbott <tabbott@ksplice.com>
> ---
>
> kernel/kprobes.c | 96 +++++++++++++++++++++++++++++++++---------------------
> 1 files changed, 58 insertions(+), 38 deletions(-)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 0b68fdc..bc9cfd0 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -100,26 +100,38 @@ static struct kprobe_blackpoint kprobe_blacklist[] = {
> * stepping on the instruction on a vmalloced/kmalloced/data page
> * is a recipe for disaster
> */
> -#define INSNS_PER_PAGE (PAGE_SIZE/(MAX_INSN_SIZE * sizeof(kprobe_opcode_t)))
> -
> struct kprobe_insn_page {
> struct list_head list;
> kprobe_opcode_t *insns; /* Page of instruction slots */
> - char slot_used[INSNS_PER_PAGE];
> int nused;
> int ngarbage;
> + char slot_used[1];

I would recommend using [] instead of [1], that would help other
developers know that it is a variable array.



> +};
> +
> +struct kprobe_insn_cache {
> + struct list_head pages; /* list of kprobe_insn_page */
> + size_t insn_size; /* size of instruction slot */
> + int nr_garbage;
> };
>
> +static int slots_per_page(struct kprobe_insn_cache *c)
> +{
> + return PAGE_SIZE/(c->insn_size * sizeof(kprobe_opcode_t));
> +}
> +
> enum kprobe_slot_state {
> SLOT_CLEAN = 0,
> SLOT_DIRTY = 1,
> SLOT_USED = 2,
> };
>
> -static DEFINE_MUTEX(kprobe_insn_mutex); /* Protects kprobe_insn_pages */
> -static LIST_HEAD(kprobe_insn_pages);
> -static int kprobe_garbage_slots;
> -static int collect_garbage_slots(void);
> +static DEFINE_MUTEX(kprobe_insn_mutex); /* Protects kprobe_insn_slots */
> +static struct kprobe_insn_cache kprobe_insn_slots = {
> + .pages = LIST_HEAD_INIT(kprobe_insn_slots.pages),
> + .insn_size = MAX_INSN_SIZE,
> + .nr_garbage = 0,
> +};
> +static int __kprobes collect_garbage_slots(struct kprobe_insn_cache *c);
>
> static int __kprobes check_safety(void)
> {
> @@ -149,32 +161,33 @@ loop_end:
> * __get_insn_slot() - Find a slot on an executable page for an instruction.
> * We allocate an executable page if there's no room on existing ones.
> */
> -static kprobe_opcode_t __kprobes *__get_insn_slot(void)
> +static kprobe_opcode_t __kprobes *__get_insn_slot(struct kprobe_insn_cache *c)
> {
> struct kprobe_insn_page *kip;
>
> retry:
> - list_for_each_entry(kip, &kprobe_insn_pages, list) {
> - if (kip->nused < INSNS_PER_PAGE) {
> + list_for_each_entry(kip, &c->pages, list) {
> + if (kip->nused < slots_per_page(c)) {
> int i;
> - for (i = 0; i < INSNS_PER_PAGE; i++) {
> + for (i = 0; i < slots_per_page(c); i++) {
> if (kip->slot_used[i] == SLOT_CLEAN) {
> kip->slot_used[i] = SLOT_USED;
> kip->nused++;
> - return kip->insns + (i * MAX_INSN_SIZE);
> + return kip->insns + (i * c->insn_size);
> }
> }
> - /* Surprise! No unused slots. Fix kip->nused. */
> - kip->nused = INSNS_PER_PAGE;
> + /* kip->nused is broken. */
> + BUG();

Does this deserve a bug, or can we get away with a WARN and find a way to
fail nicely? Is it already too late to recover?

> }
> }
>
> /* If there are any garbage slots, collect it and try again. */
> - if (kprobe_garbage_slots && collect_garbage_slots() == 0) {
> + if (c->nr_garbage && collect_garbage_slots(c) == 0)
> goto retry;
> - }
> +
> /* All out of space. Need to allocate a new page. Use slot 0. */
> - kip = kmalloc(sizeof(struct kprobe_insn_page), GFP_KERNEL);
> + kip = kmalloc(sizeof(struct kprobe_insn_page) + slots_per_page(c) - 1,

Why the '- 1'? Is it because of the char [1] above?

