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

[Jessica Yu]

+++ 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@xxxxxxxxx> wrote:
> > > On Fri, Oct 4, 2019 at 2:57 AM Matthias Maennich <maennich@xxxxxxxxxx> 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@xxxxxxxxxx/
> > > >
> > > > Reported-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
> > > > Fixes: 8651ec01daed ("module: add support for symbol namespaces.")
> > > > Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > > > Cc: Lucas De Marchi <lucas.de.marchi@xxxxxxxxx>
> > > > Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> > > > Cc: Martijn Coenen <maco@xxxxxxxxxxx>
> > > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > Cc: linux-modules@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Matthias Maennich <maennich@xxxxxxxxxx>
> > > > ---
> > > >
> > > > Please note this depends on the new ksymtab entry format proposed in
> > > > https://lore.kernel.org/lkml/20191003075826.7478-2-yamada.masahiro@xxxxxxxxxxxxx/
> > >
> > > 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@xxxxxxxxxxxxxx/),
> 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.