More on the armv4l segfault.

[Rob Landley]

On Thursday 14 December 2006 10:31 pm, Paul Brook wrote:
> > Ok, the fix is to take weak_function off of __pthread_mutex_unlock in
> > ulibc_pthread.h.  I have no idea _why_ that fixes it, but it does.
> >
> > If the prototype says that __pthread_mutex_unlock() is weak, then
> > __uclibc_mutex_unlock() never gets called (I've stuck write(2,blah)
> > commands in it and they don't happen).  Instead I get a segfault on exit. 
> > But if I take the weak off, then it's called four times (and the call 
makes
> > it down to __pthread_return_0().)
> >
> > I'm not quite sure what's going on here.  If __pthread_mutex_unlock() is
> > _not_ prototyped as a weak symbol, _then_ it winds up calling
> > __pthread_return_0(); I'm confused.  (This is gcc 4.1.1 with binutils 2.17
> > building for armv4l.)
> 
> Sounds like something is unconditionally calling __pthread_mutex_unlock() 
> without making sure it is pulled into the binary (calling a weak symbol that 
> is not defined will cause a segfault).

I stuck a write(2,"woot\n",5); in __pthread_return_0(); and when I removed 
weak_function from __pthread_mutex_unlock() it printed out 4 times at the end 
of the program.

It doesn't seem to have anything to do with the printf().  I added a second 
printf() and they both happened even in the segfault case.  It's something 
happening after the return 0; from main().

I tried today's version (svn 16946) and the plain version gave me the 
segfault, but removing weak_function from __pthread_mutex_unlock() made the 
segfault go away.  So the fix still applies to svn, I'd just like to know 
_why_ it's fixing the problem I'm seeing, and why other architectures aren't 
seeing it.

If somebody would like my test reproduction sequence, it's based on building 
everything from source and testing it under qemu.  Later today I'll probably 
upgrade my build script to use last night's uClibc snapshot with a patch to 
fix this one issue, and then I can point people at it my thing and go "remove 
the patch and watch the segfault".

Rob
-- 
"Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away." - Antoine de Saint-Exupery