Messages in this thread Patch in this message | | | Date | Sun, 29 Nov 2015 22:47:28 +0100 | From | Willy Tarreau <> | Subject | [PATCH 2.6.32 26/38] [PATCH 26/38] module: Fix locking in symbol_put_addr() |
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2.6.32-longterm review patch. If anyone has any objections, please let me know.
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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@redhat.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@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Fixes: a6e6abd575fc ("module: remove module_text_address()") Signed-off-by: Ben Hutchings <ben@decadent.org.uk> (cherry picked from commit 3895ff2d13043ecd091813b67a485ec487870b63)
Signed-off-by: Willy Tarreau <w@1wt.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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