Re: About reading /proc/*/mem

["Richard F Weber" <rfweber@link.com>]

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Eric W. Biederman wrote:

>"Richard F Weber" <rfweber@link.com> writes:
>
>>Ok, so as a rehash, the ptrace & open(),lseek() on /proc/*/mem should 
>>both work about the same.  After a lot of struggling, I've gotten the 
>>ptrace to work right & spit out the data I want/need.  However there is 
>>one small problem, SIGSTOP.
>>
>>ptrace() appears to set up the child process to do a SIGSTOP whenever 
>>any interrupt is received.  Which is kind of a bad thing for what I'm 
>>looking to do.  I guess I'm trying to write a non-intrusive debugger 
>>that can be used to view static variables stored in the heap of an 
>>application.
>>
>>On other OS's, this can be done just by popping open /proc/*/mem, and 
>>reading the data as needed, allowing the child process to continue 
>>processing away as if nothing is going on.  I'm looking to do the same 
>>sort of task under Linux. 
>>
>>Unfortunately, ptrace() probobally isn't going to allow me to do that.  
>>So my next question is does opening /proc/*/mem force the child process 
>>to stop on every interrupt (just like ptrace?)
>>
>
>
>The not stopping the child should be the major difference between
>/proc/*/mem and ptrace.
>
See this is where I start seeming to have problems.  I can open 
/proc/*/mem & lseek, but reads come back as "No such process".  However, 
if I first do a ptrace(PTRACE_ATTACH), then I can read the data, but the 
process stops.  I've kind of dug through the sys_ptrace() code under 
/usr/src/linux/arch/i386/kernel/ptrace.c, and can see and understand 
generally what it's doing, but that's getting into serious kernel-land 
stuff.  I wouldn't expect it to be this difficult to just open up 
another processes /proc/*/mem file to read data from.

Is there something obvious I'm missing?  It seems to keep pointing back 
to ptrace & /proc/*/mem are very closely related (ie: the same) 
including stopping of the child.

Thanks.

--Rich

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