Would be better to make the size of the kprobe_insn_page a macro:

#define KPROBE_INSN_SIZE offsetof(struct kbrobe_insn_page, slot_used)

and then you can do the following:

kip = kmalloc(KPROBE_INSN_SIZE + slots_per_page(c));

-- Steve


> + GFP_KERNEL);
> if (!kip)
> return NULL;
>
> @@ -189,19 +202,20 @@ static kprobe_opcode_t __kprobes *__get_insn_slot(void)
> return NULL;
> }
> INIT_LIST_HEAD(&kip->list);
> - list_add(&kip->list, &kprobe_insn_pages);
> - memset(kip->slot_used, SLOT_CLEAN, INSNS_PER_PAGE);
> + memset(kip->slot_used, SLOT_CLEAN, slots_per_page(c));
> kip->slot_used[0] = SLOT_USED;
> kip->nused = 1;
> kip->ngarbage = 0;
> + list_add(&kip->list, &c->pages);
> return kip->insns;
> }
>
> +
> kprobe_opcode_t __kprobes *get_insn_slot(void)
> {
> - kprobe_opcode_t *ret;
> + kprobe_opcode_t *ret = NULL;
> mutex_lock(&kprobe_insn_mutex);
> - ret = __get_insn_slot();
> + ret = __get_insn_slot(&kprobe_insn_slots);
> mutex_unlock(&kprobe_insn_mutex);
> return ret;
> }
> @@ -218,7 +232,7 @@ static int __kprobes collect_one_slot(struct kprobe_insn_page *kip, int idx)
> * so as not to have to set it up again the
> * next time somebody inserts a probe.
> */
> - if (!list_is_singular(&kprobe_insn_pages)) {
> + if (!list_is_singular(&kip->list)) {
> list_del(&kip->list);
> module_free(NULL, kip->insns);
> kfree(kip);
> @@ -228,7 +242,7 @@ static int __kprobes collect_one_slot(struct kprobe_insn_page *kip, int idx)
> return 0;
> }
>
> -static int __kprobes collect_garbage_slots(void)
> +static int __kprobes collect_garbage_slots(struct kprobe_insn_cache *c)
> {
> struct kprobe_insn_page *kip, *next;
> int safety;
> @@ -240,42 +254,48 @@ static int __kprobes collect_garbage_slots(void)
> if (safety != 0)
> return -EAGAIN;
>
> - list_for_each_entry_safe(kip, next, &kprobe_insn_pages, list) {
> + list_for_each_entry_safe(kip, next, &c->pages, list) {
> int i;
> if (kip->ngarbage == 0)
> continue;
> kip->ngarbage = 0; /* we will collect all garbages */
> - for (i = 0; i < INSNS_PER_PAGE; i++) {
> + for (i = 0; i < slots_per_page(c); i++) {
> if (kip->slot_used[i] == SLOT_DIRTY &&
> collect_one_slot(kip, i))
> break;
> }
> }
> - kprobe_garbage_slots = 0;
> + c->nr_garbage = 0;
> return 0;
> }
>
> -void __kprobes free_insn_slot(kprobe_opcode_t * slot, int dirty)
> +static void __kprobes __free_insn_slot(struct kprobe_insn_cache *c,
> + kprobe_opcode_t *slot, int dirty)
> {
> struct kprobe_insn_page *kip;
>
> - mutex_lock(&kprobe_insn_mutex);
> - list_for_each_entry(kip, &kprobe_insn_pages, list) {
> - if (kip->insns <= slot &&
> - slot < kip->insns + (INSNS_PER_PAGE * MAX_INSN_SIZE)) {
> - int i = (slot - kip->insns) / MAX_INSN_SIZE;
> + list_for_each_entry(kip, &c->pages, list) {
> + long idx = ((long)slot - (long)kip->insns) / c->insn_size;
> + if (idx >= 0 && idx < slots_per_page(c)) {
> + WARN_ON(kip->slot_used[idx] != SLOT_USED);
> if (dirty) {
> - kip->slot_used[i] = SLOT_DIRTY;
> + kip->slot_used[idx] = SLOT_DIRTY;
> kip->ngarbage++;
> + if (++c->nr_garbage > slots_per_page(c))
> + collect_garbage_slots(c);
> } else
> - collect_one_slot(kip, i);
> - break;
> + collect_one_slot(kip, idx);
> + return;
> }
> }
> + /* Could not free this slot. */
> + WARN_ON(1);
> +}
>
> - if (dirty && ++kprobe_garbage_slots > INSNS_PER_PAGE)
> - collect_garbage_slots();
> -
> +void __kprobes free_insn_slot(kprobe_opcode_t * slot, int dirty)
> +{
> + mutex_lock(&kprobe_insn_mutex);
> + __free_insn_slot(&kprobe_insn_slots, slot, dirty);
> mutex_unlock(&kprobe_insn_mutex);
> }
> #endif
>
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
>
> e-mail: mhiramat@redhat.com
>


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