Messages in this thread | | | Date | Wed, 5 Feb 2020 07:37:35 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] kcsan: Fix 0-sized checks |
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On Wed, Feb 05, 2020 at 11:14:19AM +0100, Marco Elver wrote: > Instrumentation of arbitrary memory-copy functions, such as user-copies, > may be called with size of 0, which could lead to false positives. > > To avoid this, add a comparison in check_access() for size==0, which > will be optimized out for constant sized instrumentation > (__tsan_{read,write}N), and therefore not affect the common-case > fast-path. > > Signed-off-by: Marco Elver <elver@google.com>
Queued, thank you!
Thanx, Paul
> --- > kernel/kcsan/core.c | 7 +++++++ > kernel/kcsan/test.c | 10 ++++++++++ > 2 files changed, 17 insertions(+) > > diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c > index e3c7d8f34f2ff..82c2bef827d42 100644 > --- a/kernel/kcsan/core.c > +++ b/kernel/kcsan/core.c > @@ -455,6 +455,13 @@ static __always_inline void check_access(const volatile void *ptr, size_t size, > atomic_long_t *watchpoint; > long encoded_watchpoint; > > + /* > + * Do nothing for 0 sized check; this comparison will be optimized out > + * for constant sized instrumentation (__tsan_{read,write}N). > + */ > + if (unlikely(size == 0)) > + return; > + > /* > * Avoid user_access_save in fast-path: find_watchpoint is safe without > * user_access_save, as the address that ptr points to is only used to > diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c > index cc6000239dc01..d26a052d33838 100644 > --- a/kernel/kcsan/test.c > +++ b/kernel/kcsan/test.c > @@ -92,6 +92,16 @@ static bool test_matching_access(void) > return false; > if (WARN_ON(matching_access(9, 1, 10, 1))) > return false; > + > + /* > + * An access of size 0 could match another access, as demonstrated here. > + * Rather than add more comparisons to 'matching_access()', which would > + * end up in the fast-path for *all* checks, check_access() simply > + * returns for all accesses of size 0. > + */ > + if (WARN_ON(!matching_access(8, 8, 12, 0))) > + return false; > + > return true; > } > > -- > 2.25.0.341.g760bfbb309-goog >
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