[PATCH-for-4.2 v3 3/3] trace: Forbid dynamic field width in event format

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Since not all trace backends support dynamic field width in
format (dtrace via stap does not), forbid them.

Add a check to refuse field width in new formats:

  $ make
  [...]
    GEN     hw/block/trace.h
  Traceback (most recent call last):
    File "scripts/tracetool.py", line 152, in <module>
      main(sys.argv)
    File "scripts/tracetool.py", line 143, in main
      events.extend(tracetool.read_events(fh, arg))
    File "scripts/tracetool/__init__.py", line 371, in read_events
      event = Event.build(line)
    File "scripts/tracetool/__init__.py", line 285, in build
      raise ValueError("Event format must not contain field width '%*'")
  ValueError: Error at hw/block/trace-events:11: Event format must not contain field width '%*'

Reported-by: Eric Blake <eblake@redhat.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1844817
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
v3:
- use better regex provided by Eric,
- instead of re.match(), use re.search() which takes unanchored regex,
- added a comment in tracing.txt
---
 docs/devel/tracing.txt        | 3 ++-
 scripts/tracetool/__init__.py | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/docs/devel/tracing.txt b/docs/devel/tracing.txt
index 8c0376fefa..6c01ce801e 100644
--- a/docs/devel/tracing.txt
+++ b/docs/devel/tracing.txt
@@ -113,7 +113,8 @@ Format strings should reflect the types defined in the trace event.  Take
 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
 respectively.  This ensures portability between 32- and 64-bit platforms.
 Format strings must not end with a newline character.  It is the responsibility
-of backends to adapt line ending for proper logging.
+of backends to adapt line ending for proper logging.  Format strings must not
+use numeric field width dynamic precision (SystemTap does not support them).
 
 Each event declaration will start with the event name, then its arguments,
 finally a format string for pretty-printing. For example:
diff --git a/scripts/tracetool/__init__.py b/scripts/tracetool/__init__.py
index 44c118bc2a..ec7fe9fa4a 100644
--- a/scripts/tracetool/__init__.py
+++ b/scripts/tracetool/__init__.py
@@ -206,6 +206,7 @@ class Event(object):
                       "\s*"
                       "(?:(?:(?P<fmt_trans>\".+),)?\s*(?P<fmt>\".+))?"
                       "\s*")
+    _DFWRE = re.compile(r"%[\d\.\- +#']*\*") # dynamic width precision
 
     _VALID_PROPS = set(["disable", "tcg", "tcg-trans", "tcg-exec", "vcpu"])
 
@@ -280,6 +281,8 @@ class Event(object):
         if fmt.endswith(r'\n"'):
             raise ValueError("Event format must not end with a newline "
                              "character")
+        if Event._DFWRE.search(fmt):
+            raise ValueError("Event format must not contain field width '%*'")
 
         if len(fmt_trans) > 0:
             fmt = [fmt_trans, fmt]
-- 
2.21.0