[PATCH v2] libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

[PATCH v2] libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

[Raphael Moreira Zinsly]

Without dwarf2 unwind tables available _Unwind_Backtrace() is not
able to return the full backtrace.
This patch adds a fallback function on powerpc to get the backtrace
by doing a backchain, this code was originally at glibc.

libgcc/ChangeLog:

	* config/rs6000/linux-unwind.h (struct rt_sigframe): Move it to
	outside of get_regs() in order to use it in another function,
	this is done twice: for __powerpc64__ and for !__powerpc64__.
	(struct trace_arg): New struct.
	(struct layout): New struct.
	(ppc_backchain_fallback): New function.
	* unwind.inc (_Unwind_Backtrace): Look for _URC_NORMAL_STOP
	code state and call MD_BACKCHAIN_FALLBACK.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/unwind-backchain.c: New test.
---
 .../gcc.target/powerpc/unwind-backchain.c     |  22 ++++
 libgcc/config/rs6000/linux-unwind.h           | 102 +++++++++++++++---
 libgcc/unwind.inc                             |  14 ++-
 3 files changed, 122 insertions(+), 16 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/powerpc/unwind-backchain.c

diff --git a/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
new file mode 100644
index 00000000000..fdce78a1f63
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
@@ -0,0 +1,22 @@
+/* { dg-do run { target { powerpc*-*-linux* } } } */
+/* { dg-options "-fno-asynchronous-unwind-tables" } */
+
+#include <execinfo.h>
+
+void
+test_backtrace()
+{
+  int addresses;
+  void *buffer[10];
+
+  addresses = backtrace(buffer, 10);
+  if(addresses != 4)
+    __builtin_abort();
+}
+
+int
+main()
+{
+  test_backtrace();
+  return 0;
+}
diff --git a/libgcc/config/rs6000/linux-unwind.h b/libgcc/config/rs6000/linux-unwind.h
index acdc948f85d..8deccc1d650 100644
--- a/libgcc/config/rs6000/linux-unwind.h
+++ b/libgcc/config/rs6000/linux-unwind.h
@@ -94,6 +94,15 @@ struct gcc_ucontext
 
 enum { SIGNAL_FRAMESIZE = 128 };
 
+struct rt_sigframe {
+  char gap[SIGNAL_FRAMESIZE];
+  struct gcc_ucontext uc;
+  unsigned long pad[2];
+  int tramp[6];
+  void *pinfo;
+  struct gcc_ucontext *puc;
+};
+
 /* If PC is at a sigreturn trampoline, return a pointer to the
    regs.  Otherwise return NULL.  */
 
@@ -136,14 +145,7 @@ get_regs (struct _Unwind_Context *context)
 #endif
 	{
 	  /* This works for 2.4.21 and later kernels.  */
-	  struct rt_sigframe {
-	    char gap[SIGNAL_FRAMESIZE];
-	    struct gcc_ucontext uc;
-	    unsigned long pad[2];
-	    int tramp[6];
-	    void *pinfo;
-	    struct gcc_ucontext *puc;
-	  } *frame = (struct rt_sigframe *) context->cfa;
+	  struct rt_sigframe *frame = (struct rt_sigframe *) context->cfa;
 	  return frame->uc.regs;
 	}
     }
@@ -154,6 +156,12 @@ get_regs (struct _Unwind_Context *context)
 
 enum { SIGNAL_FRAMESIZE = 64 };
 
+struct rt_sigframe {
+  char gap[SIGNAL_FRAMESIZE + 16];
+  char siginfo[128];
+  struct gcc_ucontext uc;
+};
+
 static struct gcc_regs *
 get_regs (struct _Unwind_Context *context)
 {
@@ -176,11 +184,7 @@ get_regs (struct _Unwind_Context *context)
     }
   else if (pc[0] == 0x38006666 || pc[0] == 0x380000AC)
     {
-      struct rt_sigframe {
-	char gap[SIGNAL_FRAMESIZE + 16];
-	char siginfo[128];
-	struct gcc_ucontext uc;
-      } *frame = (struct rt_sigframe *) context->cfa;
+      struct rt_sigframe *frame = (struct rt_sigframe *) context->cfa;
       return frame->uc.regs;
     }
   return NULL;
@@ -203,7 +207,7 @@ ppc_fallback_frame_state (struct _Unwind_Context *context,
   int i;
 
   if (regs == NULL)
-    return _URC_END_OF_STACK;
+    return _URC_NORMAL_STOP;
 
