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Subject[tip:perf/urgent] perf: Fix capabilities bitfield compatibility in 'struct perf_event_mmap_page'
Commit-ID:  fa7315871046b9a4c48627905691dbde57e51033
Gitweb: http://git.kernel.org/tip/fa7315871046b9a4c48627905691dbde57e51033
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 19 Sep 2013 10:16:42 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 20 Sep 2013 09:45:11 +0200

perf: Fix capabilities bitfield compatibility in 'struct perf_event_mmap_page'

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:

860f085b74e9 ("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 <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Also-Fixed-by: Adrian Hunter <adrian.hunter@intel.com>
Link: http://lkml.kernel.org/n/tip-zr03yxjrpXesOzzupszqglbv@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/kernel/cpu/perf_event.c | 10 +++++-----
include/uapi/linux/perf_event.h | 14 +++++++++-----
kernel/events/core.c | 21 +++++++++++++++++++++
tools/perf/arch/x86/util/tsc.c | 6 +++---
4 files changed, 38 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 8355c84..a9c606b 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1883,9 +1883,9 @@ static struct pmu pmu = {

void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
{
- userpg->cap_usr_time = 0;
- userpg->cap_usr_time_zero = 0;
- userpg->cap_usr_rdpmc = x86_pmu.attr_rdpmc;
+ userpg->cap_user_time = 0;
+ userpg->cap_user_time_zero = 0;
+ userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc;
userpg->pmc_width = x86_pmu.cntval_bits;

if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
@@ -1894,13 +1894,13 @@ void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
return;

- userpg->cap_usr_time = 1;
+ userpg->cap_user_time = 1;
userpg->time_mult = this_cpu_read(cyc2ns);
userpg->time_shift = CYC2NS_SCALE_FACTOR;
userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;

if (sched_clock_stable && !check_tsc_disabled()) {
- userpg->cap_usr_time_zero = 1;
+ userpg->cap_user_time_zero = 1;
userpg->time_zero = this_cpu_read(cyc2ns_offset);
}
}
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 7f6d584..009a655 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -380,10 +380,13 @@ struct perf_event_mmap_page {
union {
__u64 capabilities;
struct {
- __u64 cap_usr_time : 1,
- cap_usr_rdpmc : 1,
- cap_usr_time_zero : 1,
- cap_____res : 61;
+ __u64 cap_bit0 : 1, /* Always 0, deprecated, see commit 860f085b74e9 */
+ cap_bit0_is_deprecated : 1, /* Always 1, signals that bit 0 is zero */
+
+ 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 */
+ cap_____res : 59;
};
};

@@ -442,12 +445,13 @@ struct perf_event_mmap_page {
* ((rem * time_mult) >> time_shift);
*/
__u64 time_zero;
+ __u32 size; /* Header size up to __reserved[] fields. */

/*
* Hole for extension of the self monitor capabilities
*/

- __u64 __reserved[119]; /* align to 1k */
+ __u8 __reserved[118*8+4]; /* align to 1k. */

/*
* Control data for the mmap() data buffer.
diff --git a/kernel/events/core.c b/kernel/events/core.c
index dd236b6..cb4238e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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:
diff --git a/tools/perf/arch/x86/util/tsc.c b/tools/perf/arch/x86/util/tsc.c
index 9570c2b..b2519e4 100644
--- a/tools/perf/arch/x86/util/tsc.c
+++ b/tools/perf/arch/x86/util/tsc.c
@@ -32,7 +32,7 @@ u64 tsc_to_perf_time(u64 cyc, struct perf_tsc_conversion *tc)
int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
struct perf_tsc_conversion *tc)
{
- bool cap_usr_time_zero;
+ bool cap_user_time_zero;
u32 seq;
int i = 0;

@@ -42,7 +42,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
tc->time_mult = pc->time_mult;
tc->time_shift = pc->time_shift;
tc->time_zero = pc->time_zero;
- cap_usr_time_zero = pc->cap_usr_time_zero;
+ cap_user_time_zero = pc->cap_user_time_zero;
rmb();
if (pc->lock == seq && !(seq & 1))
break;
@@ -52,7 +52,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
}
}

- if (!cap_usr_time_zero)
+ if (!cap_user_time_zero)
return -EOPNOTSUPP;

return 0;

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