Re: [PATCH v8 3/6] Uprobes: Support SDT markers having reference count (semaphore)

Song Liu
2018-08-11T07:58:24Z

On Wed, Aug 8, 2018 at 9:18 PM, Ravi Bangoria
<ravi.bangoria@linux.ibm.com> wrote:
> Userspace Statically Defined Tracepoints[1] are dtrace style markers
> inside userspace applications. Applications like PostgreSQL, MySQL,
> Pthread, Perl, Python, Java, Ruby, Node.js, libvirt, QEMU, glib etc
> have these markers embedded in them. These markers are added by developer
> at important places in the code. Each marker source expands to a single
> nop instruction in the compiled code but there may be additional
> overhead for computing the marker arguments which expands to couple of
> instructions. In case the overhead is more, execution of it can be
> omitted by runtime if() condition when no one is tracing on the marker:
>
> if (reference_counter > 0) {
> Execute marker instructions;
> }
>
> Default value of reference counter is 0. Tracer has to increment the
> reference counter before tracing on a marker and decrement it when
> done with the tracing.
>
> Implement the reference counter logic in core uprobe. User will be
> able to use it from trace_uprobe as well as from kernel module. New
> trace_uprobe definition with reference counter will now be:
>
> <path>:<offset>[(ref_ctr_offset)]
>
> where ref_ctr_offset is an optional field. For kernel module, new
> variant of uprobe_register() has been introduced:
>
> uprobe_register_refctr(inode, offset, ref_ctr_offset, consumer)
>
> No new variant for uprobe_unregister() because it's assumed to have
> only one reference counter for one uprobe.
>
> [1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
>
> Note: 'reference counter' is called as 'semaphore' in original Dtrace
> (or Systemtap, bcc and even in ELF) documentation and code. But the
> term 'semaphore' is misleading in this context. This is just a counter
> used to hold number of tracers tracing on a marker. This is not really
> used for any synchronization. So we are referring it as 'reference
> counter' in kernel / perf code.
>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> [Only trace_uprobe.c]
> ---
> include/linux/uprobes.h | 5 +
> kernel/events/uprobes.c | 246 ++++++++++++++++++++++++++++++++++++++++++--
> kernel/trace/trace.c | 2 +-
> kernel/trace/trace_uprobe.c | 38 ++++++-
> 4 files changed, 280 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index bb9d2084af03..103a48a48872 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -123,6 +123,7 @@ extern unsigned long uprobe_get_swbp_addr(struct pt_regs *regs);
> extern unsigned long uprobe_get_trap_addr(struct pt_regs *regs);
> extern int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr, uprobe_opcode_t);
> extern int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
> +extern int uprobe_register_refctr(struct inode *inode, loff_t offset, loff_t ref_ctr_offset, struct uprobe_consumer *uc);
> extern int uprobe_apply(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, bool);
> extern void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
> extern int uprobe_mmap(struct vm_area_struct *vma);
> @@ -160,6 +161,10 @@ uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
> {
> return -ENOSYS;
> }
> +static inline int uprobe_register_refctr(struct inode *inode, loff_t offset, loff_t ref_ctr_offset, struct uprobe_consumer *uc)
> +{
> + return -ENOSYS;
> +}
> static inline int
> uprobe_apply(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, bool add)
> {
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index c0418ba52ba8..61b0481ef417 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -73,6 +73,7 @@ struct uprobe {
> struct uprobe_consumer *consumers;
> struct inode *inode; /* Also hold a ref to inode */
> loff_t offset;
> + loff_t ref_ctr_offset;
> unsigned long flags;
>
> /*
> @@ -88,6 +89,15 @@ struct uprobe {
> struct arch_uprobe arch;
> };
>
> +struct delayed_uprobe {
> + struct list_head list;
> + struct uprobe *uprobe;
> + struct mm_struct *mm;
> +};
> +
> +static DEFINE_MUTEX(delayed_uprobe_lock);
> +static LIST_HEAD(delayed_uprobe_list);
> +
> /*
> * Execute out of line area: anonymous executable mapping installed
> * by the probed task to execute the copy of the original instruction
> @@ -282,6 +292,157 @@ static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t
> return 1;
> }
>
> +static struct delayed_uprobe *
> +delayed_uprobe_check(struct uprobe *uprobe, struct mm_struct *mm)
> +{
> + struct delayed_uprobe *du;
> +
> + list_for_each_entry(du, &delayed_uprobe_list, list)
> + if (du->uprobe == uprobe && du->mm == mm)
> + return du;
> + return NULL;
> +}
> +
> +static int delayed_uprobe_add(struct uprobe *uprobe, struct mm_struct *mm)
> +{
> + struct delayed_uprobe *du;
> +
> + if (delayed_uprobe_check(uprobe, mm))
> + return 0;
> +
> + du = kzalloc(sizeof(*du), GFP_KERNEL);
> + if (!du)
> + return -ENOMEM;
> +
> + du->uprobe = uprobe;
> + du->mm = mm;
> + list_add(&du->list, &delayed_uprobe_list);
> + return 0;
> +}
> +
> +static void delayed_uprobe_delete(struct delayed_uprobe *du)
> +{
> + if (!du)
> + return;
Do we really need this check?

