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Message-Id: <20151129214703.980047828@1wt.eu>
Date: Sun, 29 Nov 2015 22:47:28 +0100
From: Willy Tarreau <w@....eu>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc: poma <pomidorabelisima@...il.com>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Rusty Russell <rusty@...tcorp.com.au>,
Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>
Subject: [PATCH 2.6.32 26/38] [PATCH 26/38] module: Fix locking in symbol_put_addr()
2.6.32-longterm review patch. If anyone has any objections, please let me know.
------------------
commit 275d7d44d802ef271a42dc87ac091a495ba72fc5 upstream.
Poma (on the way to another bug) reported an assertion triggering:
[<ffffffff81150529>] module_assert_mutex_or_preempt+0x49/0x90
[<ffffffff81150822>] __module_address+0x32/0x150
[<ffffffff81150956>] __module_text_address+0x16/0x70
[<ffffffff81150f19>] symbol_put_addr+0x29/0x40
[<ffffffffa04b77ad>] dvb_frontend_detach+0x7d/0x90 [dvb_core]
Laura Abbott <labbott@...hat.com> produced a patch which lead us to
inspect symbol_put_addr(). This function has a comment claiming it
doesn't need to disable preemption around the module lookup
because it holds a reference to the module it wants to find, which
therefore cannot go away.
This is wrong (and a false optimization too, preempt_disable() is really
rather cheap, and I doubt any of this is on uber critical paths,
otherwise it would've retained a pointer to the actual module anyway and
avoided the second lookup).
While its true that the module cannot go away while we hold a reference
on it, the data structure we do the lookup in very much _CAN_ change
while we do the lookup. Therefore fix the comment and add the
required preempt_disable().
Reported-by: poma <pomidorabelisima@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Fixes: a6e6abd575fc ("module: remove module_text_address()")
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
(cherry picked from commit 3895ff2d13043ecd091813b67a485ec487870b63)
Signed-off-by: Willy Tarreau <w@....eu>
---
kernel/module.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/module.c b/kernel/module.c
index 4b270e6..04aa4f1 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -923,11 +923,15 @@ void symbol_put_addr(void *addr)
if (core_kernel_text(a))
return;
- /* module_text_address is safe here: we're supposed to have reference
- * to module from symbol_get, so it can't go away. */
+ /*
+ * Even though we hold a reference on the module; we still need to
+ * disable preemption in order to safely traverse the data structure.
+ */
+ preempt_disable();
modaddr = __module_text_address(a);
BUG_ON(!modaddr);
module_put(modaddr);
+ preempt_enable();
}
EXPORT_SYMBOL_GPL(symbol_put_addr);
--
1.7.12.2.21.g234cd45.dirty
--
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