Re: fork_findpid() - Fatal trap 12: page fault while in kernel mode

On 16 Dec, Konstantin Belousov wrote:
> On Wed, Dec 16, 2015 at 12:21:16PM +0100, Fabian Keil wrote:
>> Konstantin Belousov <kostikbel_at_gmail.com> wrote:
>> > It is the values of *p and *(p->p_pgrp) that are needed, from the frame 8.
>> 
>> Unfortunately it's not available and apparently I removed the attempts
>> to get it from the previous output.
> 
>> allproc is available and the first one matches lastpid and has an invalid
>> p_pgrp, but due to trypid being optimized out as well, it's not obvious
>> (to me) that it's the right process.
> 
> p_suspcount = 0, p_xthread = 0xfffff801162819a0, p_boundary_count = 0, p_pendingcnt = 0, p_itimers = 0x0, p_procdesc = 0x0, p_treeflag = 0, p_magic = 3203398350, p_osrel = 1100090, 
>>   p_comm = 0xfffff800304df3c4 "privoxy",
> p_pgrp = 0x618b0080,
> 
>> I've changed p's declaration to static so hopefully its value will
>> be available the next time the panic occurs, but it may take a while
>> until that happens.
> 
> From the state of the process you provided, it is a new (zigote) of the
> forking process, which was already linked into allproc list.  Also,
> it seems that bzero part of the forking procedure was finished, but bcopy
> was not yet.  The p_pgrp cannot be a pointer, it is not yet initialized.
> 
> There, we have at least one issue, since zigote is linked before the
> p_pgrp is initialized, and the proctree/allproc locks are dropped.
> As result, fork_findpid() accesses memory with undefined content.
> 
> It seems that the least morbid solution is to slightly extend the scope
> of the allproc lock in do_fork(), to prevent fork_findpid() from working
> while we did not finished copying data from old to new process.

I used to have a patch the deferred linking the new process into
proctree/allproc until it was fully formed.  The motivation was to get
rid of all of the PRS_NEW stuff scattered around the source.
Unfortunately the patch bit-rotted and I'm pretty sure that I lost it.