> + list_del(&du->list);
> + kfree(du);
> +}
> +
> +static void delayed_uprobe_remove(struct uprobe *uprobe, struct mm_struct *mm)
> +{
> + struct list_head *pos, *q;
> + struct delayed_uprobe *du;
> +
> + if (!uprobe && !mm)
> + return;
And do we really need this check?

> +
> + list_for_each_safe(pos, q, &delayed_uprobe_list) {
> + du = list_entry(pos, struct delayed_uprobe, list);
> +
> + if (uprobe && mm && du->uprobe == uprobe && du->mm == mm)
> + delayed_uprobe_delete(du);
> + else if (!uprobe && du->mm == mm)
> + delayed_uprobe_delete(du);
> + else if (!mm && du->uprobe == uprobe)
> + delayed_uprobe_delete(du);
> + }
> +}
> +
> +static bool valid_ref_ctr_vma(struct uprobe *uprobe,
> + struct vm_area_struct *vma)
> +{
> + unsigned long vaddr = offset_to_vaddr(vma, uprobe->ref_ctr_offset);
> +
> + return uprobe->ref_ctr_offset &&
> + vma->vm_file &&
> + file_inode(vma->vm_file) == uprobe->inode &&
> + (vma->vm_flags & (VM_WRITE|VM_SHARED)) == VM_WRITE &&
> + vma->vm_start <= vaddr &&
> + vma->vm_end > vaddr;
> +}
> +
> +static struct vm_area_struct *
> +find_ref_ctr_vma(struct uprobe *uprobe, struct mm_struct *mm)
> +{
> + struct vm_area_struct *tmp;
> +
> + for (tmp = mm->mmap; tmp; tmp = tmp->vm_next)
> + if (valid_ref_ctr_vma(uprobe, tmp))
> + return tmp;
> +
> + return NULL;
> +}
> +
> +static int
> +__update_ref_ctr(struct mm_struct *mm, unsigned long vaddr, short d)
> +{
> + void *kaddr;
> + struct page *page;
> + struct vm_area_struct *vma;
> + int ret = 0;
It is not necessary to initialize ret here.

> + short *ptr;
> +
> + if (vaddr == 0 || d == 0)
> + return -EINVAL;
> +
> + ret = get_user_pages_remote(NULL, mm, vaddr, 1,
> + FOLL_WRITE, &page, &vma, NULL);
> + if (unlikely(ret <= 0)) {
> + /*
> + * We are asking for 1 page. If get_user_pages_remote() fails,
> + * it may return 0, in that case we have to return error.
> + */
> + ret = (ret == 0) ? -EBUSY : ret;
> + pr_warn("Failed to %s ref_ctr. (%d)\n",
> + d > 0 ? "increment" : "decrement", ret);
This warning is not really useful. Seems this function has little information
about which uprobe is failing here. Maybe we only need warning in the caller
(or caller of caller).

