This is followup to this RFC:
https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573954.html
---8<---
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.
(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.
---
libgcc/config/rs6000/linux-unwind.h | 96 ++++++++++++++++++++++++-----
libgcc/unwind.inc | 14 ++++-
2 files changed, 94 insertions(+), 16 deletions(-)
diff --git a/libgcc/config/rs6000/linux-unwind.h
b/libgcc/config/rs6000/linux-unwind.h
index acdc948f85d..af8fa01f450 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 +356,67 @@ frob_update_context (struct _Unwind_Context *context,
_Unwind_FrameState *fs ATT
}
#endif
}
+
+#define MD_BACKCHAIN_FALLBACK ppc_backchain_fallback
+
+struct trace_arg
+{
+ void **array;
+ struct unwind_link *unwind_link;
+ _Unwind_Word cfa;
+ int cnt;
+ 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 -> | %r1 last frame--------> | %r1 last frame--->... --> NULL
+ | | | |
+ | cr save | | cr save |
+ | | | |
+ | (unused) | | return address |
+ +----------------+ +-----------------+
+*/
+struct layout
+{
+ struct layout *next;
+#ifdef __powerpc64__
+ long int condition_register;
+#endif
+ void *return_address;
+};
+
+
+void ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
+{
+ struct layout *current;
+ struct trace_arg *arg = a;
+ int count;
+
+ /* Get the last address computed and start with the next. */
+ current = context->cfa;
+ current = current->next;
+
+ for (count = arg->cnt; current != NULL && count < arg->size;
+ current = current->next, count++)
+ {
+ arg->array[count] = current->return_address;
+
+ /* Check if the symbol is the signal trampoline and get the interrupted
+ * symbol address from the trampoline saved area. */
+ context->ra = current->return_address;
+ if (current->return_address != NULL && get_regs(context) != NULL)
+ {
+ 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];
+ }
+ }
+
+ arg->cnt = 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
