perf/x86: Fix USER/KERNEL tagging of samples properly

Some PMUs don't provide a full register set for their sample,
specifically 'advanced' PMUs like AMD IBS and Intel PEBS which provide
'better' than regular interrupt accuracy.

In this case we use the interrupt regs as basis and over-write some
fields (typically IP) with different information.

The perf core however uses user_mode() to distinguish user/kernel
samples, user_mode() relies on regs->cs. If the interrupt skid pushed
us over a boundary the new IP might not be in the same domain as the
interrupt.

Commit ce5c1fe9a9 ("perf/x86: Fix USER/KERNEL tagging of samples")
tried to fix this by making the perf core use kernel_ip(). This
however is wrong (TM), as pointed out by Linus, since it doesn't allow
for VM86 and non-zero based segments in IA32 mode.

Therefore, provide a new helper to set the regs->ip field,
set_linear_ip(), which massages the regs into a suitable state
assuming the provided IP is in fact a linear address.

Also modify perf_instruction_pointer() and perf_callchain_user() to
deal with segments base offsets.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1341910954.3462.102.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/perf_event.h

@@ -196,11 +196,16 @@ static inline u32 get_ibs_caps(void) { return 0; }
 extern void perf_events_lapic_init(void);
 
 /*
- * Abuse bit 3 of the cpu eflags register to indicate proper PEBS IP fixups.
+ * Abuse bits {3,5} of the cpu eflags register. These flags are otherwise
- * This flag is otherwise unused and ABI specified to be 0, so nobody should
+ * unused and ABI specified to be 0, so nobody should care what we do with
- * care what we do with it.
+ * them.
+ *
+ * EXACT - the IP points to the exact instruction that triggered the
+ *         event (HW bugs exempt).
+ * VM    - original X86_VM_MASK; see set_linear_ip().
  */
 #define PERF_EFLAGS_EXACT	(1UL << 3)
+#define PERF_EFLAGS_VM		(1UL << 5)
 
 struct pt_regs;
 extern unsigned long perf_instruction_pointer(struct pt_regs *regs);

arch/x86/kernel/cpu/perf_event.c

@@ -32,6 +32,8 @@
 #include <asm/smp.h>
 #include <asm/alternative.h>
 #include <asm/timer.h>
+#include <asm/desc.h>
+#include <asm/ldt.h>
 
 #include "perf_event.h"
 
@@ -1738,6 +1740,29 @@ valid_user_frame(const void __user *fp, unsigned long size)
 	return (__range_not_ok(fp, size, TASK_SIZE) == 0);
 }
 
+static unsigned long get_segment_base(unsigned int segment)
+{
+	struct desc_struct *desc;
+	int idx = segment >> 3;
+
+	if ((segment & SEGMENT_TI_MASK) == SEGMENT_LDT) {
+		if (idx > LDT_ENTRIES)
+			return 0;
+
+		if (idx > current->active_mm->context.size)
+			return 0;
+
+		desc = current->active_mm->context.ldt;
+	} else {
+		if (idx > GDT_ENTRIES)
+			return 0;
+
+		desc = __this_cpu_ptr(&gdt_page.gdt[0]);
+	}
+
+	return get_desc_base(desc + idx);
+}
+
 #ifdef CONFIG_COMPAT
 
 #include <asm/compat.h>
@@ -1746,13 +1771,17 @@ static inline int
 perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
 {
 	/* 32-bit process in 64-bit kernel. */
+	unsigned long ss_base, cs_base;
 	struct stack_frame_ia32 frame;
 	const void __user *fp;
 
 	if (!test_thread_flag(TIF_IA32))
 		return 0;
 
-	fp = compat_ptr(regs->bp);
+	cs_base = get_segment_base(regs->cs);
+	ss_base = get_segment_base(regs->ss);
+
+	fp = compat_ptr(ss_base + regs->bp);
 	while (entry->nr < PERF_MAX_STACK_DEPTH) {
 		unsigned long bytes;
 		frame.next_frame     = 0;
@@ -1765,8 +1794,8 @@ perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
 		if (!valid_user_frame(fp, sizeof(frame)))
 			break;
 
-		perf_callchain_store(entry, frame.return_address);
+		perf_callchain_store(entry, cs_base + frame.return_address);
-		fp = compat_ptr(frame.next_frame);
+		fp = compat_ptr(ss_base + frame.next_frame);
 	}
 	return 1;
 }
@@ -1789,6 +1818,12 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 		return;
 	}
 
+	/*
+	 * We don't know what to do with VM86 stacks.. ignore them for now.
+	 */
+	if (regs->flags & (X86_VM_MASK | PERF_EFLAGS_VM))
+		return;
+
 	fp = (void __user *)regs->bp;
 
 	perf_callchain_store(entry, regs->ip);
@@ -1816,16 +1851,50 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 	}
 }
 
