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>

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 8355c84b9729df767945c5f4c8a5f7c5db15fab8..a9c606bb494529d985abf9243237d365da194105 100644 (file)
--- 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 7f6d584c267b4d1f43f1e291755809039627bb74..009a655a5d354c51e20fb34cb12ca653e94f9b71 100644 (file)
--- 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 dd236b66ca3a8ef894386e7dcb8c7f02dbba34ae..cb4238e85b38e37886b27dd76351d2590a835924 100644 (file)
--- 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 9570c2b0f83c580454e2610cb3c07a4d7443ee19..b2519e49424f4c42bb389de846d21c8337773036 100644 (file)
--- 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;