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    SubjectRe: [PATCH v2 6/6] kernel: tracepoints: add support for relative references
    On 18 August 2017 at 12:26, 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 | 42 ++++++++++++++++++--
    > kernel/tracepoint.c | 7 +---
    > 2 files changed, 40 insertions(+), 9 deletions(-)
    >
    > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
    > index a26ffbe09e71..68701821933a 100644
    > --- a/include/linux/tracepoint.h
    > +++ b/include/linux/tracepoint.h
    > @@ -228,6 +228,42 @@ 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")
    > +
    > +struct tracepoint_entry_t {
    > + int tp_offset;
    > +};
    > +
    > +static inline
    > +struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
    > +{
    > + return (struct tracepoint *)((unsigned long)ent + ent->tp_offset);
    > +}
    > +#else
    > +#define __TRACEPOINT_ENTRY(name) \
    > + static struct tracepoint * const __tracepoint_ptr_##name __used \
    > + __attribute__((section("__tracepoints_ptrs"))) = \
    > + &__tracepoint_##name
    > +
    > +struct tracepoint_entry_t {
    > + struct tracepoint *tp;
    > +};
    > +
    > +static inline
    > +struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
    > +{
    > + return ent->tp;
    > +}
    > +#endif
    > +
    > +extern struct tracepoint_entry_t const __start___tracepoints_ptrs[];
    > +extern struct tracepoint_entry_t const __stop___tracepoints_ptrs[];
    > +

    It appears the stuff above needs to be move inside the double-include
    guard (which oddly enough does not cover the entire file)

    > /*
    > * 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 +273,9 @@ 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..21bc41454fd6 100644
    > --- a/kernel/tracepoint.c
    > +++ b/kernel/tracepoint.c
    > @@ -28,9 +28,6 @@
    > #include <linux/sched/task.h>
    > #include <linux/static_key.h>
    >
    > -extern struct tracepoint * const __start___tracepoints_ptrs[];
    > -extern struct tracepoint * const __stop___tracepoints_ptrs[];
    > -
    > /* Set to 1 to enable tracepoint debug output */
    > static const int tracepoint_debug;
    >
    > @@ -508,12 +505,12 @@ static void for_each_tracepoint_range(struct tracepoint * const *begin,
    > void (*fct)(struct tracepoint *tp, void *priv),
    > void *priv)
    > {
    > - struct tracepoint * const *iter;
    > + struct tracepoint_entry_t const *iter;
    >
    > if (!begin)
    > return;
    > for (iter = begin; iter < end; iter++)
    > - fct(*iter, priv);
    > + fct(tracepoint_from_entry(iter), priv);
    > }
    >
    > /**
    > --
    > 2.11.0
    >

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