+/*
+ * Deal with code segment offsets for the various execution modes:
+ *
+ *   VM86 - the good olde 16 bit days, where the linear address is
+ *          20 bits and we use regs->ip + 0x10 * regs->cs.
+ *
+ *   IA32 - Where we need to look at GDT/LDT segment descriptor tables
+ *          to figure out what the 32bit base address is.
+ *
+ *    X32 - has TIF_X32 set, but is running in x86_64
+ *
+ * X86_64 - CS,DS,SS,ES are all zero based.
+ */
+static unsigned long code_segment_base(struct pt_regs *regs)
+{
+	/*
+	 * If we are in VM86 mode, add the segment offset to convert to a
+	 * linear address.
+	 */
+	if (regs->flags & X86_VM_MASK)
+		return 0x10 * regs->cs;
+
+	/*
+	 * For IA32 we look at the GDT/LDT segment base to convert the
+	 * effective IP to a linear address.
+	 */
+#ifdef CONFIG_X86_32
+	if (user_mode(regs) && regs->cs != __USER_CS)
+		return get_segment_base(regs->cs);
+#else
+	if (test_thread_flag(TIF_IA32)) {
+		if (user_mode(regs) && regs->cs != __USER32_CS)
+			return get_segment_base(regs->cs);
+	}
+#endif
+	return 0;
+}
+
 unsigned long perf_instruction_pointer(struct pt_regs *regs)
 {
-	unsigned long ip;
-
 	if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
-		ip = perf_guest_cbs->get_guest_ip();
+		return perf_guest_cbs->get_guest_ip();
-	else
-		ip = instruction_pointer(regs);
 
-	return ip;
+	return regs->ip + code_segment_base(regs);
 }
 
 unsigned long perf_misc_flags(struct pt_regs *regs)
@@ -1838,7 +1907,7 @@ unsigned long perf_misc_flags(struct pt_regs *regs)
 		else
 			misc |= PERF_RECORD_MISC_GUEST_KERNEL;
 	} else {
-		if (!kernel_ip(regs->ip))
+		if (user_mode(regs))
 			misc |= PERF_RECORD_MISC_USER;
 		else
 			misc |= PERF_RECORD_MISC_KERNEL;

arch/x86/kernel/cpu/perf_event.h

@@ -516,6 +516,26 @@ static inline bool kernel_ip(unsigned long ip)
 #endif
 }
 
+/*
+ * Not all PMUs provide the right context information to place the reported IP
+ * into full context. Specifically segment registers are typically not
+ * supplied.
+ *
+ * Assuming the address is a linear address (it is for IBS), we fake the CS and
+ * vm86 mode using the known zero-based code segment and 'fix up' the registers
+ * to reflect this.
+ *
+ * Intel PEBS/LBR appear to typically provide the effective address, nothing
+ * much we can do about that but pray and treat it like a linear address.
+ */
+static inline void set_linear_ip(struct pt_regs *regs, unsigned long ip)
+{
+	regs->cs = kernel_ip(ip) ? __KERNEL_CS : __USER_CS;
+	if (regs->flags & X86_VM_MASK)
+		regs->flags ^= (PERF_EFLAGS_VM | X86_VM_MASK);
+	regs->ip = ip;
+}
+
 #ifdef CONFIG_CPU_SUP_AMD
 
 int amd_pmu_init(void);

arch/x86/kernel/cpu/perf_event_amd_ibs.c

@@ -13,6 +13,8 @@
 
 #include <asm/apic.h>
 
+#include "perf_event.h"
+
 static u32 ibs_caps;
 
 #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_AMD)
@@ -536,7 +538,7 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 	if (check_rip && (ibs_data.regs[2] & IBS_RIP_INVALID)) {
 		regs.flags &= ~PERF_EFLAGS_EXACT;
 	} else {
-		instruction_pointer_set(&regs, ibs_data.regs[1]);
+		set_linear_ip(&regs, ibs_data.regs[1]);
 		regs.flags |= PERF_EFLAGS_EXACT;
 	}
 

arch/x86/kernel/cpu/perf_event_intel_ds.c

@@ -499,7 +499,7 @@ static int intel_pmu_pebs_fixup_ip(struct pt_regs *regs)
 	 * We sampled a branch insn, rewind using the LBR stack
 	 */
 	if (ip == to) {
-		regs->ip = from;
+		set_linear_ip(regs, from);
 		return 1;
 	}
 
@@ -529,7 +529,7 @@ static int intel_pmu_pebs_fixup_ip(struct pt_regs *regs)
 	} while (to < ip);
 
 	if (to == ip) {
-		regs->ip = old_to;
+		set_linear_ip(regs, old_to);
 		return 1;
 	}
 
@@ -569,7 +569,8 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
 	 * A possible PERF_SAMPLE_REGS will have to transfer all regs.
 	 */
 	regs = *iregs;
-	regs.ip = pebs->ip;
+	regs.flags = pebs->flags;
+	set_linear_ip(&regs, pebs->ip);
 	regs.bp = pebs->bp;
 	regs.sp = pebs->sp;