[PATCH] s390: Remove backchain-based fallback from backtrace

[Florian Weimer <fweimer at redhat dot com>]

Stefan,

do you agree this change is correct?  It's not really clear to me how
the fallback unwinder can work given its dependency on _Unwind_GetIP.

I tested this on s390-linux-gnu and s390x-linux-gnu (but couldn't run
the C++ tests on s390 due to test environment limitations).

Thanks,
Florian
8<------------------------------------------------------------------8<
This backtrace variant is used when _Unwind_Backtrace is not found in
libgcc_s.  It still depends on _Unwind_GetIP from libgcc_s, so it does
not work if loading libgcc_s fails.  Therefore, as long as libgcc_s
provides a definition of _Unwind_Backtrace, the fallback is not useful.

After this change, the implementation is the same on s390 and s390x
and can be shared.

-----
 sysdeps/s390/{s390-32 => }/backtrace.c |  58 +------------
 sysdeps/s390/s390-64/backtrace.c       | 147 ---------------------------------
 2 files changed, 1 insertion(+), 204 deletions(-)

diff --git a/sysdeps/s390/s390-32/backtrace.c b/sysdeps/s390/backtrace.c
similarity index 60%
rename from sysdeps/s390/s390-32/backtrace.c
rename to sysdeps/s390/backtrace.c
index 497e4f8875..5801e5e211 100644
--- a/sysdeps/s390/s390-32/backtrace.c
+++ b/sysdeps/s390/backtrace.c
@@ -24,36 +24,6 @@
 #include <stdlib.h>
 #include <unwind.h>
 
-/* This is a global variable set at program start time.  It marks the
-   highest used stack address.  */
-extern void *__libc_stack_end;
-
-
-/* This is the stack layout we see for every non-leaf function.
-           size                    offset
-    %r15 ->    +------------------+
-             4 | back chain       |  0
-             4 | end of stack     |  4
-             8 | glue             |  8
-             8 | scratch          | 16
-            40 | save area r6-r15 | 24
-            16 | save area f4,f6  | 64
-            16 | empty            | 80
-               +------------------+
-   r14 in the save area holds the return address.
-*/
-
-struct layout
-{
-  int back_chain;
-  int end_of_stack;
-  int glue[2];
-  int scratch[2];
-  int save_grps[10];
-  int save_fp[4];
-  int empty[2];
-};
-
 struct trace_arg
 {
   void **array;
@@ -77,32 +47,6 @@ init (void)
   if (unwind_getip == NULL)
     unwind_backtrace = NULL;
 }
-
-static int
-__backchain_backtrace (void **array, int size)
-{
-  /* We assume that all the code is generated with frame pointers set.  */
-  struct layout *stack;
-  int cnt = 0;
-
-  __asm__ ("LR  %0,%%r15" : "=d" (stack) );
-  /* We skip the call to this function, it makes no sense to record it.  */
-  stack = (struct layout *) stack->back_chain;
-  while (cnt < size)
-    {
-      if (stack == NULL || (void *) stack > __libc_stack_end)
-	/* This means the address is out of range.  Note that for the
-	   toplevel we see a frame pointer with value NULL which clearly is
-	   out of range.  */
-	break;
-
-      array[cnt++] = (void *) (stack->save_grps[8] & 0x7fffffff);
-
-      stack = (struct layout *) stack->back_chain;
-    }
-
-  return cnt;
-}
 #else
 # define unwind_backtrace _Unwind_Backtrace
 # define unwind_getip _Unwind_GetIP
@@ -136,7 +80,7 @@ __backtrace (void **array, int size)
   __libc_once (once, init);
 
   if (unwind_backtrace == NULL)
-    return __backchain_backtrace (array, size);
+    return 0;
 #endif
 