   new_cfa = regs->gpr[__LIBGCC_STACK_POINTER_REGNUM__];
   fs->regs.cfa_how = CFA_REG_OFFSET;
@@ -352,3 +356,73 @@ frob_update_context (struct _Unwind_Context *context, _Unwind_FrameState *fs ATT
     }
 #endif
 }
+
+#define MD_BACKCHAIN_FALLBACK ppc_backchain_fallback
+
+struct trace_arg
+{
+  /* Stores the list of addresses.  */
+  void **array;
+  struct unwind_link *unwind_link;
+  _Unwind_Word cfa;
+  /* Number of addresses currently stored.  */
+  int count;
+  /* Maximum number of addresses.  */
+  int size;
+};
+
+/* This is the stack layout we see with every stack frame.
+   Note that every routine is required by the ABI to lay out the stack
+   like this.
+
+	    +----------------+        +-----------------+
+    %r1  -> | previous frame--------> | previous frame--->...  --> NULL
+	    |                |        |                 |
+	    | cr save        |        | cr save	        |
+	    |                |        |                 |
+	    | (unused)       |        | lr save         |
+	    +----------------+        +-----------------+
+
+  The CR save is only present on 64-bit ABIs.
+*/
+struct frame_layout
+{
+  struct frame_layout *backchain;
+#ifdef __powerpc64__
+  long int cr_save;
+#endif
+  void *lr_save;
+};
+
+
+void ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
+{
+  struct frame_layout *current;
+  struct trace_arg *arg = a;
+  int count;
+
+  /* Get the last address computed and start with the next.  */
+  current = context->cfa;
+  current = current->backchain;
+
+  for (count = arg->count; current != NULL; current = current->backchain)
+    {
+      arg->array[count] = current->lr_save;
+
+      /* Check if the symbol is the signal trampoline and get the interrupted
+	 symbol address from the trampoline saved area.  */
+      context->ra = current->lr_save;
+      if (current->lr_save && get_regs (context))
+	{
+	  struct rt_sigframe *sigframe = (struct rt_sigframe *) current;
+	  if (count + 1 == arg->size)
+	    break;
+	  arg->array[++count] = (void *) sigframe->uc.rsave.nip;
+	  current = (void *) sigframe->uc.rsave.gpr[1];
+	}
+      if (count++ >= arg->size)
+	break;
+    }
+
+  arg->count = count-1;
+}
diff --git a/libgcc/unwind.inc b/libgcc/unwind.inc
index aa48d104fd0..456a5ee682f 100644
--- a/libgcc/unwind.inc
+++ b/libgcc/unwind.inc
@@ -300,14 +300,24 @@ _Unwind_Backtrace(_Unwind_Trace_Fn trace, void * trace_argument)
 
       /* Set up fs to describe the FDE for the caller of context.  */
       code = uw_frame_state_for (&context, &fs);
-      if (code != _URC_NO_REASON && code != _URC_END_OF_STACK)
+      if (code != _URC_NO_REASON && code != _URC_END_OF_STACK
+          && code != _URC_NORMAL_STOP)
 	return _URC_FATAL_PHASE1_ERROR;
 
       /* Call trace function.  */
       if ((*trace) (&context, trace_argument) != _URC_NO_REASON)
 	return _URC_FATAL_PHASE1_ERROR;
 
-      /* We're done at end of stack.  */	
+#ifdef MD_BACKCHAIN_FALLBACK
+      /* Do a backchain if there is no DWARF data.  */
+      if (code == _URC_NORMAL_STOP)
+	{
+	  MD_BACKCHAIN_FALLBACK(&context, trace_argument);
+	  break;
+	}
+#endif
+
+      /* We're done at end of stack.  */
       if (code == _URC_END_OF_STACK)
 	break;
 
-- 
2.31.1

Re: [PATCH v2] libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

[Raphael M Zinsly]

