Re: [PATCH] binfmt_elf: core dump masking support

[Pavel Machek <pavel@ucw.cz>]

On Mon 2007-01-22 11:29:40, Kawai, Hidehiro wrote:
> Hi Pavel,
> 
> >>>>The /proc/<pid>/ approach doesn't have these demerits, and it
> >>>>has an advantage that users can change the bitmask of any process
> >>>>at anytime.
> >>>
> >>>Well... not sure if it is advantage. 
> >>
> >>For example, consider the following case:
> >>  a process forks many children and system administrator wants to
> >>  allow only one of these processes to dump shared memory.
> >>
> >>This is accomplished as follows:
> >>
> >> $ echo 1 > /proc/self/coremask
> >> $ ./some_program
> >> (fork children)
> >> $ echo 0 > /proc/<a child's pid>/coremask
> >>
> >>With the /proc/<pid>/ interface, we don't need to modify the
> >>user program.  In contrast, with the ulimit or setrlimit interface,
> >>the administrator can't do it without modifying the user program
> >>to call setrlimit.  This will not be preferred.
> > 
> > Yep, otoh process coremask setting can change while it is running,
> > that is not expected. Hmm, it can also change while it is dumping
> > core, are you sure it is not racy?
> 
> Good point, thanks.  I never thought of that.
> We can change the coremask setting while dumping the process's
> memory, and it is problematic.
> 
> maydump() function which decides a given VMA may be dumped or not
> is invoked twice per VMAs.  One is at the time of writing a program
> header for a VMA, another is at the time of writing its contents.
> If the coremask setting differs between the two, the program
> header will point wrong place in the core file as its contents.
> 
>  
> > (run echo 1 > coremask, echo 0 > coremask in a loop while dumping
> > core. Do you have enough locking to make it work as expected?)
> 
> Currently, any lock isn't acquired.  But I think the kernel only
> have to preserve the coremask setting in a local variable at the
> begining of core dumping.  I'm going to do this in the next version.

No, I do not think that is enough. At minimum, you'd need atomic_t
variable. But I'd recomend against it. Playing with locking is tricky.

								Pavel
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