   unwind_backtrace (backtrace_helper, &arg);
diff --git a/sysdeps/s390/s390-64/backtrace.c b/sysdeps/s390/s390-64/backtrace.c
deleted file mode 100644
index 5d14e01280..0000000000
--- a/sysdeps/s390/s390-64/backtrace.c
+++ /dev/null
@@ -1,147 +0,0 @@
-/* Return backtrace of current program state.  64 bit S/390 version.
-   Copyright (C) 2001-2020 Free Software Foundation, Inc.
-   Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#include <libc-lock.h>
-#include <dlfcn.h>
-#include <execinfo.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <unwind.h>
-
-
-/* This is a global variable set at program start time.  It marks the
-   highest used stack address.  */
-extern void *__libc_stack_end;
-
-
-/* This is the stack layout we see for every non-leaf function.
-           size                    offset
-    %r15 ->    +------------------+
-             8 | back chain       |   0
-             8 | end of stack     |   8
-            32 | scratch          |  16
-            80 | save area r6-r15 |  48
-            16 | save area f4,f6  | 128
-            16 | empty            | 144
-               +------------------+
-   r14 in the save area holds the return address.
-*/
-
-struct layout
-{
-  long back_chain;
-  long end_of_stack;
-  long scratch[4];
-  long save_grps[10];
-  long save_fp[2];
-  long empty[2];
-};
-
-struct trace_arg
-{
-  void **array;
-  int cnt, size;
-};
-
-#ifdef SHARED
-static _Unwind_Reason_Code (*unwind_backtrace) (_Unwind_Trace_Fn, void *);
-static _Unwind_Ptr (*unwind_getip) (struct _Unwind_Context *);
-
-static void
-init (void)
-{
-  void *handle = __libc_dlopen ("libgcc_s.so.1");
-
-  if (handle == NULL)
-    return;
-
-  unwind_backtrace = __libc_dlsym (handle, "_Unwind_Backtrace");
-  unwind_getip = __libc_dlsym (handle, "_Unwind_GetIP");
-  if (unwind_getip == NULL)
-    unwind_backtrace = NULL;
-}
-
-static int
-__backchain_backtrace (void **array, int size)
-{
-  /* We assume that all the code is generated with frame pointers set.  */
-  struct layout *stack;
-  int cnt = 0;
-
-  __asm__ ("LGR  %0,%%r15" : "=d" (stack) );
-  /* We skip the call to this function, it makes no sense to record it.  */
-  stack = (struct layout *) stack->back_chain;
-  while (cnt < size)
-    {
-      if (stack == NULL || (void *) stack > __libc_stack_end)
-	/* This means the address is out of range.  Note that for the
-	   toplevel we see a frame pointer with value NULL which clearly is
-	   out of range.  */
-	break;
-
-      array[cnt++] = (void *) stack->save_grps[8];
-
-      stack = (struct layout *) stack->back_chain;
-    }
-
-  return cnt;
-}
-#else
-# define unwind_backtrace _Unwind_Backtrace
-# define unwind_getip _Unwind_GetIP
-#endif
-
-static _Unwind_Reason_Code
-backtrace_helper (struct _Unwind_Context *ctx, void *a)
-{
-  struct trace_arg *arg = a;
-
-  /* We are first called with address in the __backtrace function.
-     Skip it.  */
-  if (arg->cnt != -1)
-    arg->array[arg->cnt] = (void *) unwind_getip (ctx);
-  if (++arg->cnt == arg->size)
-    return _URC_END_OF_STACK;
-  return _URC_NO_REASON;
-}
-
-int
-__backtrace (void **array, int size)
-{
-  struct trace_arg arg = { .array = array, .size = size, .cnt = -1 };
-
-  if (size <= 0)
-    return 0;
-
-#ifdef SHARED
-  __libc_once_define (static, once);
-
-  __libc_once (once, init);
-
-  if (unwind_backtrace == NULL)
-    return __backchain_backtrace (array, size);
-#endif
-
-  unwind_backtrace (backtrace_helper, &arg);
-
-  return arg.cnt != -1 ? arg.cnt : 0;
-}
-
-weak_alias (__backtrace, backtrace)
-libc_hidden_def (__backtrace)