Ping

On 26/08/2021 11:53, Raphael Moreira Zinsly wrote:
> Without dwarf2 unwind tables available _Unwind_Backtrace() is not
> able to return the full backtrace.
> This patch adds a fallback function on powerpc to get the backtrace
> by doing a backchain, this code was originally at glibc.
> 
> libgcc/ChangeLog:
> 
> 	* config/rs6000/linux-unwind.h (struct rt_sigframe): Move it to
> 	outside of get_regs() in order to use it in another function,
> 	this is done twice: for __powerpc64__ and for !__powerpc64__.
> 	(struct trace_arg): New struct.
> 	(struct layout): New struct.
> 	(ppc_backchain_fallback): New function.
> 	* unwind.inc (_Unwind_Backtrace): Look for _URC_NORMAL_STOP
> 	code state and call MD_BACKCHAIN_FALLBACK.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/powerpc/unwind-backchain.c: New test.
> ---
>   .../gcc.target/powerpc/unwind-backchain.c     |  22 ++++
>   libgcc/config/rs6000/linux-unwind.h           | 102 +++++++++++++++---
>   libgcc/unwind.inc                             |  14 ++-
>   3 files changed, 122 insertions(+), 16 deletions(-)
>   create mode 100644 gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
> 
> diff --git a/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
> new file mode 100644
> index 00000000000..fdce78a1f63
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
> @@ -0,0 +1,22 @@
> +/* { dg-do run { target { powerpc*-*-linux* } } } */
> +/* { dg-options "-fno-asynchronous-unwind-tables" } */
> +
> +#include <execinfo.h>
> +
> +void
> +test_backtrace()
> +{
> +  int addresses;
> +  void *buffer[10];
> +
> +  addresses = backtrace(buffer, 10);
> +  if(addresses != 4)
> +    __builtin_abort();
> +}
> +
> +int
> +main()
> +{
> +  test_backtrace();
> +  return 0;
> +}
> diff --git a/libgcc/config/rs6000/linux-unwind.h b/libgcc/config/rs6000/linux-unwind.h
> index acdc948f85d..8deccc1d650 100644
> --- a/libgcc/config/rs6000/linux-unwind.h
> +++ b/libgcc/config/rs6000/linux-unwind.h
> @@ -94,6 +94,15 @@ struct gcc_ucontext
>   
>   enum { SIGNAL_FRAMESIZE = 128 };
>   
> +struct rt_sigframe {
> +  char gap[SIGNAL_FRAMESIZE];
> +  struct gcc_ucontext uc;
> +  unsigned long pad[2];
> +  int tramp[6];
> +  void *pinfo;
> +  struct gcc_ucontext *puc;
> +};
> +
>   /* If PC is at a sigreturn trampoline, return a pointer to the
>      regs.  Otherwise return NULL.  */
>   
> @@ -136,14 +145,7 @@ get_regs (struct _Unwind_Context *context)
>   #endif
>   	{
>   	  /* This works for 2.4.21 and later kernels.  */
> -	  struct rt_sigframe {
> -	    char gap[SIGNAL_FRAMESIZE];
> -	    struct gcc_ucontext uc;
> -	    unsigned long pad[2];
> -	    int tramp[6];
> -	    void *pinfo;
> -	    struct gcc_ucontext *puc;
> -	  } *frame = (struct rt_sigframe *) context->cfa;
> +	  struct rt_sigframe *frame = (struct rt_sigframe *) context->cfa;
>   	  return frame->uc.regs;
>   	}
>       }
> @@ -154,6 +156,12 @@ get_regs (struct _Unwind_Context *context)
>   
>   enum { SIGNAL_FRAMESIZE = 64 };
>   
> +struct rt_sigframe {
> +  char gap[SIGNAL_FRAMESIZE + 16];
> +  char siginfo[128];
> +  struct gcc_ucontext uc;
> +};
> +
>   static struct gcc_regs *
>   get_regs (struct _Unwind_Context *context)
>   {
> @@ -176,11 +184,7 @@ get_regs (struct _Unwind_Context *context)
>       }
>     else if (pc[0] == 0x38006666 || pc[0] == 0x380000AC)
>       {
> -      struct rt_sigframe {
> -	char gap[SIGNAL_FRAMESIZE + 16];
> -	char siginfo[128];
> -	struct gcc_ucontext uc;
> -      } *frame = (struct rt_sigframe *) context->cfa;
> +      struct rt_sigframe *frame = (struct rt_sigframe *) context->cfa;
>         return frame->uc.regs;
>       }
>     return NULL;
