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perf: Fix capabilities bitfield compatibility in 'struct perf_event_mmap_page'

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Solve the problems around the broken definition of perf_event_mmap_page::
cap_usr_time and cap_usr_rdpmc fields which used to overlap, partially
fixed by:

  860f085b74 ("perf: Fix broken union in 'struct perf_event_mmap_page'")

The problem with the fix (merged in v3.12-rc1 and not yet released
officially), noticed by Vince Weaver is that the new behavior is
not detectable by new user-space, and that due to the reuse of the
field names it's easy to mis-compile a binary if old headers are used
on a new kernel or new headers are used on an old kernel.

To solve all that make this change explicit, detectable and self-contained,
by iterating the ABI the following way:

 - Always clear bit 0, and rename it to usrpage->cap_bit0, to at least not
   confuse old user-space binaries. RDPMC will be marked as unavailable
   to old binaries but that's within the ABI, this is a capability bit.

 - Rename bit 1 to ->cap_bit0_is_deprecated and always set it to 1, so new
   libraries can reliably detect that bit 0 is deprecated and perma-zero
   without having to check the kernel version.

 - Use bits 2, 3, 4 for the newly defined, correct functionality:

	cap_user_rdpmc		: 1, /* The RDPMC instruction can be used to read counts */
	cap_user_time		: 1, /* The time_* fields are used */
	cap_user_time_zero	: 1, /* The time_zero field is used */

 - Rename all the bitfield names in perf_event.h to be different from the
   old names, to make sure it's not possible to mis-compile it
   accidentally with old assumptions.

The 'size' field can then be used in the future to add new fields and it
will act as a natural ABI version indicator as well.

Also adjust tools/perf/ userspace for the new definitions, noticed by
Adrian Hunter.

Reported-by: Vince Weaver <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Also-Fixed-by: Adrian Hunter <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Peter Zijlstra 2013-09-19 10:16:42 +02:00 • committed by Ingo Molnar
commit fa73158710
4 changed files with 38 additions and 13 deletions
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21
kernel/events/core.c
View file

@ -3660,6 +3660,26 @@ static void calc_timer_values(struct perf_event *event,
*running = ctx_time - event->tstamp_running;
}
static void perf_event_init_userpage(struct perf_event *event)
{
struct perf_event_mmap_page *userpg;
struct ring_buffer *rb;
rcu_read_lock();
rb = rcu_dereference(event->rb);
if (!rb)
goto unlock;
userpg = rb->user_page;
/* Allow new userspace to detect that bit 0 is deprecated */
userpg->cap_bit0_is_deprecated = 1;
userpg->size = offsetof(struct perf_event_mmap_page, __reserved);
unlock:
rcu_read_unlock();
}
void __weak arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
{
}
@ -4044,6 +4064,7 @@ again:
ring_buffer_attach(event, rb);
rcu_assign_pointer(event->rb, rb);
perf_event_init_userpage(event);
perf_event_update_userpage(event);
unlock: