[Libunwind-devel] [PATCH] Correct handling of DW_CFA_GNU_args_size

[Dave Watson]

When resuming execution, DW_CFA_GNU_args_size from the current frame
must be added back to the stack pointer.  Clang now generates these frequently
at -O3.  A simple repro for x86_64, that will crash with clang ~3.9 or newer:

void f(int, int,int,int,int,int,int,int,int);

int main() {
  try {
    f(0,1,2,3,4,5,6,7,8);
  } catch (int) {
    return 0;
  }
  return 1;
}

Where f is something that throws an int, but in a different translation unit to
prevent optimization.

This results in cfi instructions before the call:
 .cfi_escape 0x2e, 0x20

Grabbing the args_size means fully parsing the cfi in the current frame, which
is unfortunate because it means nearly twice the work at each step.  The logic
to grab args_size can be in unw_step or get_proc_info (since this is always
called before resuming in stack unwinding).  Putting it in get_proc_info allows
the more common unw_step code to remain fast.

It would potentially fit in nicely with a proc info cache (as mentioned in the
if0 comment block)
---
 src/dwarf/Gparser.c  | 16 ++++++++++++++--
 src/x86/Gresume.c    |  9 +++++++++
 src/x86_64/Gresume.c |  9 +++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/src/dwarf/Gparser.c b/src/dwarf/Gparser.c
index 3a47255..5336a2a 100644
--- a/src/dwarf/Gparser.c
+++ b/src/dwarf/Gparser.c
@@ -925,7 +925,19 @@ dwarf_make_proc_info (struct dwarf_cursor *c)
   if (c->as->caching_policy == UNW_CACHE_NONE
       || get_cached_proc_info (c) < 0)
 #endif
-    /* Lookup it up the slow way... */
-    return fetch_proc_info (c, c->ip, 0);
+  dwarf_state_record_t sr;
+  int ret;
+
+  /* Lookup it up the slow way... */
+  if ((ret = fetch_proc_info (c, c->ip, 1)) < 0)
+    return ret;
+  /* Also need to check if current frame contains
+     args_size, and set cursor appropriatly.  Only
+     needed for unw_resume */
+  if ((ret = dwarf_create_state_record (c, &sr)) < 0)
+    return ret;
+  put_unwind_info (c, &c->pi);
+  c->args_size = sr.args_size;
+
   return 0;
 }
diff --git a/src/x86/Gresume.c b/src/x86/Gresume.c
index cb3fb31..9bec497 100644
--- a/src/x86/Gresume.c
+++ b/src/x86/Gresume.c
@@ -63,6 +63,15 @@ establish_machine_state (struct cursor *c)
             (*access_reg) (as, reg, &val, 1, arg);
         }
     }
+
+  if (c->dwarf.args_size)
+    {
+      if (tdep_access_reg (c, RSP, &val, 0) >= 0)
+        {
+          val += c->dwarf.args_size;
+          (*access_reg) (as, RSP, &val, 1, arg);
+        }
+    }
   return 0;
 }
 
diff --git a/src/x86_64/Gresume.c b/src/x86_64/Gresume.c
index 6880d94..b880b01 100644
--- a/src/x86_64/Gresume.c
+++ b/src/x86_64/Gresume.c
@@ -95,6 +95,15 @@ establish_machine_state (struct cursor *c)
             (*access_reg) (as, reg, &val, 1, arg);
         }
     }
+
+  if (c->dwarf.args_size)
+    {
+      if (tdep_access_reg (c, RSP, &val, 0) >= 0)
+        {
+          val += c->dwarf.args_size;
+          (*access_reg) (as, RSP, &val, 1, arg);
+        }
+    }
   return 0;
 }
 
-- 
2.8.0-rc2