Implement Solaris-like link_map l_refname member.

The implementation is based on the public documentation, in particular
dlinfo(3) from Solaris.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

lib/libc/gen/dlinfo.3

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 19, 2020
+.Dd May 21, 2020
 .Dt DLINFO 3
 .Os
 .Sh NAME
@@ -111,6 +111,7 @@ const void      *l_ld;     /* Pointer to .dynamic in memory */
 struct link_map *l_next,   /* linked list of mapped libs */
                 *l_prev;
 caddr_t         l_addr;     /* Load Offset of library */
+const char      *l_refname; /* Object this one filters for */
 .Ed
 .Bl -tag -width ".Va l_addr"
 .It Va l_base
@@ -133,6 +134,11 @@ structure on the link-map list.
 The load offset of the object, that is, the difference between
 the actual load address and the base virtual address the object
 was linked at.
+.It Va l_refname
+A name of the object this object filters for, if any.
+If there are more then one filtee, a name from the first
+.Dv DT_FILTER
+dynamic entry is supplied.
 .El
 .It Dv RTLD_DI_SERINFO
 Retrieve the library search paths associated with the given

libexec/rtld-elf/rtld.c

@@ -1207,6 +1207,9 @@ digest_dynamic1(Obj_Entry *obj, int early, const Elf_Dyn **dyn_rpath,
 
 		*needed_filtees_tail = nep;
 		needed_filtees_tail = &nep->next;
+
+		if (obj->linkmap.l_refname == NULL)
+		    obj->linkmap.l_refname = (char *)dynp->d_un.d_val;
 	    }
 	    break;
 
@@ -1402,6 +1405,10 @@ digest_dynamic1(Obj_Entry *obj, int early, const Elf_Dyn **dyn_rpath,
 	}
 	obj->dynsymcount += obj->symndx_gnu;
     }
+
+    if (obj->linkmap.l_refname != NULL)
+	obj->linkmap.l_refname = obj->strtab + (unsigned long)obj->
+	  linkmap.l_refname;
 }
 
 static bool

sys/sys/link_elf.h

@@ -65,6 +65,7 @@ typedef struct link_map {
 	const void	*l_ld;			/* Pointer to .dynamic in memory */
 	struct link_map	*l_next, *l_prev;	/* linked list of of mapped libs */
 	caddr_t		l_addr;			/* Load Offset of library */
+	const char	*l_refname;		/* object we are filtering for */
 } Link_map;
 
 struct r_debug {