[Libguestfs] [PATCH nbdkit] server: Add -D nbdkit.environ=1 to dump the environment

[Richard W.M. Jones]

On Mon, May 08, 2023 at 08:54:48AM +0200, Laszlo Ersek wrote:
> On 5/7/23 12:43, Richard W.M. Jones wrote:
> > This is not secure so should not be used routinely.  Also we do not
> > attempt to filter environment variables, so even ones containing
> > multiple lines or special characters are all sent to nbdkit_debug.
> > 
> > The reason for adding this is to allow for debugging the new
> > nbd_set_socket_activation_name(3) API added in libnbd 1.16.
> > ---
> >  docs/nbdkit.pod | 10 ++++++++++
> >  server/main.c   | 28 ++++++++++++++++++++++++++++
> >  2 files changed, 38 insertions(+)
> > 
> > diff --git a/docs/nbdkit.pod b/docs/nbdkit.pod
> > index 83cf6a11c..22350cc08 100644
> > --- a/docs/nbdkit.pod
> > +++ b/docs/nbdkit.pod
> > @@ -619,6 +619,16 @@ are numerous and not usually very interesting.
> >  S<I<-D nbdkit.backend.controlpath=0>> suppresses the non-datapath
> >  commands (config, open, close, can_write, etc.)
> >  
> > +=item B<-D nbdkit.environ=1>
> > +
> > +(not Windows)
> > +
> > +Print nbdkit's environment variables in the debug output at start up.
> > +This is insecure because environment variables may contain both
> > +sensitive and user-controlled information, so it should not be used
> > +routinely.  But it is useful for tracking down problems related to
> > +environment variables.
> > +
> >  =item B<-D nbdkit.tls.log=>N
> >  
> >  Enable TLS logging.  C<N> can be in the range 0 (no logging) to 99.
> > diff --git a/server/main.c b/server/main.c
> > index 1df5d69ac..7b6adc3cc 100644
> > --- a/server/main.c
> > +++ b/server/main.c
> > @@ -210,6 +210,26 @@ dump_config (void)
> >  #endif
> >  }
> >  
> > +#ifndef WIN32
> > +
> > +/* -D nbdkit.environ=1 to dump the environment at start up. */
> > +NBDKIT_DLL_PUBLIC int nbdkit_debug_environ;
> > +
> > +#ifndef HAVE_ENVIRON_DECL
> > +extern char **environ;
> > +#endif
> > +
> > +static void
> > +dump_environment (void)
> > +{
> > +  size_t i;
> > +
> > +  for (i = 0; environ[i]; ++i)
> > +    nbdkit_debug ("%s", environ[i]);
> > +}
> > +
> > +#endif /* !WIN32 */
> > +
> >  int
> >  main (int argc, char *argv[])
> >  {
> > @@ -662,6 +682,14 @@ main (int argc, char *argv[])
> >    /* Check all debug flags were used, and free them. */
> >    free_debug_flags ();
> >  
> > +#ifndef WIN32
> > +  /* Dump the environment if asked.  This is the earliest we can do it
> > +   * because it uses a debug flag.
> > +   */
> > +  if (nbdkit_debug_environ && verbose)
> > +    dump_environment ();
> > +#endif
> > +
> >    if (help) {
> >      struct backend *b;
> >  
> 
> I was surprised not to see any assignment to the new global variable,
> but then I found "server/debug-flags.c"; that's some serious wizardry.

With the side effect, as it turns out, that it's very hard to
implement for language plugins.  Rust is fine.  OCaml can be done by
adding a C object to the link.  Other languages, not really :-(

> Side thought: should we not make both "nbdkit_debug_environ" (and
> friends), and the target of the "sym" pointer in "apply_debug_flags",
> volatile? Aliasing doesn't get much more obscure than this, and I wonder
> if this has the potential to blow up later, when compilers (and
> processors) get even more clever.
> 
> Of course POSIX could always say "this is just required to work", which
> I guess would be fine, but currently it doesn't seem to say it.

I guess Eric will know for sure, but isn't it generally true that all
global variables can be modified by any function call?  My model is
that they can modified at any time by any called function; even
through a pointer so not really visible to the compiler.

> Anyway:
> 
> Acked-by: Laszlo Ersek <lersek at redhat.com>
> 
> Thanks for writing the patch!
> Laszlo

Rich.

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