> + return ret;
> + }
> +
> + kaddr = kmap_atomic(page);
> + ptr = kaddr + (vaddr & ~PAGE_MASK);
> +
> + if (unlikely(*ptr + d < 0)) {
> + pr_warn("ref_ctr going negative. vaddr: 0x%lx, "
> + "curr val: %d, delta: %d\n", vaddr, *ptr, d);
> + ret = -EINVAL;
> + goto out;
> + }
> +
> + *ptr += d;
> + ret = 0;
> +out:
> + kunmap_atomic(kaddr);
> + put_page(page);
> + return ret;
> +}
> +
> +static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm,
> + bool is_register)
What's the reason of bool is_register here vs. short d in __update_ref_ctr()?
Can we use short for both?

> +{
> + struct vm_area_struct *rc_vma;
> + unsigned long rc_vaddr;
> + int ret = 0;
> +
> + rc_vma = find_ref_ctr_vma(uprobe, mm);
> +
> + if (rc_vma) {
> + rc_vaddr = offset_to_vaddr(rc_vma, uprobe->ref_ctr_offset);
> + ret = __update_ref_ctr(mm, rc_vaddr, is_register ? 1 : -1);
> +
> + if (is_register)
> + return ret;
> + }
Mixing __update_ref_ctr() here and delayed_uprobe_add() in the same
function is a little confusing (at least for me). How about we always use
delayed uprobe for uprobe_mmap() and use non-delayed in other case(s)?

> +
> + mutex_lock(&delayed_uprobe_lock);
> + if (is_register)
> + ret = delayed_uprobe_add(uprobe, mm);
> + else
> + delayed_uprobe_remove(uprobe, mm);
> + mutex_unlock(&delayed_uprobe_lock);
> +
> + return ret;
> +}
> +
> /*
> * NOTE:
> * Expect the breakpoint instruction to be the smallest size instruction for
> @@ -302,9 +463,13 @@ static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t
> int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
> unsigned long vaddr, uprobe_opcode_t opcode)
> {
> + struct uprobe *uprobe;
> struct page *old_page, *new_page;
> struct vm_area_struct *vma;
> - int ret;
> + int ret, is_register, ref_ctr_updated = 0;
> +
> + is_register = is_swbp_insn(&opcode);
> + uprobe = container_of(auprobe, struct uprobe, arch);
>
> retry:
> /* Read the page with vaddr into memory */
> @@ -317,6 +482,15 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
> if (ret <= 0)
> goto put_old;
>
> + /* We are going to replace instruction, update ref_ctr. */
> + if (!ref_ctr_updated && uprobe->ref_ctr_offset) {
> + ret = update_ref_ctr(uprobe, mm, is_register);
> + if (ret)
> + goto put_old;
> +
> + ref_ctr_updated = 1;
> + }
> +
> ret = anon_vma_prepare(vma);
> if (ret)
> goto put_old;
> @@ -337,6 +511,11 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>
> if (unlikely(ret == -EAGAIN))
> goto retry;
> +
> + /* Revert back reference counter if instruction update failed. */
> + if (ret && is_register && ref_ctr_updated)
> + update_ref_ctr(uprobe, mm, false);
> +
> return ret;
> }
>
> @@ -378,8 +557,15 @@ static struct uprobe *get_uprobe(struct uprobe *uprobe)
