Re: [uml-devel] [smcc@CSAIL.MIT.EDU: Bug#271069: gdb startup fails in tracefork check]

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

On Saturday 11 September 2004 00:26, Matt Zimmerman wrote:
> ----- Forwarded message from Stephen McCamant <smcc@CSAIL.MIT.EDU> -----
>
> Date: Fri, 10 Sep 2004 18:09:07 -0400
> From: Stephen McCamant <smcc@CSAIL.MIT.EDU>
> Resent-From: Stephen McCamant <smcc@CSAIL.MIT.EDU>
> To: submit@bugs.debian.org
> Subject: Bug#271069: gdb startup fails in tracefork check
>
> Package: user-mode-linux
> Version: 2.4.26-2um-1
>
> With the current versions of UML and GDB in sarge, with a 2.4.x host
> kernel (no SKAS support, so this is in tt mode), gdb runs into an
> error when UML tries to start it as requested by a "debug" option. For
> instance, given the invocation

> where the line 1 the error refers to is "att 1". Despite the message,
> though, I don't think the error comes from the init file, but rather
> from part of GDB's startup process: linux_test_for_tracefork(), from
> the file linux-nat.c in the GDB source, is used to check whether the
> kernel supports various options to ptrace which, if present, allow GDB
> to follow children that fork. To do this, GDB forks a child process,
> waits for the child to stop itself, then sets some ptrace options and
> has the child fork. When run on its own, the test executes without
> problems, but when gdb is started by UML, the waitpid call that gdb
> uses to wait for the first child to call PTRACE_TRACEME is
> unexpectedly interrupted; the EINTR return value is what makes gdb
> error out.

Well, when gdb receives EINTR, it should just retry the failed call. It's a 
general rule. In fact, in these days we are applying a lot of such patches to 
fix such bugs for UML.
So, file a bug-report to gdb developers.

UML may have his part in triggering this bug, so we may fix it, too.

> If I edit the source to gdb so that the function immediately returns a
> "not-available" result without actually doing the test, and use that
> modified gdb instead of the regular one, debugging the UML kernel
> works as I would expect.

Try fixing it as I suggest above. I.e., when call fails and errno == EINTR, 
retry the same identical call (without changing *anything*).

Bye
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729


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