Re: [RFC PATCH for 4.18 3/5] rseq: uapi: declare rseq_cs field as union, update includes

----- On Jul 6, 2018, at 12:02 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

----- On Jul 5, 2018, at 2:05 PM, Mathieu Desnoyers
mathieu.desnoyers@efficios.com wrote:

[...]

The 0-day bot noticed that __get_user() is unimplemented for 64-bit
values on arm32 (although get_user() is implemented).

The following diff fixes this discrepancy, and allows this rseq patch
to build on arm32:

For -rc, I would favor the following simpler approach. Or I could even
just use get_user() instead. Thoughts ?

    rseq: implement work-around for missing 8-byte __get_user on arm
    
    Now that rseq uses __u64 for its pointer fields, 32-bit architectures
    need to read this 64-bit value from user-space.
    
    __get_user is used to read this value, given that its access check has
    already been performed with access_ok() on rseq registration.
    
    arm does not implement 8-byte __get_user. Work-around this limitation
    by using get_user() on ARM instead, with its redundant access check.
    
    Signed-off-by: Mathieu Desnoyers [off-list ref]
    CC: Thomas Gleixner [off-list ref]
    Cc: Joel Fernandes [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Catalin Marinas [off-list ref]
    Cc: Dave Watson [off-list ref]
    Cc: Will Deacon [off-list ref]
    Cc: Andi Kleen [off-list ref]
    Cc: "H . Peter Anvin" [off-list ref]
    Cc: Chris Lameter [off-list ref]
    Cc: Russell King [off-list ref]
    Cc: Andrew Hunter [off-list ref]
    Cc: Michael Kerrisk [off-list ref]
    Cc: "Paul E . McKenney" [off-list ref]
    Cc: Paul Turner [off-list ref]
    Cc: Boqun Feng [off-list ref]
    Cc: Josh Triplett [off-list ref]
    Cc: Steven Rostedt [off-list ref]
    Cc: Ben Maurer [off-list ref]
    Cc: linux-api@vger.kernel.org
    CC: linux-arm-kernel@lists.infradead.org
    Cc: Andy Lutomirski [off-list ref]
    Cc: Andrew Morton [off-list ref]
    Cc: Linus Torvalds [off-list ref]

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 3081e67..0e67625 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c

@@ -18,6 +18,16 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/rseq.h>
 
+/*
+ * ARM does not implement 8 bytes __get_user. Use get_user on that
+ * architecture instead.
+ */
+#ifdef CONFIG_ARM
+#define __rseq_get_user                get_user
+#else
+#define __rseq_get_user                __get_user
+#endif
+
 #define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
                                       RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
 

@@ -120,7 +130,7 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rs
        u32 sig;
        int ret;
 
-       ret = __get_user(ptr, &t->rseq->rseq_cs.ptr64);
+       ret = __rseq_get_user(ptr, &t->rseq->rseq_cs.ptr64);
        if (ret)
                return ret;
        if (!ptr) {

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com