arm: Use _dl_find_object on __gnu_Unwind_Find_exidx (BZ 31405)

Instead of __dl_iterate_phdr. On ARM dlfo_eh_frame/dlfo_eh_count
maps to PT_ARM_EXIDX vaddr start / length.

On a Neoverse N1 machine with 160 cores, the following program:

  $ cat test.c
  #include <stdlib.h>
  #include <pthread.h>
  #include <assert.h>

  enum {
    niter = 1024,
    ntimes = 128,
  };

  static void *
  tf (void *arg)
  {
    int a = (int) arg;

    for (int i = 0; i < niter; i++)
      {
        void *p[ntimes];
        for (int j = 0; j < ntimes; j++)
  	p[j] = malloc (a * 128);
        for (int j = 0; j < ntimes; j++)
  	free (p[j]);
      }

    return NULL;
  }

  int main (int argc, char *argv[])
  {
    enum { nthreads = 16 };
    pthread_t t[nthreads];

    for (int i = 0; i < nthreads; i ++)
      assert (pthread_create (&t[i], NULL, tf, (void *) i) == 0);

    for (int i = 0; i < nthreads; i++)
      {
        void *r;
        assert (pthread_join (t[i], &r) == 0);
        assert (r == NULL);
      }

    return 0;
  }
  $ arm-linux-gnueabihf-gcc -fsanitize=address test.c -o test

Improves from ~15s to 0.5s.

Checked on arm-linux-gnueabihf.

elf/Makefile

@@ -34,6 +34,7 @@ routines = \
   dl-addr \
   dl-addr-obj \
   dl-early_allocate \
+  dl-find_object \
   dl-iteratephdr \
   dl-libc \
   dl-origin \
@@ -60,7 +61,6 @@ dl-routines = \
   dl-deps \
   dl-exception \
   dl-execstack \
-  dl-find_object \
   dl-fini \
   dl-init \
   dl-load \

elf/dl-find_object.c

@@ -356,7 +356,7 @@ _dlfo_lookup (uintptr_t pc, struct dl_find_object_internal *first1, size_t size)
 }
 
 int
-_dl_find_object (void *pc1, struct dl_find_object *result)
+__dl_find_object (void *pc1, struct dl_find_object *result)
 {
   uintptr_t pc = (uintptr_t) pc1;
 
@@ -463,7 +463,8 @@ _dl_find_object (void *pc1, struct dl_find_object *result)
         return -1;
     } /* Transaction retry loop.  */
 }
-rtld_hidden_def (_dl_find_object)
+hidden_def (__dl_find_object)
+weak_alias (__dl_find_object, _dl_find_object)
 
 /* _dlfo_process_initial is called twice.  First to compute the array
    sizes from the initial loaded mappings.  Second to fill in the

include/dlfcn.h

@@ -4,7 +4,8 @@
 #include <link.h>		/* For ElfW.  */
 #include <stdbool.h>
 
-rtld_hidden_proto (_dl_find_object)
+extern __typeof (_dl_find_object) __dl_find_object;
+hidden_proto (__dl_find_object)
 
 /* Internally used flag.  */
 #define __RTLD_DLOPEN	0x80000000

sysdeps/arm/find_exidx.c

@@ -16,64 +16,15 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <link.h>
-#include <unwind.h>
-
-struct unw_eh_callback_data
-{
-  _Unwind_Ptr pc;
-  _Unwind_Ptr exidx_start;
-  int exidx_len;
-};
-
-
-/* Callback to determines if the PC lies within an object, and remember the
-   location of the exception index table if it does.  */
-
-static int
-find_exidx_callback (struct dl_phdr_info * info, size_t size, void * ptr)
-{
-  struct unw_eh_callback_data * data;
-  const ElfW(Phdr) *phdr;
-  int i;
-  int match;
-  _Unwind_Ptr load_base;
-
-  data = (struct unw_eh_callback_data *) ptr;
-  load_base = info->dlpi_addr;
-  phdr = info->dlpi_phdr;
-
-  match = 0;
-  for (i = info->dlpi_phnum; i > 0; i--, phdr++)
-    {
-      if (phdr->p_type == PT_LOAD)
-        {
-          _Unwind_Ptr vaddr = phdr->p_vaddr + load_base;
-          if (data->pc >= vaddr && data->pc < vaddr + phdr->p_memsz)
-            match = 1;
-        }
-      else if (phdr->p_type == PT_ARM_EXIDX)
-	{
-	  data->exidx_start = (_Unwind_Ptr) (phdr->p_vaddr + load_base);
-	  data->exidx_len = phdr->p_memsz;
-	}
-    }
-
-  return match;
-}
-
 
 /* Find the exception index table containing PC.  */
 
 _Unwind_Ptr
 __gnu_Unwind_Find_exidx (_Unwind_Ptr pc, int * pcount)
 {
-  struct unw_eh_callback_data data;
-
-  data.pc = pc;
-  data.exidx_start = 0;
-  if (__dl_iterate_phdr (find_exidx_callback, &data) <= 0)
+  struct dl_find_object data;
+  if (__dl_find_object ((void *) pc, &data) < 0)
     return 0;
-
-  *pcount = data.exidx_len / 8;
-  return data.exidx_start;
+  *pcount = data.dlfo_eh_count;
+  return (_Unwind_Ptr) data.dlfo_eh_frame;
 }