[Bug c/24724] New: _Unwind_Backtrace() calls malloc

[Bug c/24724] New: _Unwind_Backtrace() calls malloc

[arun dot sharma at google dot com]

As this stacktrace shows:

#3  0x00000000004044e2 in malloc (size=36024) at tcmalloc.cc:1314
#4  0x000000000047a938 in search_object ()
#5  0x000000000047b189 in _Unwind_Find_FDE ()
#6  0x0000000000478049 in uw_frame_state_for ()
#7  0x0000000000478eca in uw_init_context_1 ()
#8  0x00000000004790b0 in _Unwind_Backtrace ()

there are code paths from _Unwind_Backtrace to malloc. This makes the unwinder
deadlock prone when called from applications that have their own customized
malloc.


-- 
           Summary: _Unwind_Backtrace() calls malloc
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: arun dot sharma at google dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2005-11-08 00:23 -------
What is your malloc doing special and why would it dead lock?  (if you are
throwing from inside malloc I think that is an invalid thing to do).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[arun dot sharma at google dot com]

------- Comment #2 from arun dot sharma at google dot com  2005-11-08 00:48 -------

It deadlocks because malloc is holding a lock and then calls the unwinder.
No, we're not throwing exceptions. One reason why malloc might want to use the
unwinder is to do heap profiling.

http://goog-perftools.sourceforge.net/doc/heap_profiler.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2005-11-08 00:51 -------
You know that glibc has an backtrace function which might be more friendly for
your purpose?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2005-11-08 00:53 -------
I really doubt we can remove it because this is also used in the undwinding for
exceptions.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[arun dot sharma at google dot com]

------- Comment #5 from arun dot sharma at google dot com  2005-11-08 00:55 -------
(In reply to comment #3)
> You know that glibc has an backtrace function which might be more friendly for
> your purpose?
> 

glibc backtrace dlopens libgcc and uses _Unwind_Backtrace() on amd64. glibc
backtrace has it's own problems (i.e. mallocs) which is why we're not using it.

See: 

http://sources.redhat.com/bugzilla/show_bug.cgi?id=1579


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2005-11-08 01:02 -------
Hmm, You could try libunwind instead, it should work on x86_64:
http://www.hpl.hp.com/research/linux/libunwind/

They show you how to use libunwind to generate a normal backtrace:
http://www.hpl.hp.com/research/linux/libunwind/man/libunwind(3).php

Though I doubt that none of these will remove the use of malloc though.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[arun dot sharma at google dot com]

------- Comment #7 from arun dot sharma at google dot com  2005-11-08 01:07 -------
(In reply to comment #4)
> I really doubt we can remove it because this is also used in the undwinding for
> exceptions.
> 

It must be possible to do stack unwinding without any mallocs. If the exception
throwing code path requires mallocs, that's fine by us.

The particular malloc in question is coming from start_fde_sort() in
unwind-dw2-fde.c. Perhaps the sorting can be done earlier i.e. before
_Unwind_Backtrace() is called?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[arun dot sharma at google dot com]

------- Comment #8 from arun dot sharma at google dot com  2005-11-08 01:09 -------
(In reply to comment #6)
> Hmm, You could try libunwind instead, it should work on x86_64:
> http://www.hpl.hp.com/research/linux/libunwind/
> 
> They show you how to use libunwind to generate a normal backtrace:
> http://www.hpl.hp.com/research/linux/libunwind/man/libunwind(3).php
> 
> Though I doubt that none of these will remove the use of malloc though.
> 

libunwind doesn't pass unit tests on amd64. davidm thinks that the problems are
outside of libunwind. I think he has a couple of bugs open against gcc/glibc.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2005-11-08 01:10 -------
(In reply to comment #8)
> libunwind doesn't pass unit tests on amd64. davidm thinks that the problems are
> outside of libunwind. I think he has a couple of bugs open against gcc/glibc.