>
> static void put_uprobe(struct uprobe *uprobe)
> {
> - if (atomic_dec_and_test(&uprobe->ref))
> + if (atomic_dec_and_test(&uprobe->ref)) {
> + /*
> + * If application munmap(exec_vma) before uprobe_unregister()
> + * gets called, we don't get a chance to remove uprobe from
> + * delayed_uprobe_list in remove_breakpoint(). Do it here.
> + */
> + delayed_uprobe_remove(uprobe, NULL);
> kfree(uprobe);
> + }
> }
>
> static int match_uprobe(struct uprobe *l, struct uprobe *r)
> @@ -484,7 +670,8 @@ static struct uprobe *insert_uprobe(struct uprobe *uprobe)
> return u;
> }
>
> -static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset)
> +static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset,
> + loff_t ref_ctr_offset)
> {
> struct uprobe *uprobe, *cur_uprobe;
>
> @@ -494,6 +681,7 @@ static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset)
>
> uprobe->inode = inode;
> uprobe->offset = offset;
> + uprobe->ref_ctr_offset = ref_ctr_offset;
> init_rwsem(&uprobe->register_rwsem);
> init_rwsem(&uprobe->consumer_rwsem);
>
> @@ -895,7 +1083,7 @@ EXPORT_SYMBOL_GPL(uprobe_unregister);
> * else return 0 (success)
> */
> static int __uprobe_register(struct inode *inode, loff_t offset,
> - struct uprobe_consumer *uc)
> + loff_t ref_ctr_offset, struct uprobe_consumer *uc)
> {
> struct uprobe *uprobe;
> int ret;
> @@ -912,7 +1100,7 @@ static int __uprobe_register(struct inode *inode, loff_t offset,
> return -EINVAL;
>
> retry:
> - uprobe = alloc_uprobe(inode, offset);
> + uprobe = alloc_uprobe(inode, offset, ref_ctr_offset);
> if (!uprobe)
> return -ENOMEM;
> /*
> @@ -938,10 +1126,17 @@ static int __uprobe_register(struct inode *inode, loff_t offset,
> int uprobe_register(struct inode *inode, loff_t offset,
> struct uprobe_consumer *uc)
> {
> - return __uprobe_register(inode, offset, uc);
> + return __uprobe_register(inode, offset, 0, uc);
> }
> EXPORT_SYMBOL_GPL(uprobe_register);
>
> +int uprobe_register_refctr(struct inode *inode, loff_t offset,
> + loff_t ref_ctr_offset, struct uprobe_consumer *uc)
> +{
> + return __uprobe_register(inode, offset, ref_ctr_offset, uc);
> +}
> +EXPORT_SYMBOL_GPL(uprobe_register_refctr);
> +
> /*
> * uprobe_apply - unregister a already registered probe.
> * @inode: the file in which the probe has to be removed.
> @@ -1060,6 +1255,31 @@ static void build_probe_list(struct inode *inode,
> spin_unlock(&uprobes_treelock);
> }
>
> +static int delayed_uprobe_install(struct vm_area_struct *vma)
This function name is confusing. How about we call it delayed_ref_ctr_incr() or
something similar? Also, we should add comments to highlight this is vma is not
the vma containing the uprobe, but the vma containing the ref_ctr.

