Re: UBSAN: Undefined behaviour in kernel/rcu/tree_plugin.h in 4.20-rc1

[Paul E. McKenney]

On Wed, Nov 14, 2018 at 03:43:05PM +0100, Christophe LEROY wrote:
> 
> 
> Le 09/11/2018 à 21:10, Paul E. McKenney a écrit :
> >On Fri, Nov 09, 2018 at 06:11:20PM +0100, Christophe LEROY wrote:
> >>(Resending due to error in Paul's address)
> >>
> >>Paul
> >>
> >>I get the following UBSAN reports in 4.20-rc1 on an MPC8321E
> >>(powerpc/book3s/32)
> >>
> >>I bisected it to 3e31009898699dfc ("rcu: Defer reporting RCU-preempt
> >>quiescent states when disabled")
> >
> >Fixed by dfdc33585b0a ("rcu: Avoid signed integer overflow in
> >rcu_preempt_deferred_qs()") in my -rcu tree and in -next, which I intend
> >to push into the next merge window.
> 
> Thanks, I confirm it fixes the issue.
> 
> Do you intend to push it into 4.20-rc3 or do you mean 4.21 ?

The next merge window, which will be either v4.21 or v5.0.  The v4.20
merge window is over and done.  ;-)

Please note that the gcc command-line arguments used by the Linux kernel
prevent the compiler from taking advantage of the C-standard signed
integer overflow aspect of undefined behavior, so this is a aesthetic
issue rather than a failure case.  Plus the C++ standards committee just
voted in a change that gets rid of signed integer overflow completely.
It is not clear whether the C language will also make this change, but
it does require that the usual compilers have the ability to operate in
this manner.

							Thanx, Paul

> Christophe
> 
> >							Thanx, Paul
> >
> >>Thanks
> >>Christophe
> >>
> >>[    4.919995] ================================================================================
> >>[    4.928428] UBSAN: Undefined behaviour in kernel/rcu/tree_plugin.h:623:28
> >>[    4.935198] signed integer overflow:
> >>[    4.938766] 0 - -2147483648 cannot be represented in type 'int'
> >>[    4.944678] CPU: 0 PID: 119 Comm: mkdir Not tainted
> >>4.19.0-rc1-s3k-dev-00005-g5a60513 #214
> >>[    4.952908] Call Trace:
> >>[    4.955382] [dec4fd20] [c02cb0d0] ubsan_epilogue+0x18/0x74 (unreliable)
> >>[    4.962003] [dec4fd30] [c02cb5e0] handle_overflow+0xd0/0xe0
> >>[    4.967588] [dec4fdb0] [c007b424] rcu_preempt_deferred_qs+0xc0/0xc8
> >>[    4.973857] [dec4fdd0] [c007be28] rcu_note_context_switch+0x74/0x608
> >>[    4.980217] [dec4fe10] [c064b790] __schedule+0x58/0x6e0
> >>[    4.985448] [dec4fe50] [c064bfdc] preempt_schedule_common+0x48/0x9c
> >>[    4.991717] [dec4fe70] [c01308c8] handle_mm_fault+0x10fc/0x1ecc
> >>[    4.997639] [dec4fee0] [c001339c] do_page_fault+0x10c/0x760
> >>[    5.003225] [dec4ff40] [c001234c] handle_page_fault+0x14/0x40
> >>[    5.008968] --- interrupt: 401 at 0xff9cff8
> >>[    5.008968]     LR = 0xfeefd78
> >>[    5.016170] ================================================================================
> >>[    5.024591] ================================================================================
> >>[    5.033005] UBSAN: Undefined behaviour in kernel/rcu/tree_plugin.h:627:28
> >>[    5.039775] signed integer overflow:
> >>[    5.043342] -2147483648 + -2147483648 cannot be represented in type 'int'
> >>[    5.050118] CPU: 0 PID: 119 Comm: mkdir Not tainted
> >>4.19.0-rc1-s3k-dev-00005-g5a60513 #214
> >>[    5.058348] Call Trace:
> >>[    5.060813] [dec4fd20] [c02cb0d0] ubsan_epilogue+0x18/0x74 (unreliable)
> >>[    5.067433] [dec4fd30] [c02cb5e0] handle_overflow+0xd0/0xe0
> >>[    5.073014] [dec4fdb0] [c007b408] rcu_preempt_deferred_qs+0xa4/0xc8
> >>[    5.079283] [dec4fdd0] [c007be28] rcu_note_context_switch+0x74/0x608
> >>[    5.085640] [dec4fe10] [c064b790] __schedule+0x58/0x6e0
> >>[    5.090871] [dec4fe50] [c064bfdc] preempt_schedule_common+0x48/0x9c
> >>[    5.097139] [dec4fe70] [c01308c8] handle_mm_fault+0x10fc/0x1ecc
> >>[    5.103059] [dec4fee0] [c001339c] do_page_fault+0x10c/0x760
> >>[    5.108642] [dec4ff40] [c001234c] handle_page_fault+0x14/0x40
> >>[    5.114385] --- interrupt: 401 at 0xff9cff8
> >>[    5.114385]     LR = 0xfeefd78
> >>[    5.121588] ================================================================================
> >>
>