[PATCH v2 3/5] mm, uaccess: trigger might_sleep() in might_fault() when pagefaults are disabled

From: David Hildenbrand
Date: Wed Dec 10 2014 - 09:24:43 EST

Commit 662bbcb2747c ("mm, sched: Allow uaccess in atomic with
pagefault_disable()") removed might_sleep() checks for all user access code
(that uses might_fault()).

The reason was to disable wrong "sleep in atomic" warnings in the following
rc = copy_to_user(...)

Which is valid, as pagefault_disable() increments the preempt counter and
therefore disables the pagefault handler. copy_to_user() will not sleep and return
an invalid return code if a page is not available.

However, as all might_sleep() checks are removed, CONFIG_DEBUG_ATOMIC_SLEEP
would no longer detect the following scenario:
rc = copy_to_user(...)

If the kernel is compiled with preemption turned on, the preempt counter would
be incremented and copy_to_user() would never sleep. However, with preemption
turned off, the preempt counter will not be touched, we will therefore sleep in
atomic context. We really want to enable CONFIG_DEBUG_ATOMIC_SLEEP checks for
user access functions again, otherwise horrible deadlocks might be hard to debug.

Root of all evil is that pagefault_disable() acted almost as preempt_disable(),
depending on preemption being turned on/off.

As we now have pagefault_disabled(), we can use it to distingusih whether user
acces functions might sleep.

Convert might_fault() into a makro that calls __might_fault(), to allow proper
file + line messages in case of a might_sleep() warning. We can't move the code
directly into kernel.h for now, as that results in ugly header recursions we
can't avoid for now.

Signed-off-by: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
include/linux/kernel.h | 3 ++-
mm/memory.c | 18 ++++++------------
2 files changed, 8 insertions(+), 13 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 446d76a..7e65a55 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -232,7 +232,8 @@ static inline u32 reciprocal_scale(u32 val, u32 ep_ro)

#if defined(CONFIG_MMU) && \
-void might_fault(void);
+#define might_fault() __might_fault(__FILE__, __LINE__)
+void __might_fault(const char *file, int line);
static inline void might_fault(void) { }
diff --git a/mm/memory.c b/mm/memory.c
index 0b3f6c7..563720a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3686,7 +3686,7 @@ void print_vma_addr(char *prefix, unsigned long ip)

-void might_fault(void)
+void __might_fault(const char *file, int line)
* Some code (nfs/sunrpc) uses socket ops on kernel memory while
@@ -3696,21 +3696,15 @@ void might_fault(void)
if (segment_eq(get_fs(), KERNEL_DS))
- /*
- * it would be nicer only to annotate paths which are not under
- * pagefault_disable, however that requires a larger audit and
- * providing helpers like get_user_atomic.
- */
- if (in_atomic())
+ if (pagefault_disabled())
- __might_sleep(__FILE__, __LINE__, 0);
+ __might_sleep(file, line, 0);
if (current->mm)


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