elf: Add SFrame support to _dl_find_object function

The SFrame provides information to be able to do stack trace is now
well defined and implemented in Binutils 2.41.  The format simply
contains enough information to be able to do stack trace given a
program counter (PC) value, the stack pointer, and the frame pointer.
The SFrame information is stored in a .sframe ELF section, which is
loaded into its own PT_GNU_SFRAME segment. We consider for this support
SFrame version 2.

This patch adds the bits to _dl_find_object to recognize and store in
struct dl_find_object the necessary info about SFrame section.

Signed-off-by: Claudiu Zissulescu <claudiu.zissulescu-ianculescu@oracle.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>

dlfcn/dlfcn.h

@@ -217,15 +217,21 @@ struct dl_find_object
   int dlfo_eh_count;		/* Number of exception handling entries.  */
   unsigned int __dlfo_eh_count_pad;
 # endif
-  __extension__ unsigned long long int __dflo_reserved[7];
+  void *dlfo_sframe;		/* SFrame stack trace data of the object.  */
+#if __WORDSIZE == 32
+  unsigned int __dlfo_sframe_pad;
+#endif
+  __extension__ unsigned long long int __dlfo_reserved[6];
 };
 
 /* If ADDRESS is found in an object, fill in *RESULT and return 0.
    Otherwise, return -1.  */
 int _dl_find_object (void *__address, struct dl_find_object *__result) __THROW;
 
-#endif /* __USE_GNU */
+/* SFrame stack trace data is valid.  */
+#define DLFO_FLAG_SFRAME      (1ULL << 0)
 
+#endif /* __USE_GNU */
 
 __END_DECLS
 

elf/dl-find_object.h

@@ -43,6 +43,7 @@ struct dl_find_object_internal
 #if DLFO_STRUCT_HAS_EH_COUNT
   int eh_count;
 #endif
+  void *sframe;
 };
 
 /* Create a copy of *SOURCE in *COPY using relaxed MO loads and
@@ -67,13 +68,14 @@ _dl_find_object_internal_copy (const struct dl_find_object_internal *source,
   atomic_store_relaxed (&copy->eh_count,
                         atomic_load_relaxed (&source->eh_count));
 #endif
+  atomic_store_relaxed (&copy->sframe,
+                        atomic_load_relaxed (&source->sframe));
 }
 
 static inline void
 _dl_find_object_to_external (struct dl_find_object_internal *internal,
                              struct dl_find_object *external)
 {
-  external->dlfo_flags = 0;
   external->dlfo_map_start = (void *) internal->map_start;
   external->dlfo_map_end = (void *) internal->map_end;
   external->dlfo_link_map = internal->map;
@@ -84,6 +86,11 @@ _dl_find_object_to_external (struct dl_find_object_internal *internal,
 # if DLFO_STRUCT_HAS_EH_COUNT
   external->dlfo_eh_count = internal->eh_count;
 # endif
+  external->dlfo_sframe = internal->sframe;
+  if (internal->sframe != NULL)
+    external->dlfo_flags = DLFO_FLAG_SFRAME;
+  else
+    external->dlfo_flags = 0;
 }
 
 /* Extract the object location data from a link map and writes it to
@@ -92,6 +99,9 @@ static void __attribute__ ((unused))
 _dl_find_object_from_map (struct link_map *l,
                           struct dl_find_object_internal *result)
 {
+  /* A mask to find out which segment has been read out.  */
+  unsigned int read_seg = 0;
+
   atomic_store_relaxed (&result->map_start, (uintptr_t) l->l_map_start);
   atomic_store_relaxed (&result->map_end, (uintptr_t) l->l_map_end);
   atomic_store_relaxed (&result->map, l);
@@ -100,23 +110,39 @@ _dl_find_object_from_map (struct link_map *l,
   atomic_store_relaxed (&result->eh_dbase, (void *) l->l_info[DT_PLTGOT]);
 #endif
 
-  for (const ElfW(Phdr) *ph = l->l_phdr, *ph_end = l->l_phdr + l->l_phnum;
-       ph < ph_end; ++ph)
-    if (ph->p_type == DLFO_EH_SEGMENT_TYPE)
-      {
-        atomic_store_relaxed (&result->eh_frame,
-                              (void *) (ph->p_vaddr + l->l_addr));
-#if DLFO_STRUCT_HAS_EH_COUNT
-        atomic_store_relaxed (&result->eh_count, ph->p_memsz / 8);
-#endif
-        return;
-      }
-
-  /* Object has no exception handling segment.  */
+  /* Initialize object's exception handling segment and SFrame segment
+     data.  */
+  atomic_store_relaxed (&result->sframe, NULL);
   atomic_store_relaxed (&result->eh_frame, NULL);
 #if DLFO_STRUCT_HAS_EH_COUNT
   atomic_store_relaxed (&result->eh_count, 0);
 #endif
+
+  for (const ElfW(Phdr) *ph = l->l_phdr, *ph_end = l->l_phdr + l->l_phnum;
+       ph < ph_end; ++ph)
+    {
+      switch (ph->p_type)
+        {
+        case DLFO_EH_SEGMENT_TYPE:
+          atomic_store_relaxed (&result->eh_frame,
+                                (void *) (ph->p_vaddr + l->l_addr));
+#if DLFO_STRUCT_HAS_EH_COUNT
+          atomic_store_relaxed (&result->eh_count, ph->p_memsz / 8);
+#endif
+          read_seg |= 1;
+          break;
+
+        case PT_GNU_SFRAME:
+          atomic_store_relaxed (&result->sframe,
+                                (void *) (ph->p_vaddr + l->l_addr));
+          read_seg |= 2;
+          /* Fall through.  */
+        default:
+          break;
+        }
+      if (read_seg == 3)
+        return;
+   }
 }
 
 /* Called by the dynamic linker to set up the data structures for the

manual/dynlink.texi

@@ -545,7 +545,8 @@ result data to the caller.
 
 @table @code
 @item unsigned long long int dlfo_flags
-Currently unused and always 0.
+Bit zero signals if SFrame stack data is valid.  See
+@code{DLFO_FLAG_SFRAME} below.
 
 @item void *dlfo_map_start
 The start address of the inspected mapping.  This information comes from
@@ -562,6 +563,11 @@ This member contains a pointer to the link map of the object.
 This member contains a pointer to the exception handling data of the
 object.  See @code{DLFO_EH_SEGMENT_TYPE} below.
 
+@item void *dlfo_sframe
+This member points to the SFrame stack trace data associated with the
+object.  It is valid only when @code{DLFO_FLAG_SFRAME} is set in
+@code{dlfo_flags}; otherwise, it may be null or undefined.
+
 @end table
 
 This structure is a GNU extension.
@@ -639,6 +645,15 @@ information is processed.
 This function is a GNU extension.
 @end deftypefun
 
+The following flag masks are defined for use with @code{dlfo_flags}:
+
+@table @code
+@item DLFO_FLAG_SFRAME
+A bit mask used to signal that the object contains SFrame data.  See
+@code{dlfo_sframe} above.
+
+@end table
+
 @node Dynamic Linker Hardening
 @section Avoiding Unexpected Issues With Dynamic Linking