> @@ -203,7 +207,7 @@ ppc_fallback_frame_state (struct _Unwind_Context *context,
>     int i;
>   
>     if (regs == NULL)
> -    return _URC_END_OF_STACK;
> +    return _URC_NORMAL_STOP;
>   
>     new_cfa = regs->gpr[__LIBGCC_STACK_POINTER_REGNUM__];
>     fs->regs.cfa_how = CFA_REG_OFFSET;
> @@ -352,3 +356,73 @@ frob_update_context (struct _Unwind_Context *context, _Unwind_FrameState *fs ATT
>       }
>   #endif
>   }
> +
> +#define MD_BACKCHAIN_FALLBACK ppc_backchain_fallback
> +
> +struct trace_arg
> +{
> +  /* Stores the list of addresses.  */
> +  void **array;
> +  struct unwind_link *unwind_link;
> +  _Unwind_Word cfa;
> +  /* Number of addresses currently stored.  */
> +  int count;
> +  /* Maximum number of addresses.  */
> +  int size;
> +};
> +
> +/* This is the stack layout we see with every stack frame.
> +   Note that every routine is required by the ABI to lay out the stack
> +   like this.
> +
> +	    +----------------+        +-----------------+
> +    %r1  -> | previous frame--------> | previous frame--->...  --> NULL
> +	    |                |        |                 |
> +	    | cr save        |        | cr save	        |
> +	    |                |        |                 |
> +	    | (unused)       |        | lr save         |
> +	    +----------------+        +-----------------+
> +
> +  The CR save is only present on 64-bit ABIs.
> +*/
> +struct frame_layout
> +{
> +  struct frame_layout *backchain;
> +#ifdef __powerpc64__
> +  long int cr_save;
> +#endif
> +  void *lr_save;
> +};
> +
> +
> +void ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
> +{
> +  struct frame_layout *current;
> +  struct trace_arg *arg = a;
> +  int count;
> +
> +  /* Get the last address computed and start with the next.  */
> +  current = context->cfa;
> +  current = current->backchain;
> +
> +  for (count = arg->count; current != NULL; current = current->backchain)
> +    {
> +      arg->array[count] = current->lr_save;
> +
> +      /* Check if the symbol is the signal trampoline and get the interrupted
> +	 symbol address from the trampoline saved area.  */
> +      context->ra = current->lr_save;
> +      if (current->lr_save && get_regs (context))
> +	{
> +	  struct rt_sigframe *sigframe = (struct rt_sigframe *) current;
> +	  if (count + 1 == arg->size)
> +	    break;
> +	  arg->array[++count] = (void *) sigframe->uc.rsave.nip;
> +	  current = (void *) sigframe->uc.rsave.gpr[1];
> +	}
> +      if (count++ >= arg->size)
> +	break;
> +    }
> +
> +  arg->count = count-1;
> +}
> diff --git a/libgcc/unwind.inc b/libgcc/unwind.inc
> index aa48d104fd0..456a5ee682f 100644
> --- a/libgcc/unwind.inc
> +++ b/libgcc/unwind.inc
> @@ -300,14 +300,24 @@ _Unwind_Backtrace(_Unwind_Trace_Fn trace, void * trace_argument)
>   
>         /* Set up fs to describe the FDE for the caller of context.  */
>         code = uw_frame_state_for (&context, &fs);
> -      if (code != _URC_NO_REASON && code != _URC_END_OF_STACK)
> +      if (code != _URC_NO_REASON && code != _URC_END_OF_STACK
> +          && code != _URC_NORMAL_STOP)
>   	return _URC_FATAL_PHASE1_ERROR;
>   
>         /* Call trace function.  */
>         if ((*trace) (&context, trace_argument) != _URC_NO_REASON)
>   	return _URC_FATAL_PHASE1_ERROR;
>   
> -      /* We're done at end of stack.  */	
> +#ifdef MD_BACKCHAIN_FALLBACK
> +      /* Do a backchain if there is no DWARF data.  */
> +      if (code == _URC_NORMAL_STOP)
> +	{
> +	  MD_BACKCHAIN_FALLBACK(&context, trace_argument);
> +	  break;
> +	}
> +#endif
> +
> +      /* We're done at end of stack.  */
>         if (code == _URC_END_OF_STACK)
>   	break;
>   
> 

