Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules

From: Paul E. McKenney
Date: Sun Apr 07 2019 - 13:10:13 EST

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 b/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 and srcu
> > > > torture works fine with rcutorture built as a module. Putting further prints
> > > > in kernel/module.c verified that the kernel is able to find the srcu structs
> > > > just fine. You could squash the below patch into this one or apply it on top
> > > > 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 RO_DATA_SECTION()
> > > 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.

> Let me know if you would like me to send a patch separately, or if the
> 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!

> > And may I have your Tested-by?
>
> Absolutely, please do and thanks!

Done, and thank you for giving it a go!

Thanx, Paul

> - Joel
>
>
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit a365bb5f6eafb220a1448674054b05c250829313
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> > Date: Fri Apr 5 16:15:00 2019 -0700
> >
> > srcu: Allocate per-CPU data for DEFINE_SRCU() in modules
> >
> > Adding DEFINE_SRCU() or DEFINE_STATIC_SRCU() to a loadable module requires
> > that the size of the reserved region be increased, which is not something
> > we want to be doing all that often. One approach would be to require
> > that loadable modules define an srcu_struct and invoke init_srcu_struct()
> > from their module_init function and cleanup_srcu_struct() from their
> > module_exit function. However, this is more than a bit user unfriendly.
> >
> > This commit therefore creates an ___srcu_struct_ptrs linker section,
> > and pointers to srcu_struct structures created by DEFINE_SRCU() and
> > DEFINE_STATIC_SRCU() within a module are placed into that module's
> > ___srcu_struct_ptrs section. The required init_srcu_struct() and
> > cleanup_srcu_struct() functions are then automatically invoked as needed
> > when that module is loaded and unloaded, thus allowing modules to continue
> > to use DEFINE_SRCU() and DEFINE_STATIC_SRCU() while avoiding the need
> > to increase the size of the reserved region.
> >
> > Many of the algorithms and some of the code was cheerfully cherry-picked
> > from other code making use of linker sections, perhaps most notably from
> > tracepoints. All bugs are nevertheless the sole property of the author.
> >
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > [ paulmck: Use __section() and use "default" in srcu_module_notify()'s
> > "switch" statement as suggested by Joel Fernandes. ]
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/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 = .; \
> > } \
> > \
> > .rodata1 : AT(ADDR(.rodata1) - LOAD_OFFSET) { \
> > diff --git a/include/linux/module.h b/include/linux/module.h
> > index 5bf5dcd91009..921443a026dd 100644
> > --- a/include/linux/module.h
> > +++ b/include/linux/module.h
> > @@ -21,6 +21,7 @@
> > #include <linux/rbtree_latch.h>
> > #include <linux/error-injection.h>
> > #include <linux/tracepoint-defs.h>
> > +#include <linux/srcu.h>
> >
> > #include <linux/percpu.h>
> > #include <asm/module.h>
> > @@ -448,6 +449,10 @@ struct module {
> > unsigned int num_tracepoints;
> > tracepoint_ptr_t *tracepoints_ptrs;
> > #endif
> > +#ifdef CONFIG_TREE_SRCU
> > + unsigned int num_srcu_structs;
> > + struct srcu_struct **srcu_struct_ptrs;
> > +#endif
> > #ifdef CONFIG_BPF_EVENTS
> > unsigned int num_bpf_raw_events;
> > struct bpf_raw_event_map *bpf_raw_events;