Yes and the ones against gcc are only about eplogue or prologue so it should
not matter for what you are doing.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #10 from pinskia at gcc dot gnu dot org  2005-11-08 01:12 -------
(In reply to comment #9)
> Yes and the ones against gcc are only about eplogue or prologue so it should
> not matter for what you are doing.

PR 18748 and PR 18749 both are about prologue and eplogue code which should not
matter with the backtrace at all.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2005-11-08 01:23 -------
(In reply to comment #7)
> The particular malloc in question is coming from start_fde_sort() in
> unwind-dw2-fde.c. Perhaps the sorting can be done earlier i.e. before
> _Unwind_Backtrace() is called?

If you do that, the start up time is high and every time you load a shared
library it stalls and you keep around stuff which you don't need at all.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[arun dot sharma at google dot com]

------- Comment #12 from arun dot sharma at google dot com  2005-11-08 01:30 -------
(In reply to comment #10)
> (In reply to comment #9)
> > Yes and the ones against gcc are only about eplogue or prologue so it should
> > not matter for what you are doing.
> 
> PR 18748 and PR 18749 both are about prologue and eplogue code which should not
> matter with the backtrace at all.
> 

ok, will try to root cause our problems with libunwind (they show up as bad
pointer dereferences in libunwind) and get back to you.

Thanks.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[rth at gcc dot gnu dot org]

------- Comment #13 from rth at gcc dot gnu dot org  2010-08-04 23:08 -------
There are two solutions to this:

(1) Make sure your binary provides PT_GNU_EH_FRAME.  This is the quickest
    path through the unwinder, since the table is pre-sorted by the linker.

(2) Have your malloc detect the recursion and return NULL.  This will cause
    the unwinder to perform a linear search through the unsorted tables.
    It should not fail due to the fake out-of-memory condition, since it
    was designed to handle throwing an exception during a true OOM condition.


-- 

rth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WONTFIX


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

[Bug other/24724] _Unwind_Backtrace() calls malloc

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
   Last reconfirmed|                            |2012-09-13
         Resolution|WONTFIX                     |
     Ever Confirmed|0                           |1

--- Comment #14 from H.J. Lu <hjl.tools at gmail dot com> 2012-09-13 13:43:42 UTC ---
(In reply to comment #13)
> There are two solutions to this:
> 
> (1) Make sure your binary provides PT_GNU_EH_FRAME.  This is the quickest
>     path through the unwinder, since the table is pre-sorted by the linker.

This isn't the problem.

> (2) Have your malloc detect the recursion and return NULL.  This will cause
>     the unwinder to perform a linear search through the unsorted tables.
>     It should not fail due to the fake out-of-memory condition, since it
>     was designed to handle throwing an exception during a true OOM condition.

The problem is _Unwind_Find_FDE in unwind-dw2-fde.c
calls search_object to find FDE in the registered objects,
which is loaded unsorted from .eh_frame section.  Can
we use .eh_frame_hdr section to load the sorted table
directly?

[Bug other/24724] _Unwind_Backtrace() calls malloc

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

Ian Lance Taylor <ian at airs dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ian at airs dot com

--- Comment #15 from Ian Lance Taylor <ian at airs dot com> 2012-09-13 14:06:03 UTC ---
On a system whose linker supports --eh-frame-hdr, we will use the version of
_Unwind_Find_FDE in unwind-dw2-fde-dip.c.  It will override the version in
unwind-dw2-fde.c by renaming it via #define.  This file is selected by
libgcc/config/t-eh-dw2-dip.  It will still call the version of
_Unwind_Find_FDE, but that function will only look through files registered by
__register_frame_info_bases.  __register_frame_info_bases is called by
crtstuff.c, but it is only called on systems whose linker does not support
--eh-frame-hdr.

So on what system are you actually seeing a call to qsort?  Does that system
have a linker that supports --eh-frame-hdr?

[Bug other/24724] _Unwind_Backtrace() calls malloc

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24724

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |WONTFIX

--- Comment #16 from H.J. Lu <hjl.tools at gmail dot com> 2012-09-13 14:20:21 UTC ---
It is an Android target bug.