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SubjectRe: Weirdness in /proc/<pid>/maps and /proc/<pid>/stat.
Date
> 
> With commit d899bf7b, the behavior of field 28 of /proc/<pid>/stat
> was changed as was /proc/<pid>/maps. I don't know if that change was
> correct, but its resulting behavior is much more difficult to explain.
> I was wondering if we could determine what the "correct" behavior is
> before I spend much more time trying to give it the wrong behavior.
>
> My test program is attached below. Essentially:
> fork() -> pthread_create() -> fork()
>
> x86_64 2.6.32 stable kernel:
> Step stat-28 maps-threadstack
> p (parent) 0x7fff5607ddc0 N/A
> c (child) 0x7fff55c7dc50 N/A
> ppthread 0x7f2cf5c9bff0 0x7f2cf5c9d000:007feff0
> ppthread+fork 0x7f2cf589be30 0x7f2cf5c9d000:003fee30
> cpthread 0x7f2cf589be30 0x7f2cf5c9d000:007feff0
> cpthread+fork 0x7f2cf589be30 0x7f2cf5c9d000:003fee30
> Note: For all of the above, the /proc/<pids>/task/*/maps files had the
> stack line like:
> 7fff55c7d000-7fff56081000 rw-p 00000000 00:00 0 [stack]
>
> The problems I see are:
> 1) In the fork() case, we are using the current userland stack
> pointer for task->stack_start. This appears wrong as the
> function which called fork() may be returned to and may
> further return to higher level callers, finding sp now
> beyond the value reported in /proc/self/stat. Additionally,
> the value reported for the child of the fork has no relationship
> to the stack size rlimit any longer.

BUG.


> 2) In the pthread + fork case, in addition to the problem
> above, the size information in /proc/self/maps
> is incorrect as it does not take into consideration
> the same return paths.

BUG.

Robin, do you really need this feature? if not, I'll revert this one.


sidenote: if anyone really need to know thread stack range, I think we need to
prevent vma consoliation of thread stack, iow need to implement MAP_STACK.


>
> The problem I am running into is coming up with any way to
> make the task->stack_start value usable.







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