[PATCH v2] elf: Refine direct extern access diagnostics to protected symbol

[Fangrui Song]

Refine commit 349b0441dab375099b1d7f6909c1742286a67da9:

1. Copy relocations for extern protected data do not work properly,
regardless whether GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS is used.
It makes sense to produce a warning unconditionally.  When the defining
shared object has GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS, report
an error to satisfy the "no copy relocations" enforcement intended by
this GNU property.

2. Non-zero value of an undefined function symbol may break pointer
equality, but may be benign in many cases (many programs don't take the
address in the shared object then compare it with the address in the
executable).  Reword the diagnostic to be clearer.

3. Remove the unneeded condition !(undef_map->l_1_needed &
GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS). If the executable has
GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS (can only occur in error
cases), the diagnostic should be emitted as well.

--
Changes from v1:
* Keep 2 as an error in the presence of GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS
  as requested by H.J. Lu.
---
 sysdeps/generic/dl-protected.h | 50 ++++++++++++++++++----------------
 1 file changed, 27 insertions(+), 23 deletions(-)

diff --git a/sysdeps/generic/dl-protected.h b/sysdeps/generic/dl-protected.h
index 88cb8ec917..38386b5200 100644
--- a/sysdeps/generic/dl-protected.h
+++ b/sysdeps/generic/dl-protected.h
@@ -26,29 +26,33 @@ _dl_check_protected_symbol (const char *undef_name,
 			    const struct link_map *map,
 			    int type_class)
 {
-  if (undef_map != NULL
-      && undef_map->l_type == lt_executable
-      && !(undef_map->l_1_needed
-	   & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS)
-      && (map->l_1_needed
-	  & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS))
-    {
-      if ((type_class & ELF_RTYPE_CLASS_COPY))
-	/* Disallow copy relocations in executable against protected
-	   data symbols in a shared object which needs indirect external
-	   access.  */
-	_dl_signal_error (0, map->l_name, undef_name,
-			  N_("copy relocation against non-copyable protected symbol"));
-      else if (ref->st_value != 0
-	       && ref->st_shndx == SHN_UNDEF
-	       && (type_class & ELF_RTYPE_CLASS_PLT))
-	/* Disallow non-zero symbol values of undefined symbols in
-	   executable, which are used as the function pointer, against
-	   protected function symbols in a shared object with indirect
-	   external access.  */
-	_dl_signal_error (0, map->l_name, undef_name,
-			  N_("non-canonical reference to canonical protected function"));
-    }
+  if (undef_map == NULL || undef_map->l_type != lt_executable)
+    return;
+
+  if (type_class & ELF_RTYPE_CLASS_COPY)
+    /* Disallow copy relocations in executable against protected
+       data symbols in a shared object which needs indirect external
+       access.  */
+    _dl_error_printf ("warning: copy relocation against non-copyable "
+		      "protected symbol `%s' in `%s'\n",
+		      undef_name, map->l_name);
+  else if ((type_class & ELF_RTYPE_CLASS_PLT) && ref->st_value != 0
+	   && ref->st_shndx == SHN_UNDEF)
+    /* Disallow non-zero symbol values of undefined symbols in
+       executable, which are used as the function pointer, against
+       protected function symbols in a shared object with indirect
+       external access.  */
+    _dl_error_printf (
+	"warning: direct reference to "
+	"protected function `%s' in `%s' may break pointer equality\n",
+	undef_name, map->l_name);
+  else
+    return;
+
+  if (map->l_1_needed & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS)
+    _dl_signal_error (
+	0, map->l_name, undef_name,
+	N_ ("error due to GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS"));
 }
 
 #endif /* _DL_PROTECTED_H */
-- 
2.36.1.255.ge46751e96f-goog