> +{
> + struct list_head *pos, *q;
> + struct delayed_uprobe *du;
> + unsigned long vaddr;
> + int ret = 0, err = 0;
> +
> + mutex_lock(&delayed_uprobe_lock);
> + list_for_each_safe(pos, q, &delayed_uprobe_list) {
> + du = list_entry(pos, struct delayed_uprobe, list);
> +
> + if (!valid_ref_ctr_vma(du->uprobe, vma))
> + continue;
> +
> + vaddr = offset_to_vaddr(vma, du->uprobe->ref_ctr_offset);
> + ret = __update_ref_ctr(vma->vm_mm, vaddr, 1);
> + /* Record an error and continue. */
> + if (ret && !err)
> + err = ret;
I think this is a good place (when ret != 0) to call pr_warn(). I guess we can
print which mm get error for which uprobe (inode+offset).

> + delayed_uprobe_delete(du);
> + }
> + mutex_unlock(&delayed_uprobe_lock);
> + return err;
> +}
> +
> /*
> * Called from mmap_region/vma_adjust with mm->mmap_sem acquired.
> *
> @@ -1072,7 +1292,15 @@ int uprobe_mmap(struct vm_area_struct *vma)
> struct uprobe *uprobe, *u;
> struct inode *inode;
>
> - if (no_uprobe_events() || !valid_vma(vma, true))
> + if (no_uprobe_events())
> + return 0;
> +
> + if (vma->vm_file &&
> + (vma->vm_flags & (VM_WRITE|VM_SHARED)) == VM_WRITE &&
> + test_bit(MMF_HAS_UPROBES, &vma->vm_mm->flags))
> + delayed_uprobe_install(vma);
> +
> + if (!valid_vma(vma, true))
> return 0;
>
> inode = file_inode(vma->vm_file);
> @@ -1246,6 +1474,10 @@ void uprobe_clear_state(struct mm_struct *mm)
> {
> struct xol_area *area = mm->uprobes_state.xol_area;
>
> + mutex_lock(&delayed_uprobe_lock);
> + delayed_uprobe_remove(NULL, mm);
> + mutex_unlock(&delayed_uprobe_lock);
> +
> if (!area)
> return;
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 823687997b01..616160b85860 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4620,7 +4620,7 @@ static const char readme_msg[] =
> "place (kretprobe): [<module>:]<symbol>[+<offset>]|<memaddr>\n"
> #endif
> #ifdef CONFIG_UPROBE_EVENTS
> - "\t place: <path>:<offset>\n"
> + " place (uprobe): <path>:<offset>[(ref_ctr_offset)]\n"
> #endif
> "\t args: <name>=fetcharg[:type]\n"
> "\t fetcharg: %<register>, @ , @<symbol>[+|-<offset>],\n"
> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> index bf89a51e740d..bf2be098eb08 100644
> --- a/kernel/trace/trace_uprobe.c
> +++ b/kernel/trace/trace_uprobe.c
> @@ -59,6 +59,7 @@ struct trace_uprobe {
> struct inode *inode;
> char *filename;
> unsigned long offset;
> + unsigned long ref_ctr_offset;
> unsigned long nhit;
> struct trace_probe tp;
> };
> @@ -364,10 +365,10 @@ static int register_trace_uprobe(struct trace_uprobe *tu)
> static int create_trace_uprobe(int argc, char **argv)
> {
> struct trace_uprobe *tu;
> - char *arg, *event, *group, *filename;
> + char *arg, *event, *group, *filename, *rctr, *rctr_end;
> char buf[MAX_EVENT_NAME_LEN];
> struct path path;
> - unsigned long offset;
> + unsigned long offset, ref_ctr_offset;
> bool is_delete, is_return;
> int i, ret;
>
> @@ -376,6 +377,7 @@ static int create_trace_uprobe(int argc, char **argv)
> is_return = false;
> event = NULL;
> group = NULL;
> + ref_ctr_offset = 0;
>
> /* argc must be >= 1 */
> if (argv[0][0] == '-')
> @@ -450,6 +452,26 @@ static int create_trace_uprobe(int argc, char **argv)
> goto fail_address_parse;
> }
>
> + /* Parse reference counter offset if specified. */
> + rctr = strchr(arg, '(');
> + if (rctr) {
> + rctr_end = strchr(rctr, ')');
> + if (rctr > rctr_end || *(rctr_end + 1) != 0) {
> + ret = -EINVAL;
> + pr_info("Invalid reference counter offset.\n");
> + goto fail_address_parse;
> + }
> +
> + *rctr++ = '\0';
> + *rctr_end = '\0';
> + ret = kstrtoul(rctr, 0, &ref_ctr_offset);
> + if (ret) {
> + pr_info("Invalid reference counter offset.\n");
> + goto fail_address_parse;
> + }
> + }
> +
> + /* Parse uprobe offset. */
> ret = kstrtoul(arg, 0, &offset);
> if (ret)
> goto fail_address_parse;
> @@ -484,6 +506,7 @@ static int create_trace_uprobe(int argc, char **argv)
> goto fail_address_parse;
> }
> tu->offset = offset;
> + tu->ref_ctr_offset = ref_ctr_offset;
> tu->path = path;
> tu->filename = kstrdup(filename, GFP_KERNEL);
>
> @@ -602,6 +625,9 @@ static int probes_seq_show(struct seq_file *m, void *v)
> trace_event_name(&tu->tp.call), tu->filename,
> (int)(sizeof(void *) * 2), tu->offset);
>
> + if (tu->ref_ctr_offset)
> + seq_printf(m, "(0x%lx)", tu->ref_ctr_offset);
> +
> for (i = 0; i < tu->tp.nr_args; i++)
> seq_printf(m, " %s=%s", tu->tp.args[i].name, tu->tp.args[i].comm);
>
> @@ -917,7 +943,13 @@ probe_event_enable(struct trace_uprobe *tu, struct trace_event_file *file,
>
> tu->consumer.filter = filter;
> tu->inode = d_real_inode(tu->path.dentry);
> - ret = uprobe_register(tu->inode, tu->offset, &tu->consumer);
> + if (tu->ref_ctr_offset) {
> + ret = uprobe_register_refctr(tu->inode, tu->offset,
> + tu->ref_ctr_offset, &tu->consumer);
> + } else {
> + ret = uprobe_register(tu->inode, tu->offset, &tu->consumer);
> + }
> +
> if (ret)
> goto err_buffer;
>
> --
> 2.14.4
>