Re: [PATCH] depmod: create and use System.map.no_namespaces

Jessica Yu
2019-10-07T11:26:06Z

+++ Matthias Maennich [07/10/19 07:46 +0100]:
>Hi!
>
>On Sat, Oct 05, 2019 at 04:53:56PM +0900, Masahiro Yamada wrote:
>>On Sat, Oct 5, 2019 at 3:25 AM Lucas De Marchi
>><lucas.de.marchi@gmail.com> wrote:
>>>
>>>On Fri, Oct 4, 2019 at 2:57 AM Matthias Maennich <maennich@google.com> wrote:
>>>>
>>>> depmod in its current version is not aware of symbol namespace in
>>>> ksymtab entries introduced with 8651ec01daed ("module: add support for
>>>> symbol namespaces."). They have the form
>>>>
>>>> __ksymtab_NAMESPACE.symbol_name
>>>>
>>>> A fix for kmod's depmod has been proposed [1]. In order to support older
>>>> versions of depmod as well, create a System.map.no_namespaces during
>>>> scripts/depmod.sh that has the pre-namespaces format. That way users do
>>>> not immediately upgrade the userspace tool.
>>>>
>>>> [1] https://lore.kernel.org/linux-modules/20191004094136.166621-1-maennich@google.com/
>>>>
>>>> Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
>>>> Fixes: 8651ec01daed ("module: add support for symbol namespaces.")
>>>> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
>>>> Cc: Lucas De Marchi <lucas.de.marchi@gmail.com>
>>>> Cc: Jessica Yu <jeyu@kernel.org>
>>>> Cc: Martijn Coenen <maco@android.com>
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Cc: linux-modules@vger.kernel.org
>>>> Signed-off-by: Matthias Maennich <maennich@google.com>
>>>> ---
>>>>
>>>> Please note this depends on the new ksymtab entry format proposed in
>>>> https://lore.kernel.org/lkml/20191003075826.7478-2-yamada.masahiro@socionext.com/
>>>
>>>I don't really agree with that thought, more below.
>>>
>>>>
>>>> That is likely to be merged soon as well as it fixes problems in 5.4-rc*, hence
>>>> this patch depends on it.
>>>>
>>>> Cheers,
>>>> Matthias
>>>>
>>>> .gitignore | 1 +
>>>> scripts/depmod.sh | 8 +++++++-
>>>> 2 files changed, 8 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/.gitignore b/.gitignore
>>>> index 70580bdd352c..5ed58a7cb433 100644
>>>> --- a/.gitignore
>>>> +++ b/.gitignore
>>>> @@ -59,6 +59,7 @@ modules.order
>>>> /vmlinux-gdb.py
>>>> /vmlinuz
>>>> /System.map
>>>> +/System.map.no_namespaces
>>>> /Module.markers
>>>> /modules.builtin.modinfo
>>>>
>>>> diff --git a/scripts/depmod.sh b/scripts/depmod.sh
>>>> index e083bcae343f..602e1af072c7 100755
>>>> --- a/scripts/depmod.sh
>>>> +++ b/scripts/depmod.sh
>>>> @@ -39,7 +39,13 @@ if $depmod_hack_needed; then
>>>> KERNELRELEASE=99.98.$KERNELRELEASE
>>>> fi
>>>>
>>>> -set -- -ae -F System.map
>>>> +# Older versions of depmod do not support symbol namespaces in ksymtab entries,
>>>> +# hence create an alternative System.map with namespace patched out to use for
>>>> +# depmod. I.e. transform entries as follows:
>>>> +# __ksymtab_NAMESPACE.symbol_name -> __ksymtab_symbol_name
>>>> +sed 's/__ksymtab_.*\./__ksymtab_/' System.map > System.map.no_namespaces
>>>
>>>So people with old kmod will have to know they need to pass
>>>System.map.no_namespaces rather than the usual
>>>System.map. Also, distros will need to be update to also copy the new
>>>file to the kernel package (or upgrade/patch kmod).
>>>
>>>I'd rather maintain the current format and fix the bug that patch is
>>>fixing. The namespace
>>>in the end IMO is just a small annoyance with a reason to exist.
>>
>>I agree, this fix is bad.
>>We should not bother kmod or any tools.
>>And System.map.no_namespaces is a cheesy workaround.
>
>Thanks for the feedback!
>
>Based on the discussion from last year about changing kmod/depmod for
>this change to the ksymtab entries,
>(https://lore.kernel.org/lkml/CAKi4VA+wvVxPxVSP6ruSR++TYvavKPNAQ5XhREF_9ZxeOKQirg@mail.gmail.com/),
>I assumed this approach would be acceptable. The workaround (yeah, it is
>a hack) is a way to mitigate the issue for people that can't update
>depmod so easily. System.map.no_namespaces was not intended to be part
>of any distribution, but rather used locally in depmod.sh. I could have
>made this more clear.
>
>>
>>BTW, I expressed my negative opinion in the review process
>>for the patch set. I am still not convinced with the
>>namespace feature, but anyway it was merged
>>(with poor review and test).
>>
>>
>>
>>Get back on track, probably the right fix would be to
>>stop using __ksymtab_<namespace>.<symbol>.
>>
>>It is not used for any purposes but passing
>><namespace> / <symbol> pairs to modpost.
>>
>>
>>For example, __kstrtabns_##sym points to
>>the namespace string, so it would be possible
>>to parse it from modpost?
>>
>>Then, asm("__ksymtab_" #ns NS_SEPARATOR #sym)
>>will go away.
>
>I will give this a try.

That sounds like a good idea. Maybe one way of going about it is to
take the symbol value of __kstrtabns_##sym, subtract the section address
(the section referred to by the st_shndx) from it to obtain the offset
into __ksymtab_strings, and read in the namespace string from that
offset.