usdt: permit to enable probes from a pid and a particular binary

Currently, for usdt, the commands where both a pid and a binary path
are specified are not well supported. For example,
```
funccount -p <pid> 'u:<binary_path>:probe'
```
will count `probe` occurances for all binary paths in `pid`, not just
`<binary_path>`. The command
```
argdist -p <pid> 'u:<binary_path>:probe():s64:arg1'
```
will also count `probe` occurances for all binary paths in `pid`
with my previous patch.

Furthermore, suppose user want to trace linker `setjmp` probe point
with command
```
trace.py -p <pid> 'u:/usr/lib64/ld-2.17.so:setjmp'
```
Without my previous patch, user will have incorrect results as both
`libc:setjmp` and `rtld:setjmp` exists and the bcc just picks the first
one which is `libc:setjmp`. My last patch will cause `enable_probe`
failures if in the same usdt context, two probes have the same
probe_name but different provider name.

To fix all these issues, this patch passes additional binary path
to the pid-based usdt context, so that only probes from that particular
binary will be added to the context. This solved all the above
mentioned issues.
Signed-off-by: Yonghong Song <yhs@fb.com>

src/cc/bcc_usdt.h

@@ -22,7 +22,7 @@ extern "C" {
 
 #include <stdint.h>
 
-void *bcc_usdt_new_frompid(int pid);
+void *bcc_usdt_new_frompid(int pid, const char *path);
 void *bcc_usdt_new_frompath(const char *path);
 void bcc_usdt_close(void *usdt);
 

src/cc/link_all.cc

@@ -13,7 +13,7 @@ namespace {
       if (::getenv("bar") != (char *)-1)
         return;
 
-      (void)bcc_usdt_new_frompid(-1);
+      (void)bcc_usdt_new_frompid(-1, nullptr);
       (void)bcc_usdt_new_frompath(nullptr);
       (void)bcc_usdt_close(nullptr);
     }

src/cc/usdt.h

@@ -249,6 +249,7 @@ class Context {
 public:
   Context(const std::string &bin_path);
   Context(int pid);
+  Context(int pid, const std::string &bin_path);
   ~Context();
 
   optional<int> pid() const { return pid_; }

src/cc/usdt/usdt.cc

@@ -365,6 +365,22 @@ Context::Context(int pid) : pid_(pid), pid_stat_(pid),
     probe->finalize_locations();
 }
 
+Context::Context(int pid, const std::string &bin_path)
+    : pid_(pid), pid_stat_(pid),
+      mount_ns_instance_(new ProcMountNS(pid)), loaded_(false) {
+  std::string full_path = resolve_bin_path(bin_path);
+  if (!full_path.empty()) {
+    if (bcc_elf_foreach_usdt(full_path.c_str(), _each_probe, this) == 0) {
+      cmd_bin_path_ = ebpf::get_pid_exe(pid);
+      if (cmd_bin_path_.empty())
+        return;
+      loaded_ = true;
+    }
+  }
+  for (const auto &probe : probes_)
+    probe->finalize_locations();
+}
+
 Context::~Context() {
   if (pid_stat_ && !pid_stat_->is_stale()) {
     for (auto &p : probes_) p->disable();
@@ -374,8 +390,13 @@ Context::~Context() {
 
 extern "C" {
 
-void *bcc_usdt_new_frompid(int pid) {
-  USDT::Context *ctx = new USDT::Context(pid);
+void *bcc_usdt_new_frompid(int pid, const char *path) {
+  USDT::Context *ctx;
+
+  if (!path)
+    ctx = new USDT::Context(pid);
+  else
+    ctx = new USDT::Context(pid, path);
   if (!ctx->loaded()) {
     delete ctx;
     return nullptr;

src/python/bcc/libbcc.py

@@ -179,7 +179,7 @@ lib.bcc_symcache_refresh.restype = None
 lib.bcc_symcache_refresh.argtypes = [ct.c_void_p]
 
 lib.bcc_usdt_new_frompid.restype = ct.c_void_p
-lib.bcc_usdt_new_frompid.argtypes = [ct.c_int]
+lib.bcc_usdt_new_frompid.argtypes = [ct.c_int, ct.c_char_p]
 
 lib.bcc_usdt_new_frompath.restype = ct.c_void_p
 lib.bcc_usdt_new_frompath.argtypes = [ct.c_char_p]

src/python/bcc/usdt.py

@@ -127,7 +127,10 @@ class USDT(object):
     def __init__(self, pid=None, path=None):
         if pid and pid != -1:
             self.pid = pid
-            self.context = lib.bcc_usdt_new_frompid(pid)
+            if path:
+                self.context = lib.bcc_usdt_new_frompid(pid, path.encode('ascii'))
+            else:
+                self.context = lib.bcc_usdt_new_frompid(pid, ct.c_char_p(0))
             if self.context == None:
                 raise USDTException("USDT failed to instrument PID %d" % pid)
         elif path: