Support for global variable arguments in USDT probes

I previously thought it wouldn't be possible to probe arbitrary user-mode addresses,
but it turns out that it is. This was required to support global variable arguments
in USDT probes. For example, there are a few probes in libpthread whose arguments
refer to the mp_ global variable. These are now supported.

Additionally, refactored process-related utils into a ProcUtils class in procstat.py,
and implemented a version of `which` (`ProcUtils.which`) that is as portable as
possible -- it only relies on traversing the PATH environment variable.

src/python/bcc/__init__.py

@@ -26,7 +26,7 @@ import sys
 basestring = (unicode if sys.version_info[0] < 3 else str)
 
 from .libbcc import lib, _CB_TYPE
-from .procstat import ProcStat
+from .procstat import ProcStat, ProcUtils
 from .table import Table
 from .tracepoint import Perf, Tracepoint
 from .usyms import ProcessSymbols

src/python/bcc/procstat.py

@@ -31,3 +31,95 @@ class ProcStat(object):
                 return os.popen("cut -d' ' -f 22 /proc/%d/stat" %
                                 self.pid).read()
 
+class ProcUtils(object):
+        @staticmethod
+        def get_load_address(pid, bin_path):
+                """
+                get_load_address(pid, bin_path)
+
+                Returns the address at which the specified module is loaded
+                in the specified process. The module path must match exactly
+                the file system path, not a symbolic link.
+                """
+                with open("/proc/%d/maps" % pid) as m:
+                        maps = m.readlines()
+                addrs = map(lambda l: l.split('-')[0],
+                            filter(lambda l: bin_path in l, maps)
+                            )
+                if len(addrs) == 0:
+                        raise ValueError("lib %s not loaded in pid %d"
+                                        % (bin_path, pid))
+                return int(addrs[0], 16)
+
+        @staticmethod
+        def get_modules(pid):
+                """
+                get_modules(pid)
+
+                Returns a list of all the modules loaded into the specified
+                process. Modules are enumerated by looking at /proc/$PID/maps
+                and returning the module name for regions that contain
+                executable code.
+                """
+                with open("/proc/%d/maps" % pid) as f:
+                        maps = f.readlines()
+                modules = []
+                for line in maps:
+                        parts = line.strip().split()
+                        if len(parts) < 6:
+                                continue
+                        if parts[5][0] == '[' or not 'x' in parts[1]:
+                                continue
+                        modules.append(parts[5])
+                return modules
+
+        @staticmethod
+        def is_shared_object(bin_path):
+                """
+                is_shared_object(bin_path)
+
+                Returns whether the specified binary is a shared object, rather
+                than an executable. If it is neither, an error is raised.
+                """
+                mime_type = os.popen("file --mime-type -b %s" % bin_path
+                                ).read().strip()
+                if mime_type == "application/x-sharedlib":
+                        return True
+                if mime_type == "application/x-executable":
+                        return False
+                raise ValueError("invalid mime type %s for binary %s" %
+                                (mime_type, bin_path))
+
+        @staticmethod
+        def traverse_symlink(path):
+                """Returns the actual path behind the specified symlink."""
+                return os.popen("readlink -f %s" % path).read().strip()
+
+        @staticmethod
+        def which(bin_path):
+                """
+                which(bin_path)
+
+                Traverses the PATH environment variable, looking for the first
+                directory that contains an executable file named bin_path, and
+                returns the full path to that file, or None if no such file
+                can be found. This is meant to replace invocations of the
+                "which" shell utility, which doesn't have portable semantics
+                for skipping aliases.
+                """
+                # Source: http://stackoverflow.com/a/377028
+                def is_exe(fpath):
+                        return os.path.isfile(fpath) and \
+                               os.access(fpath, os.X_OK)
+
+                fpath, fname = os.path.split(bin_path)
+                if fpath:
+                        if is_exe(bin_path):
+                                return bin_path
+                else:
+                        for path in os.environ["PATH"].split(os.pathsep):
+                                path = path.strip('"')
+                                exe_file = os.path.join(path, bin_path)
+                                if is_exe(exe_file):
+                                        return exe_file
+                return None

tools/argdist.py

@@ -12,7 +12,7 @@
 # Licensed under the Apache License, Version 2.0 (the "License")
 # Copyright (C) 2016 Sasha Goldshtein.
 
-from bcc import BPF, Tracepoint, Perf, USDTReader
+from bcc import BPF, Tracepoint, Perf, ProcUtils, USDTReader
 from time import sleep, strftime
 import argparse
 import re
@@ -392,12 +392,9 @@ QUALIFIER int PROBENAME(struct pt_regs *ctx SIGNATURE)
         def _attach_u(self):
                 libpath = BPF.find_library(self.library)
                 if libpath is None:
-                        with os.popen(("which --skip-alias %s " +
-                                "2>/dev/null") % self.library) as w:
-                                libpath = w.read().strip()
+                        libpath = ProcUtils.which(self.library)
                 if libpath is None or len(libpath) == 0:
-                        self._bail("unable to find library %s" %
-                                   self.library)
+                        self._bail("unable to find library %s" % self.library)
 
                 if self.probe_type == "u":
                         for i, location in enumerate(self.usdt.locations):

tools/trace.py

@@ -331,7 +331,8 @@ BPF_PERF_OUTPUT(%s);
                         prefix = self.tp.generate_get_struct()
                 elif self.probe_type == "u":
                         signature += ", int __loc_id"
-                        prefix = self.usdt.generate_usdt_cases()
+                        prefix = self.usdt.generate_usdt_cases(
+                                pid=Probe.pid if Probe.pid != -1 else None)
                         qualifier = "static inline"
 
                 data_fields = ""