[PATCH v2 00/14] Fix signal-unsafe logging

Chase Douglas chase.douglas at canonical.com
Mon Jun 4 10:20:44 PDT 2012

On 06/03/2012 09:20 PM, Peter Hutterer wrote:
> On Wed, Apr 11, 2012 at 06:33:26PM -0700, Chase Douglas wrote:
>> On 04/11/2012 04:53 PM, Peter Hutterer wrote:
>>> On Mon, Apr 09, 2012 at 11:17:26AM -0700, Chase Douglas wrote:
> [...]
>>> - the API is rather shocking, replacing one vararg function with a number of
>>>   mixed calls makes it really hard to grep for the message.
>>>   a poor man's printf parsing that searches for %d, %s and %x would help
>>>   here.
>> Variadic argument implementation is some of the blackest magic of C
>> code, and varies a great deal even between architectures of glibc. I
>> don't know if we can be sure that variadic arguments will always be
>> signal safe. They aren't mentioned in the POSIX signal(7) man page as
>> safe functions.
>> I doubt we would actually support Interix SUA, for example, but it is
>> POSIX compliant and its man page says va_* functions are not signal
>> safe: http://www.suacommunity.com/man/3/varargs.3.html.
>>> - given that input drivers almost always use signal-sensitive processing,
>>>   making xf86IDrvMsg signal safe would go a long way 
>> It's variadic, so I don't think that will be possible. I know there's
>> bound to be some pain here that reaches device drivers, but I don't see
>> a way around it.
> sorry to pick this up so late. I've been banging my head against the wacom
> driver, finding a good way to log debug messages while parsing events and
> the crux of the matter is the vararg stuff. I went searching again and found
> that signal(7) lists execle() and that function takes a variable number of
> parameters. execle is a POSIX requirement, va_list and friends must be
> vararg-compatible, which means we can have a normal implementation of the
> log messaging functions.

Your logic seems sound, albeit not an exceptionally strong proof that
varargs are always signal safe :). At this point, I don't have the time
to rewrite the patches again. Can I hand them off to you to finish?

-- Chase

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