gary is currently certified at Master level.

Name: Gary Benson
Member since: 2001-05-31 00:39:54
Last Login: 2012-10-24 12:18:42

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I work for Red Hat.


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Recent blog entries by gary

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Judgement Day

GDB will be the weapon we fight with if we accidentally build Skynet.

Syndicated 2015-04-17 20:07:19 from

libiberty demangler fuzzer

I wrote a quick fuzzer for libiberty’s demangler this morning. Here’s how to get and use it:

$ mkdir workdir
$ cd workdir
$ git clone src
$ cd src
$ git co demangler
$ cd ..
$ mkdir build
$ cd build
$ CFLAGS="-g -O0" ../src/configure --with-separate-debug-dir=/usr/lib/debug
$ make
$ ../src/ 
+ /home/gary/workdir/build/fuzzer
../src/ line 12: 22482 Segmentation fault      (core dumped) $fuzzer
# copy the executable and PID from the two lines above
$ gdb -batch /home/gary/workdir/build/fuzzer core.22482 -ex "bt"
Core was generated by `/home/gary/workdir/build/fuzzer'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000000000040d5f2 in op_is_new_cast (op=0x7ffffac19930) at libiberty/cp-demangle.c:3064
3064	  const char *code = op->u.s_operator.op->code;
#0  0x000000000040d5f2 in op_is_new_cast (op=0x7ffffac19930) at libiberty/cp-demangle.c:3064
#1  0x000000000040dc6f in d_expression_1 (di=0x7ffffac19c90) at libiberty/cp-demangle.c:3232
#2  0x000000000040dfbc in d_expression (di=0x7ffffac19c90) at libiberty/cp-demangle.c:3315
#3  0x000000000040cff6 in d_array_type (di=0x7ffffac19c90) at libiberty/cp-demangle.c:2821
#4  0x000000000040c260 in cplus_demangle_type (di=0x7ffffac19c90) at libiberty/cp-demangle.c:2330
#5  0x000000000040cd70 in d_parmlist (di=0x7ffffac19c90) at libiberty/cp-demangle.c:2718
#6  0x000000000040ceb8 in d_bare_function_type (di=0x7ffffac19c90, has_return_type=0) at libiberty/cp-demangle.c:2772
#7  0x000000000040a9b2 in d_encoding (di=0x7ffffac19c90, top_level=1) at libiberty/cp-demangle.c:1287
#8  0x000000000040a6be in cplus_demangle_mangled_name (di=0x7ffffac19c90, top_level=1) at libiberty/cp-demangle.c:1164
#9  0x0000000000413131 in d_demangle_callback (mangled=0x7ffffac19ea0 "_Z1-Av23*;cG~Wo2Vu", options=259, callback=0x40ed11 , opaque=0x7ffffac19d70)
    at libiberty/cp-demangle.c:5862
#10 0x0000000000413267 in d_demangle (mangled=0x7ffffac19ea0 "_Z1-Av23*;cG~Wo2Vu", options=259, palc=0x7ffffac19dd8) at libiberty/cp-demangle.c:5913
#11 0x00000000004132d6 in cplus_demangle_v3 (mangled=0x7ffffac19ea0 "_Z1-Av23*;cG~Wo2Vu", options=259) at libiberty/cp-demangle.c:6070
#12 0x0000000000401a87 in cplus_demangle (mangled=0x7ffffac19ea0 "_Z1-Av23*;cG~Wo2Vu", options=259) at libiberty/cplus-dem.c:858
#13 0x0000000000400ea0 in main (argc=1, argv=0x7ffffac1a0a8) at /home/gary/workdir/src/fuzzer.c:26

The symbol that crashed it is one frame up from the bottom of the backtrace.

Syndicated 2014-05-15 13:20:10 from

Run-time linker interface

If you’re debugging an application that loads thousands of shared libraries then be sure to read the LinkerInterface page on the GDB wiki.

Syndicated 2013-10-25 15:52:14 from


I’ve been trying to figure out how to get information about libraries loaded with dlmopen out of glibc‘s runtime linker and into GDB.

The current interface uses a structure called r_debug that’s defined in link.h. If the executable’s dynamic section has a DT_DEBUG element, the runtime linker sets that element’s value to the address where this structure can be found. I tried to discover where this interface originated, but I didn’t get very far. The only mention of it I found anywhere in any standard is in the System V Application Binary Interface, where it says:

If an object file participates in dynamic linking, its program header table will have an element of type PT_DYNAMIC. This “segment” contains the .dynamic section. A special symbol, _DYNAMIC, labels the section…

and later:

This member is used for debugging. Its contents are not specified for the ABI; programs that access this entry are not ABI-conforming.

No help there then. In glibc, r_debug looks like this:

struct r_debug
  int r_version;              /* Version number for this protocol.  */

  struct link_map *r_map;     /* Head of the chain of loaded objects.  */

  /* This is the address of a function internal to the run-time linker,
     that will always be called when the linker begins to map in a
     library or unmap it, and again when the mapping change is complete.
     The debugger can set a breakpoint at this address if it wants to
     notice shared object mapping changes.  */
  ElfW(Addr) r_brk;
      /* This state value describes the mapping change taking place when
         the `r_brk' address is called.  */
      RT_CONSISTENT,          /* Mapping change is complete.  */
      RT_ADD,                 /* Beginning to add a new object.  */
      RT_DELETE               /* Beginning to remove an object mapping.  */
    } r_state;

  ElfW(Addr) r_ldbase;        /* Base address the linker is loaded at.  */

With glibc, r_version == 1. At least some versions of Solaris have r_version == 2, and when this is the case there are three extra fields, r_ldsomap, r_rdevent, r_flags. GDB uses r_ldsomap if r_version == 2; the other two seem to be the interface with librtld_db. That’s not documented anywhere to my knowledge, and may not even be fixed: applications are supposed to use the external interface to librtld_db as documented here.

Here is the problem: r_debug, as it stands, has no way to access more than one namespace. The objects in r_map are the default namespace, directly linked, or opened with dlopen, or opened with dlmopen with lmid set to LM_ID_BASE. The r_ldsomap field in Solaris’s r_debug gives access to the linker’s namespace, opened with dlmopen with lmid set to LM_ID_LDSO, but you still can’t see any other namespaces.

glibc uses multiple r_debug structures internally, one per namespace. It would be trivial to add a “next r_debug” link to r_debug if it were possible to extend the structure, but to do this you’d need to set r_version > 2. Applications could arguably expect a runtime linker with r_version > 2 to support the version 2 interface in full, but it wouldn’t be possible to do that in glibc without reverse engineering Solaris’s implementation. glibc is therefore stuck at r_version == 1, and the r_debug structure is effectively immutable for all time.

Syndicated 2013-08-30 11:03:04 from

Garden office

Garden office and view

Since two weeks ago my office has been this hillside hut. It’s a bit of a change from the pokey junkroom I’ve worked from this past twelve years. It’s a bit spartan inside right now–just a desk, a chair and a computer–but I’ll post some pictures later when I’ve jazzed it up a bit.

Syndicated 2013-07-11 12:43:25 from

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