[uClinux-dev] binfmt_flat.c and multiple processes with the same executable file

Avila Mathieu mathieu.avila at laposte.net
Mon Oct 3 03:24:26 EDT 2005


Greg Ungerer wrote:

> Hi Mathieu,
>
> Avila Mathieu wrote:
>
>> I am currently trying to run multiple processes from the same 
>> executable file. It seems that in that case, i get always the same 
>> TEXT/DATA/BSS segments.
>
>
> Can you explain further, give your example?
> What architecture are you working with?
>
I am making a port of uClinux to the PowerPC (based on a 2.6.13 kernel), 
specifically the 403GA in a NCD machine (explora401). However, it should 
work for any other PowerPC.
My intent is to give it back to the community when it is finished.
Actually, i am able to boot, go through memory initialization, and run a 
program; All relocations work fine, and my kernel is able to run a 
Busybox shell, compiled together with uClibc.

My troubles now are coming from vfork and mmapping.

uClibc for PowerPC is using fork instead of vfork because :
    /* Sigh.  The vfork system call on powerpc
     * seems to be completely broken.  So just
     * use fork instead */
Do you have any idea for this ?

That should be the reason why it used to get the same mapping for the 
same file (it didn't copy some pages/data, i think)

I have forced it to use vfork, and it seems ok.

But when the 2nd process returns, unmmap crashes (nommu.c); it seems 
some deallocations has already been done somewhere else. Or maybe it has 
to be done in the arch-dependant code ? (i don't see things like this 
for m68knommu)

When i comment that code (exit_mmap), it runs ok (although it's not a 
solution). However, i have read that after a vfork, the forking program 
waits until the 2nd one makes a "exit" or  "execve". When it makes exit 
directly, the 1st program gets back (ok), but it still blocks when the 
2nd program makes a execve, even if it returns. Do you have any idea ? 
Do i have to change it's running state, or read its return code or 
something like this ? Shouldn't it be done automatically by a signal 
sent to the parent process ?

Thanks for your help; I hope to give a uClinux for PowerPC soon ;)

--
Mathieu

>
>> I tried using a gzipped version of my file, hoping it would be copied 
>> elsewhere each time it is being run. But it's not the case.
>
>
> Can you run:
>
>     flthdr <yourprogram>
>
> Lets see what flags are set.
>
>
>> "binfmt_flat.c" contains a mapping of the file, which it seems is 
>> released only when the process ends, meaning another process gets the 
>> same mapping (is it possible ?) Maybe this has been corrected in the 
>> latest versions of uclinux ? I am currently running the one coming 
>> from kernel 2.6.9.
>
>
> Regards
> Greg
>
>
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