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

[Eric Blake <eblake@redhat.com>]

On 11/18/19 3:04 PM, Philippe Mathieu-Daudé wrote:
> 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>
> ---

> +++ 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).

Reads awkwardly - a dynamic precision is not numeric in the format 
string (but '*' instead).  Better might be:

Format strings must not use dynamic field width or precision ('*'), as 
at least SystemTap does not support them.

Or even:

Format strings may use numeric field width or precision, but must not 
use dynamic forms ('*') as at least SystemTap does not support that.

>   
>   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

The comment is slightly off - this catches both dynamic field width (any 
'*' before '.') and dynamic precision (any '*' after '.'), maybe the fix 
is just s/width/width or/

>   
>       _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 '%*'")

and I don't know if you want to tweak the error message, maybe:
  Event format must not use dynamic '*'

If we're trying to get stuff in 4.2-rc2, patch 1 and 2 are actual bug 
fixes and deserve to go in; patch 3 is nice-to-have but doesn't affect 
the build if it is omitted (as there are no other offenders left), so 
slipping it into 5.0 for a v4 to clean it up slightly doesn't hurt.  I 
don't know who would send the pull request, though, and slipping 1 and 2 
into -rc3 just because of 3 is not ideal.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org