On 03/28/2011 09:14 PM, Mark Kettenis wrote:
Date: Mon, 28 Mar 2011 10:21:06 -0700
From: Alan Coopersmith<alan.coopersm...@oracle.com>
This commit fixes only the heap corruption and sporadic crashes. It's
still possible that XauFileName returns a badly formed filename string
if called from multiple threads. For example, changing contents of
HOME environment variable could make the returned string to be
malformed. However, there shouldn't be crashes.
Perhaps we just need to define a new function to do this, that doesn't
use a static global variable of any sort, and declare that multi-threaded
code needs to use it.
That does make more sense than adding hacks to limit the potential
damage. The current interface simply is inherently thread-unsafe.
I was actually thinking about implementing XauGetFileName as an
alternative for XauFileName, that would return dynamically allocated
filename string and require it to be freed by callers. However, the real
problem is that getenv has to be called at some point to get XAUTHORITY
or HOME, and that can't be done safely inside a library in any portable way.
The only way to solve the problem in a thread safe manner would be to
pass copies of XAUTHORITY and HOME to Xlib/Xcb when creating a new
connection. Alternatively the semantics of connection creation should be
changed so that environment variables would be completely ignored. Both
of these options aren't very good from backward compatibility point of view.
We chose to fix the common case and make connection creation safe as
long as environment is not modified at the same time. We didn't
implement a new function, because if we offer a new function for
multithreaded clients, it should be also protected against environment
changes. That would again require a lot of changes in users of libXau.
In other words, the fix avoids a real problem and while not perfect,
it's good enough.
[http://en.wikipedia.org/wiki/Perfect_is_the_enemy_of_good :)]
Could you give your opinion about the environment problem? If a new
function is created for multithreaded clients, what should be done about
reading XAUTHORITY and HOME in a safe manner? Alternatively, do you
think that protecting against environment changes is not that important
and fixing the allocation problems are enough?
-- Rami
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