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    SubjectRe: [PATCH 5/5] kernel: tracepoints: add support for relative references
    On Mon, 14 Aug 2017 11:52:31 +0100
    Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:

    > To avoid the need for relocating absolute references to tracepoint
    > structures at boot time when running relocatable kernels (which may
    > take a disproportionate amount of space), add the option to emit
    > these tables as relative references instead.
    >
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > Cc: Ingo Molnar <mingo@redhat.com>
    > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    > ---
    > include/linux/tracepoint.h | 19 +++++++++++++++----
    > kernel/tracepoint.c | 19 ++++++++++++++-----
    > 2 files changed, 29 insertions(+), 9 deletions(-)
    >
    > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
    > index a26ffbe09e71..d02bf1a695e8 100644
    > --- a/include/linux/tracepoint.h
    > +++ b/include/linux/tracepoint.h
    > @@ -228,6 +228,19 @@ extern void syscall_unregfunc(void);
    > return static_key_false(&__tracepoint_##name.key); \
    > }
    >
    > +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
    > +#define __TRACEPOINT_ENTRY(name) \
    > + asm(" .section \"__tracepoints_ptrs\", \"a\" \n" \
    > + " .balign 4 \n" \
    > + " .long " VMLINUX_SYMBOL_STR(__tracepoint_##name) " - .\n" \
    > + " .previous \n")
    > +#else
    > +#define __TRACEPOINT_ENTRY(name) \
    > + static struct tracepoint * const __tracepoint_ptr_##name __used \
    > + __attribute__((section("__tracepoints_ptrs"))) = \
    > + &__tracepoint_##name
    > +#endif
    > +
    > /*
    > * We have no guarantee that gcc and the linker won't up-align the tracepoint
    > * structures, so we create an array of pointers that will be used for iteration
    > @@ -237,11 +250,9 @@ extern void syscall_unregfunc(void);
    > static const char __tpstrtab_##name[] \
    > __attribute__((section("__tracepoints_strings"))) = #name; \
    > struct tracepoint __tracepoint_##name \
    > - __attribute__((section("__tracepoints"))) = \
    > + __attribute__((section("__tracepoints"), used)) = \
    > { __tpstrtab_##name, STATIC_KEY_INIT_FALSE, reg, unreg, NULL };\
    > - static struct tracepoint * const __tracepoint_ptr_##name __used \
    > - __attribute__((section("__tracepoints_ptrs"))) = \
    > - &__tracepoint_##name;
    > + __TRACEPOINT_ENTRY(name);
    >
    > #define DEFINE_TRACE(name) \
    > DEFINE_TRACE_FN(name, NULL, NULL);
    > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
    > index 685c50ae6300..1c6689603764 100644
    > --- a/kernel/tracepoint.c
    > +++ b/kernel/tracepoint.c
    > @@ -28,8 +28,8 @@
    > #include <linux/sched/task.h>
    > #include <linux/static_key.h>
    >
    > -extern struct tracepoint * const __start___tracepoints_ptrs[];
    > -extern struct tracepoint * const __stop___tracepoints_ptrs[];
    > +extern const unsigned char __start___tracepoints_ptrs[];
    > +extern const unsigned char __stop___tracepoints_ptrs[];

    Does this have to be converted to a char array?

    >
    > /* Set to 1 to enable tracepoint debug output */
    > static const int tracepoint_debug;
    > @@ -503,17 +503,26 @@ static __init int init_tracepoints(void)
    > __initcall(init_tracepoints);
    > #endif /* CONFIG_MODULES */
    >
    > -static void for_each_tracepoint_range(struct tracepoint * const *begin,
    > - struct tracepoint * const *end,
    > +static void for_each_tracepoint_range(const void *begin, const void *end,
    > void (*fct)(struct tracepoint *tp, void *priv),
    > void *priv)
    > {
    > +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
    > + const signed int *iter;
    > +
    > + if (!begin)
    > + return;
    > + for (iter = begin; iter < (signed int *)end; iter++) {

    Isn't the typecasts here sufficient, even if the above end is defined
    as a tracepoint * const *?

    -- Steve

    > + fct((struct tracepoint *)((unsigned long)iter + *iter), priv);
    > + }
    > +#else
    > struct tracepoint * const *iter;
    >
    > if (!begin)
    > return;
    > - for (iter = begin; iter < end; iter++)
    > + for (iter = begin; iter < (struct tracepoint * const *)end; iter++)
    > fct(*iter, priv);
    > +#endif
    > }
    >
    > /**

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