[PATCH] perf: x86: fix callgraphs of 32-bit processes on 64-bit kernels.

["Török Edwin" <edwintorok@gmail.com>]

When profiling a 32-bit process on a 64-bit kernel, callgraph tracing
stopped after the first function, because it has seen a garbage memory address
(tried to interpret the frame pointer, and return address as a 64-bit pointer).

Fix this by using a struct stack_frame with 32-bit pointers when the TIF_IA32 flag is set.

Note that TIF_IA32 flag must be used, and not is_compat_task(), because the
latter is only set when the 32-bit process is executing a syscall,
which may not always be the case (when tracing page fault events for example).

Signed-off-by: Török Edwin <edwintorok@gmail.com>
---
 arch/x86/kernel/cpu/perf_event.c |   33 +++++++++++++++++++++++++++++++++
 1 files changed, 33 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 8c1c070..13ee83a 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -2401,6 +2401,20 @@ static int copy_stack_frame(const void __user *fp, struct stack_frame *frame)
 	return bytes == sizeof(*frame);
 }
 
+struct stack_frame_ia32 {
+    u32 next_frame;
+    u32 return_address;
+};
+
+static int copy_stack_frame_ia32(u32 fp, struct stack_frame_ia32 *frame)
+{
+	unsigned long bytes;
+
+	bytes = copy_from_user_nmi(frame, (const void __user*)(unsigned long)fp, sizeof(*frame));
+
+	return bytes == sizeof(*frame);
+}
+
 static void
 perf_callchain_user(struct pt_regs *regs, struct perf_callchain_entry *entry)
 {
@@ -2414,6 +2428,25 @@ perf_callchain_user(struct pt_regs *regs, struct perf_callchain_entry *entry)
 
 	callchain_store(entry, PERF_CONTEXT_USER);
 	callchain_store(entry, regs->ip);
+	if (test_thread_flag(TIF_IA32)) {
+	    /* 32-bit process in 64-bit kernel. */
+	    u32 fp = regs->bp;
+	    struct stack_frame_ia32 frame;
+	    while (entry->nr < PERF_MAX_STACK_DEPTH) {
+		frame.next_frame     = 0;
+		frame.return_address = 0;
+
+		if (!copy_stack_frame_ia32(fp, &frame))
+			break;
+
+		if ((unsigned long)fp < regs->sp)
+			break;
+
+		callchain_store(entry, frame.return_address);
+		fp = frame.next_frame;
+	    }
+	    return;
+	}
 
 	while (entry->nr < PERF_MAX_STACK_DEPTH) {
 		frame.next_frame	     = NULL;
-- 
1.7.0