BUG: Linux 2.6.25 ptrace leaks struct_task

[Joris van Rantwijk]

I think sys32_ptrace() is leaking struct_task.

In arch/x86/kernel/ptrace.c, function sys32_ptrace(), there
is a call to ptrace_get_task_struct(). In some cases (such as
PTRACE_GETREGS), there is no matching call to put_task_struct().

Test case: fork many childs, calling PTRACE_GETREGS on each child.
Indeed this causes unbounded growth of the task_struct allocation
in /proc/slabinfo, and it also causes physical memory to disappear
from /proc/meminfo.

I have tested this on Linux 2.6.25.4 for x86_64, but the
relevant code has not been changed in 2.6.25.9.
This bug appears to be fixed in 2.6.26-rc8: the code looks
different and I can't reproduce the issue there.

Oops, just discovered this problem has already been reported
in http://lkml.org/lkml/2008/5/29/266 although not much
seems to have been done about it.

Should this be fixed for 2.6.25.10 ?

Is it likely that this bug is related to the mysterious
disappearance of memory from /proc/meminfo as reported in
http://lkml.org/lkml/2008/6/24/15 ?

Even so, how is it possible that memory just disappears
from /proc/meminfo? It can't be the task_struct cache itself,
because that is all covered under Slab, right ?

Greetings, Joris.
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