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index 7f7c8c050f63..8af1824c46a8 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -120,9 +120,17 @@ struct srcu_struct {
> > *
> > * See include/linux/percpu-defs.h for the rules on per-CPU variables.
> > */
> > -#define __DEFINE_SRCU(name, is_static) \
> > - static DEFINE_PER_CPU(struct srcu_data, name##_srcu_data);\
> > - is_static struct srcu_struct name = __SRCU_STRUCT_INIT(name, name##_srcu_data)
> > +#ifdef MODULE
> > +# define __DEFINE_SRCU(name, is_static) \
> > + is_static struct srcu_struct name; \
> > + struct srcu_struct *__srcu_struct_##name \
> > + __section("___srcu_struct_ptrs") = &name
> > +#else
> > +# define __DEFINE_SRCU(name, is_static) \
> > + static DEFINE_PER_CPU(struct srcu_data, name##_srcu_data); \
> > + is_static struct srcu_struct name = \
> > + __SRCU_STRUCT_INIT(name, name##_srcu_data)
> > +#endif
> > #define DEFINE_SRCU(name) __DEFINE_SRCU(name, /* not static */)
> > #define DEFINE_STATIC_SRCU(name) __DEFINE_SRCU(name, static)
> >
> > diff --git a/kernel/module.c b/kernel/module.c
> > index 0b9aa8ab89f0..524da609c884 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -3093,6 +3093,11 @@ static int find_module_sections(struct module *mod, struct load_info *info)
> > sizeof(*mod->tracepoints_ptrs),
> > &mod->num_tracepoints);
> > #endif
> > +#ifdef CONFIG_TREE_SRCU
> > + mod->srcu_struct_ptrs = section_objs(info, "___srcu_struct_ptrs",
> > + sizeof(*mod->srcu_struct_ptrs),
> > + &mod->num_srcu_structs);
> > +#endif
> > #ifdef CONFIG_BPF_EVENTS
> > mod->bpf_raw_events = section_objs(info, "__bpf_raw_tp_map",
> > sizeof(*mod->bpf_raw_events),
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 9b761e546de8..2ded2614a2f4 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -1310,3 +1310,68 @@ void __init srcu_init(void)
> > queue_work(rcu_gp_wq, &ssp->work.work);
> > }
> > }
> > +
> > +#ifdef CONFIG_MODULES
> > +
> > +/* Initialize any global-scope srcu_struct structures used by this module. */
> > +static int srcu_module_coming(struct module *mod)
> > +{
> > + int i;
> > + struct srcu_struct **sspp = mod->srcu_struct_ptrs;
> > + int ret;
> > +
> > + for (i = 0; i < mod->num_srcu_structs; i++) {
> > + ret = init_srcu_struct(*(sspp++));
> > + if (WARN_ON_ONCE(ret))
> > + return ret;
> > + }
> > + return 0;
> > +}
> > +
> > +/* Clean up any global-scope srcu_struct structures used by this module. */
> > +static void srcu_module_going(struct module *mod)
> > +{
> > + int i;
> > + struct srcu_struct **sspp = mod->srcu_struct_ptrs;
> > +
> > + for (i = 0; i < mod->num_srcu_structs; i++)
> > + cleanup_srcu_struct(*(sspp++));
> > +}
> > +
> > +/* Handle one module, either coming or going. */
> > +static int srcu_module_notify(struct notifier_block *self,
> > + unsigned long val, void *data)
> > +{
> > + struct module *mod = data;
> > + int ret = 0;
> > +
> > + switch (val) {
> > + case MODULE_STATE_COMING:
> > + ret = srcu_module_coming(mod);
> > + break;
> > + case MODULE_STATE_GOING:
> > + srcu_module_going(mod);
> > + break;
> > + default:
> > + break;
> > + }
> > + return ret;
> > +}
> > +
> > +static struct notifier_block srcu_module_nb = {
> > + .notifier_call = srcu_module_notify,
> > + .priority = 0,
> > +};
> > +
> > +static __init int init_srcu_module_notifier(void)
> > +{
> > + int ret;
> > +
> > + ret = register_module_notifier(&srcu_module_nb);
> > + if (ret)
> > + pr_warn("Failed to register srcu module notifier\n");
> > + return ret;
> > +}
> > +late_initcall(init_srcu_module_notifier);
> > +
> > +#endif /* #ifdef CONFIG_MODULES */
> >
>