Re: [PATCH 2/4] Add Jeff Mahoney's py-crash patches.

[Kieran Bingham <kieranbingham at gmail dot com>]

On 02/02/16 02:55, Jeff Mahoney wrote:
> On 2/1/16 5:22 PM, Doug Evans wrote:
>> On Sun, Jan 31, 2016 at 1:44 PM, Ales Novak <alnovak@suse.cz>
>> wrote:
>>> --- gdb/Makefile.in              |  12 ++ 
>>> gdb/python/py-minsymbol.c    | 353
>>> +++++++++++++++++++++++++++++++++++++ gdb/python/py-objfile.c
>>> |  29 +++- gdb/python/py-section.c      | 401
>>> +++++++++++++++++++++++++++++++++++++++++++ 
>>> gdb/python/py-symbol.c       |  52 ++++-- 
>>> gdb/python/python-internal.h |  14 ++ gdb/python/python.c
>>> |   7 +- 7 files changed, 853 insertions(+), 15 deletions(-) 
>>> create mode 100644 gdb/python/py-minsymbol.c create mode 100644
>>> gdb/python/py-section.c
> 
> 
>> Hi.
> 
> Hi Doug -
> 
>> Part of what this patch is doing is exporting bfd to python. E.g.,
>> all the SEC_* constants.
> 
>> As a rule we absolutely discourage people from using bfd outside
>> of the the binutils+gdb source tree. Either this rule needs to
>> change, or I don't think we can allow this patch. I'd be interested
>> to hear what others in the community think.
> 
> That's unfortunate.  The Linux kernel uses ELF sections for a number
> of purposes.  Most notably is the definition of per-cpu variables.
> Without the ELF section, we can't resolve the addresses for the
> variables.  So, from our perspective, it's a requirement.

Jeff,

I haven't yet looked into your code specifically to check your per-cpu
implementation detail yet, so I'll just speculate for a moment:

Have you seen that we can obtain per_cpu variable from
linux.git/scripts/gdb/linux/cpus.py ?


def per_cpu(var_ptr, cpu):
    if cpu == -1:
        cpu = get_current_cpu()
    if utils.is_target_arch("sparc:v9"):
        offset = gdb.parse_and_eval(
            "trap_block[{0}].__per_cpu_base".format(str(cpu)))
    else:
        try:
            offset = gdb.parse_and_eval(
                "__per_cpu_offset[{0}]".format(str(cpu)))
        except gdb.error:
            # !CONFIG_SMP case
            offset = 0
    pointer = var_ptr.cast(utils.get_long_type()) + offset
    return pointer.cast(var_ptr.type).dereference()



>> For myself, I would much rather export ELF separately (e.g., a
>> separate python API one can use independent of any particular tool,
>> including gdb), and then have gdb provide the necessary glue to use
>> this API. [I can imagine some compromises being needed, at least
>> for now; e.g., it'd be cumbersome to read in all ELF symbols
>> twice. But fixing that is just an optimization.]
> 
> Ok, that's doable.  As it is, the section code mixes GDB and BFD
> pretty heavily.  It shouldn't be too difficult to separate the two out
> and push the section stuff into a new BFD python interface and
> associate the objfiles with it.

Some of our further work (/stretch goals) on Linux Kernel Awareness,
will also utilise this so I will be interested to see how it goes.

> -Jeff
> 
> 
>>> ... +  if (PyModule_AddIntConstant (gdb_module, "SEC_NO_FLAGS",
>>> SEC_NO_FLAGS) < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_ALLOC", SEC_ALLOC) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_LOAD", SEC_LOAD) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module, "SEC_RELOC", SEC_RELOC) < 0 
>>> +      || PyModule_AddIntConstant (gdb_module, "SEC_READONLY",
>>> SEC_READONLY) < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_CODE", SEC_CODE) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_DATA", SEC_DATA) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module, "SEC_ROM", SEC_ROM) < 0 +
>>> || PyModule_AddIntConstant (gdb_module, "SEC_CONSTRUCTOR", +
>>> SEC_CONSTRUCTOR) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_HAS_CONTENTS", +
>>> SEC_HAS_CONTENTS) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_NEVER_LOAD", +
>>> SEC_NEVER_LOAD) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_THREAD_LOCAL", +
>>> SEC_THREAD_LOCAL) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_HAS_GOT_REF", +
>>> SEC_HAS_GOT_REF) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_IS_COMMON", +
>>> SEC_IS_COMMON) < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_DEBUGGING", +                                 SEC_DEBUGGING)
>>> < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_IN_MEMORY", +                                 SEC_IN_MEMORY)
>>> < 0 +      || PyModule_AddIntConstant (gdb_module, "SEC_EXCLUDE",
>>> SEC_EXCLUDE) < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_SORT_ENTRIES", +
>>> SEC_SORT_ENTRIES) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_LINK_ONCE", +
>>> SEC_LINK_ONCE) < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_LINK_DUPLICATES", +
>>> SEC_LINK_DUPLICATES) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_LINK_DUPLICATES_DISCARD", +
>>> SEC_LINK_DUPLICATES_DISCARD) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module,
>>> "SEC_LINK_DUPLICATES_ONE_ONLY", +
>>> SEC_LINK_DUPLICATES_ONE_ONLY) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module,
>>> "SEC_LINK_DUPLICATES_SAME_SIZE", +
>>> SEC_LINK_DUPLICATES_SAME_SIZE) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module, "SEC_LINKER_CREATED", +
>>> SEC_LINKER_CREATED) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_KEEP", SEC_KEEP) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module, "SEC_SMALL_DATA", +
>>> SEC_SMALL_DATA) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_MERGE", SEC_MERGE) < 0 +      ||
>>> PyModule_AddIntConstant (gdb_module, "SEC_STRNGS", SEC_STRINGS) <
>>> 0 +      || PyModule_AddIntConstant (gdb_module, "SEC_GROUP",
>>> SEC_GROUP) < 0 +      || PyModule_AddIntConstant (gdb_module,
>>> "SEC_COFF_SHARED_LIBRARY", +
>>> SEC_COFF_SHARED_LIBRARY) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_ELF_REVERSE_COPY", +
>>> SEC_ELF_REVERSE_COPY) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_COFF_SHARED", +
>>> SEC_COFF_SHARED) < 0 +      || PyModule_AddIntConstant
>>> (gdb_module, "SEC_COFF_NOREAD", +
>>> SEC_COFF_NOREAD) < 0) +    return -1; ...
> 
> 
> 
>