[PATCH] libdw: Resolve alt file on first use.

[Mark Wielaard]

On Mon, 2018-01-22 at 12:30 +0100, Mark Wielaard wrote:
> +static void
> +find_debug_altlink (Dwarf *dbg)
> +{
> +  const char *altname;
> +  const void *build_id;
> +  ssize_t build_id_len = INTUSE(dwelf_dwarf_gnu_debugaltlink) (dbg,
> > +							       &altname,
> > +							       &build_id);
> +
> +  /* Couldn't even get the debugaltlink.  It probably doesn't exist.  */
> +  if (build_id_len < 0)
> +    return;

Testing under valgrind showed that this should be build_id_len <= 0.
When dwelf_dwarf_gnu_debugaltlink returns 0 it means there was no
.gnu_debugaltlink section at all. A negative return value means the
data found was corrupt. In both cases neither altname nor build_id
might be set up. So I added the following fixup:

diff --git a/libdw/dwarf_getalt.c b/libdw/dwarf_getalt.c
index 7b41a2b..3e5af15 100644
--- a/libdw/dwarf_getalt.c
+++ b/libdw/dwarf_getalt.c
@@ -117,7 +117,7 @@ find_debug_altlink (Dwarf *dbg)
                                                               &build_id);
 
   /* Couldn't even get the debugaltlink.  It probably doesn't exist.  */
-  if (build_id_len < 0)
+  if (build_id_len <= 0)
     return;
 
   const uint8_t *id = (const uint8_t *) build_id;