On Sun, Apr 07, 2019 at 10:05:14AM -0700, Paul E. McKenney wrote:
On Sun, Apr 07, 2019 at 03:46:13PM +0000, Joel Fernandes wrote:
> On Sun, Apr 07, 2019 at 06:59:37AM -0700, Paul E. McKenney wrote:
> > On Sun, Apr 07, 2019 at 06:39:41AM -0700, Paul E. McKenney wrote:
> > > On Sat, Apr 06, 2019 at 07:06:13PM -0400, Joel Fernandes wrote:
> > [ . . . ]
> > > > > diff --git a/include/asm-generic/vmlinux.lds.h
> > > > > index f8f6f04c4453..c2d919a1566e 100644
> > > > > --- a/include/asm-generic/vmlinux.lds.h
> > > > > +++ b/include/asm-generic/vmlinux.lds.h
> > > > > @@ -338,6 +338,10 @@
> > > > > KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */
> > > > > __stop___tracepoints_ptrs = .; \
> > > > > *(__tracepoints_strings)/* Tracepoints: strings */ \
> > > > > + . = ALIGN(8); \
> > > > > + __start___srcu_struct = .; \
> > > > > + *(___srcu_struct_ptrs) \
> > > > > + __end___srcu_struct = .; \
> > > > > } \
> > > >
> > > > This vmlinux linker modification is not needed. I tested without it
> > > > torture works fine with rcutorture built as a module. Putting further
> > > > in kernel/module.c verified that the kernel is able to find the srcu
> > > > just fine. You could squash the below patch into this one or apply it
> > > > of the dev branch.
> > >
> > > Good point, given that otherwise FORTRAN named common blocks would not
> > > work.
> > >
> > > But isn't one advantage of leaving that stuff in the
> > > macro that it can be mapped read-only? Or am I suffering from excessive
> > > optimism?
> > And to answer the other question, in the case where I am suffering from
> > excessive optimism, it should be a separate commit. Please see below
> > for the updated original commit thus far.
> Actually the vmlinux.lds.h file is unused for module building. For ex, if you
> delete include/asm-generic/vmlinux.lds.h , then you can still build
> rcutorture.ko. Did I miss something obvious? In that case the vmlinux.lds.h
> are not needed since the __section annotations automatically place the srcu
> structs in a separate section.
Hard to argue given that I just deleted include/asm-generic/vmlinux.lds.h,
touched kernel/rcu/rcutorture.c, and rebuilt the corresponding .ko
without errors. ;-)
Hmmm... Is there some way to place a section into a read-only page,
for example, tagged onto the text section for that module? That would
get rid of a class of bugs, if nothing else.
Strictly speaking, the array of pointers in the new srcu section are fixed up
at runtime because the srcu_struct(s) they point to can be loaded at a
dynamic location in memory. The srcu_struct(s) are themselves in the .bss
section of the module and their locations depend on where the .bss section of
the module is loaded in memory at load time.
I agree that after such relocation fixups are done, there is no reason to keep
the array-of-pointers section readable but unfortunately I couldn't figure a
way out to make them read-only post the relocations.
I copied Jessica Yu who maintains module loading for any input. Jessica, as a
summary, we are trying to create a custom ELF section of srcu_struct pointers
in kernels modules, and then make the module loader do SRCU initialization
from structs pointed to by this section. The srcu_struct themselves are defined
on the .bss section. Is there any way we can make this pointer array section
read-only *after* the relocation fixups of the array are completed?
> Let me know if you would like me to send a patch separately, or
> appended patch for the same in my previous email suffices.
Please do resend as a formal patch with the above in the commit log.
I doubt that I am the only one needing a bit of module-build education!
And thank you for providing that education, by the way!
Sounds great, I will go ahead and send out a patch in the morning for this
> > And may I have your Tested-by?
> Absolutely, please do and thanks!
Done, and thank you for giving it a go!
You're very welcome. thanks,