-- 
Raphael Moreira Zinsly

Re: [PATCH v2] libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

[Segher Boessenkool]

Hi!

On Thu, Aug 26, 2021 at 11:53:24AM -0300, Raphael Moreira Zinsly wrote:
> Without dwarf2 unwind tables available _Unwind_Backtrace() is not
> able to return the full backtrace.
> This patch adds a fallback function on powerpc to get the backtrace
> by doing a backchain, this code was originally at glibc.

Okay, the backchain as fallback if other (better!) methods cannot work.

> 	* config/rs6000/linux-unwind.h (struct rt_sigframe): Move it to
> 	outside of get_regs() in order to use it in another function,
> 	this is done twice: for __powerpc64__ and for !__powerpc64__.
> 	(struct trace_arg): New struct.
> 	(struct layout): New struct.
> 	(ppc_backchain_fallback): New function.
> 	* unwind.inc (_Unwind_Backtrace): Look for _URC_NORMAL_STOP
> 	code state and call MD_BACKCHAIN_FALLBACK.

Changelog lines wrap at 80 chars, not 70 or so.  The emails from commits
(to bugzilla) are a bit malformed (it counts the number of columns for
leading tabs wrong it seems), but the actual commits are just fine.

> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
> @@ -0,0 +1,22 @@
> +/* { dg-do run { target { powerpc*-*-linux* } } } */

Don't say such targets in gcc.target/powerpc/ tests please.  Everything
in gcc.target is for powerpc*-*-* already, so if you really want to
limit to powerpc*-*-linux* just write *-*-linux*.  But there are better
ways to get what you want, like, testing for the actual feature you want
(which is if backtrace() works?)  But such an improvement can be done
later (and needs more testing etc).

But please write some simple comment saying why you need -linux* in the
test.

> +void
> +test_backtrace()
> +{
> +  int addresses;
> +  void *buffer[10];
> +
> +  addresses = backtrace(buffer, 10);
> +  if(addresses != 4)
> +    __builtin_abort();
> +}

Does that work?!  Has this been tested on all powerpc*-linux configs?
Importantly also BE and 32-bit.

Okay for trunk with the testcase fix, if all testing works out.  Thanks!


Segher

Re: [PATCH v2] libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

[Raphael M Zinsly]

On 28/09/2021 16:50, Segher Boessenkool wrote:
> Hi!
> 
> On Thu, Aug 26, 2021 at 11:53:24AM -0300, Raphael Moreira Zinsly wrote:
>> Without dwarf2 unwind tables available _Unwind_Backtrace() is not
>> able to return the full backtrace.
>> This patch adds a fallback function on powerpc to get the backtrace
>> by doing a backchain, this code was originally at glibc.
> 
> Okay, the backchain as fallback if other (better!) methods cannot work.
> 
>> 	* config/rs6000/linux-unwind.h (struct rt_sigframe): Move it to
>> 	outside of get_regs() in order to use it in another function,
>> 	this is done twice: for __powerpc64__ and for !__powerpc64__.
>> 	(struct trace_arg): New struct.
>> 	(struct layout): New struct.
>> 	(ppc_backchain_fallback): New function.
>> 	* unwind.inc (_Unwind_Backtrace): Look for _URC_NORMAL_STOP
>> 	code state and call MD_BACKCHAIN_FALLBACK.
> 
> Changelog lines wrap at 80 chars, not 70 or so.  The emails from commits
> (to bugzilla) are a bit malformed (it counts the number of columns for
> leading tabs wrong it seems), but the actual commits are just fine.
> 
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
>> @@ -0,0 +1,22 @@
>> +/* { dg-do run { target { powerpc*-*-linux* } } } */
> 
> Don't say such targets in gcc.target/powerpc/ tests please.  Everything
> in gcc.target is for powerpc*-*-* already, so if you really want to
> limit to powerpc*-*-linux* just write *-*-linux*.  But there are better
> ways to get what you want, like, testing for the actual feature you want
> (which is if backtrace() works?)  But such an improvement can be done
> later (and needs more testing etc).
> 

Ok, I'll change to *-*-linux*.
I used powerpc*-*-linux* because most of the tests in this directory do 
that way.

> But please write some simple comment saying why you need -linux* in the
> test.
> 
>> +void
>> +test_backtrace()
>> +{
>> +  int addresses;
>> +  void *buffer[10];
>> +
>> +  addresses = backtrace(buffer, 10);
>> +  if(addresses != 4)
>> +    __builtin_abort();
>> +}
> 
> Does that work?!  Has this been tested on all powerpc*-linux configs?
> Importantly also BE and 32-bit.
> 

I haven't tested on BE, will do.

> Okay for trunk with the testcase fix, if all testing works out.  Thanks!
> 
> 
> Segher
> 

Thanks for the review,
-- 
Raphael Moreira Zinsly

Re: [PATCH v2] libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

[Segher Boessenkool]

On Wed, Sep 29, 2021 at 11:14:55AM -0300, Raphael M Zinsly wrote:
> On 28/09/2021 16:50, Segher Boessenkool wrote:
> >>+/* { dg-do run { target { powerpc*-*-linux* } } } */
> >
> >Don't say such targets in gcc.target/powerpc/ tests please.  Everything
> >in gcc.target is for powerpc*-*-* already, so if you really want to
> >limit to powerpc*-*-linux* just write *-*-linux*.  But there are better
> >ways to get what you want, like, testing for the actual feature you want
> >(which is if backtrace() works?)  But such an improvement can be done
> >later (and needs more testing etc).
> 
> Ok, I'll change to *-*-linux*.
> I used powerpc*-*-linux* because most of the tests in this directory do 
> that way.

Yes, that is the problem with cut-and-paste.  The dejagnu directives are
not magic incantations, they have a pretty clear meaning (and are
documented even).  Maybe we need a cookbook though :-)


Segher