Re: [uml-devel] should there be os_clone() analogous to os_getpid() ?

[Jeff Dike <jdike@addtoit.com>]

On Sat, Dec 08, 2007 at 08:24:57PM -0800, John Reiser wrote:
>In source file arch/um/os-Linux/process.c there is a warning:
>-----
>/* Don't use the glibc version, which caches the result in TLS. It misses some
> * syscalls, and also breaks with clone(), which does not unshare the TLS.
> */
> 
>int os_getpid(void)

AFAIK, there are two reasons that we make sure we call the system call
getpid -
	when we're testing ptrace, we want getpid to result in exactly
one system call
	there was a brief bug in libc where getpid returned the wrong
answer due to caching across a clone

This bug did get into the wild, and people did try to run UML on such
systems, so that's why the code is careful about making the system call.

> I see no os_clone(), yet the glibc clone() does the same caching of pid in
> ThreadLocalStorage [TLS], and the TLS still may be shared.  If nobody reads
> glibc's shared TLS slot for PID then an actual bug will be avoided.  However,
> it is unsafe to leave such a tempting pitfall.

What's the actual bug, exactly?  As long as libc's getpid gives us the
right answer, we're happy.

> Also, if you are ptrace()ing
> through a glibc clone(), then in many cases you will see syscall(__NR_getpid)
> *from glibc* immediately following!  There is an "extra" getpid()
> that the tracking logic might not expect.

Where do we care about how clone translates into a system call?

>    arch/um/drivers/ubd_user.c
>    arch/um/kernel/tt/tracer.c
>    arch/um/os/tt.c
>    arch/um/os/start_up.c
>    arch/um/os/skas/process.c

These guys all just want a new process - they don't care how it
happens.

Is something in here causing valgrind some trouble?

				Jeff

-- 
Work email - jdike at linux dot intel dot com

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