Re: [PATCH] eblopenback: Warn when unable to find ebl backend

[Mark Wielaard <mjw at redhat dot com>]

On Wed, 2015-11-25 at 23:51 +0100, Ben Gamari wrote:
> Mark Wielaard <mjw@redhat.com> writes:
> > Yes, could you describe what kind of calls you were doing that made this
> > really messy?
> >
> Well, much of the time I spent debugging wasn't even against GHC. I
> spent quite a while trying to figure out why `readelf -r` thought all of
> my relocations were invalid. Since there was no debug output I had no
> choice but to step through the executable in GDB. It took me the better
> part of an hour to eventually realize that there was a typo in my
> LD_LIBRARY_PATH :(.

The problem is that ebl_openbackend never fails (except if you give it
an Elf from which the ehdr cannot be read), it returns a generic
"backend" when the backend cannot be found or there is some error
loading it. I wonder if we should just do this:

diff --git a/libebl/eblopenbackend.c b/libebl/eblopenbackend.c
index b301400..ed16ef5 100644
--- a/libebl/eblopenbackend.c
+++ b/libebl/eblopenbackend.c
@@ -370,25 +370,11 @@ openbackend (Elf *elf, const char *emulation, GElf_Half machine)
            /* Not the module we need.  */
            (void) dlclose (h);
          }
-
-       /* We cannot find a DSO but the emulation/machine ID matches.
-          Return that information.  */
-       result->dlhandle = NULL;
-       result->elf = elf;
-       result->name = machines[cnt].prefix;
-       fill_defaults (result);
-
-       return result;
       }
 
-  /* Nothing matched.  We use only the default callbacks.   */
-  result->dlhandle = NULL;
-  result->elf = elf;
-  result->emulation = "<unknown>";
-  result->name = "<unknown>";
-  fill_defaults (result);
-
-  return result;
+  /* Unknown emulation/machine ID or we cannot find a DSO.  */
+  free (result);
+  return NULL;
 }
 
The default openbackend should handle ebl == NULL just fine.
Then we just have to adjust the callers to either error out or just warn
about the unknown backend.

Cheers,

Mark