linux-kernel - Trouble with ptrace self-attach rule since kernel

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Message-Id: <200608312305.47515.ah2@delair.de>
Date:	Thu, 31 Aug 2006 23:05:47 +0200
From:	Andreas Hobein <ah2@...air.de>
To:	linux-kernel@...r.kernel.org
Subject: Trouble with ptrace self-attach rule since kernel > 2.6.14

Hi !

I have some trouble with the restriction of the ptrace functionality assumably 
introduced into the linux kernel  with the patch from 9. 11.2006 
1105_2_ptrace-self-attach.patch.

My multithreaded application tries to write callstacks of all threads (some 
sort of built-in mini debugger) in case of abnormal situations or failure. 
With the newer linux kernel (> 2.6.14) self-attaching to processes of the 
same thread group does not work any longer. Any call to ptrace results in a 
EPERM result.

I have worked around this problem by first forking the process, than creating 
the callstack output in the forked child process - which works without the 
above mentioned problem - and terminating the child process just after this 
operation.

Anyway this solution is somehow dirty and I would prefer the way it was 
implemented before. My question is: Why may a sibling thread not 
ptrace_attach another process of the same thread group, while at the same 
time a forked child process of the same thread is allowed to do this 
operation? Is there any replacement like pthread_suspend, which is available 
on other Unixes?

(A short program for the demonstration of this effect is attached. Use Option 
-f to enable forking)

Best regards,

        Andreas

View attachment "trace.c" of type "text/x-csrc" (1812 bytes)