linux-kernel - Re: [RFC PATCH for 4.18 3/5] rseq: uapi: declare rseq

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Date:   Fri, 6 Jul 2018 15:23:39 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: 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@...icios.com wrote:

> ----- On Jul 5, 2018, at 2:05 PM, Mathieu Desnoyers
> mathieu.desnoyers@...icios.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 <mathieu.desnoyers@...icios.com>
    CC: Thomas Gleixner <tglx@...utronix.de>
    Cc: Joel Fernandes <joelaf@...gle.com>
    Cc: Peter Zijlstra <peterz@...radead.org>
    Cc: Catalin Marinas <catalin.marinas@....com>
    Cc: Dave Watson <davejwatson@...com>
    Cc: Will Deacon <will.deacon@....com>
    Cc: Andi Kleen <andi@...stfloor.org>
    Cc: "H . Peter Anvin" <hpa@...or.com>
    Cc: Chris Lameter <cl@...ux.com>
    Cc: Russell King <linux@....linux.org.uk>
    Cc: Andrew Hunter <ahh@...gle.com>
    Cc: Michael Kerrisk <mtk.manpages@...il.com>
    Cc: "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
    Cc: Paul Turner <pjt@...gle.com>
    Cc: Boqun Feng <boqun.feng@...il.com>
    Cc: Josh Triplett <josh@...htriplett.org>
    Cc: Steven Rostedt <rostedt@...dmis.org>
    Cc: Ben Maurer <bmaurer@...com>
    Cc: linux-api@...r.kernel.org
    CC: linux-arm-kernel@...ts.infradead.org
    Cc: Andy Lutomirski <luto@...capital.net>
    Cc: Andrew Morton <akpm@...ux-foundation.org>
    Cc: Linus Torvalds <torvalds@...ux-foundation.org>

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