[PATCH 2/3] ARM: perf: disable the pagefault handler when reading from user space

[Jean Pihet]

As done on other architectures (ARM64, x86, Sparc etc.).

This prevents a deadlock on down_read in do_page_fault when unwinding
using fp and triggering on kernel tracepoints:

  INFO: task stress:2116 blocked for more than 120 seconds.
        Not tainted 3.15.0-rc4-00364-g3401dfb-dirty #43
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  stress          D c04b41e8     0  2116   2115 0x00000000
  [<c04b41e8>] (__schedule) from [<c04b46dc>] (schedule+0x40/0x90)
  [<c04b46dc>] (schedule) from [<c04b6ec8>] (__down_read+0xc4/0xfc)
  [<c04b6ec8>] (__down_read) from [<c04b69c0>] (down_read+0x18/0x1c)
  [<c04b69c0>] (down_read) from [<c001d41c>] (do_page_fault+0xac/0x420)
  [<c001d41c>] (do_page_fault) from [<c0008444>] (do_DataAbort+0x44/0xa8)
  [<c0008444>] (do_DataAbort) from [<c00136b8>] (__dabt_svc+0x38/0x60)
  Exception stack(0xecbc3af8 to 0xecbc3b40)
  3ae0:                                                       ecbc3b74 b6d72ff4
  3b00: ffffffec 00000000 b6d72ff4 ec0fc000 00000000 ec0fc000 0000007e 00000000
  3b20: ecbc2000 ecbc3bac 00000014 ecbc3b44 c0019e78 c021ef44 00000013 ffffffff
  [<c00136b8>] (__dabt_svc) from [<c021ef44>] (__copy_from_user+0xa4/0x3a0)

Signed-off-by: Jean Pihet <jean.pihet at linaro.org>
Cc: Will Deacon <will.deacon at arm.com>
---
 arch/arm/kernel/perf_event.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index 6493c4c..f5aeca2 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -560,11 +560,16 @@ user_backtrace(struct frame_tail __user *tail,
 	       struct perf_callchain_entry *entry)
 {
 	struct frame_tail buftail;
+	unsigned long err;
 
-	/* Also check accessibility of one struct frame_tail beyond */
 	if (!access_ok(VERIFY_READ, tail, sizeof(buftail)))
 		return NULL;
-	if (__copy_from_user_inatomic(&buftail, tail, sizeof(buftail)))
+
+	pagefault_disable();
+	err = __copy_from_user_inatomic(&buftail, tail, sizeof(buftail));
+	pagefault_enable();
+
+	if (err)
 		return NULL;
 
 	perf_callchain_store(entry, buftail.lr);
-- 
1.8.1.2