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kcsan: Fix misreporting if concurrent races on same address

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If there are at least 4 threads racing on the same address, it can
happen that one of the readers may observe another matching reader in
other_info. To avoid locking up, we have to consume 'other_info'
regardless, but skip the report. See the added comment for more details.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Marco Elver 2020-02-10 15:56:39 +01:00 committed by Ingo Molnar
parent 80d4c47752
commit 3a5b45e503
1 changed files with 38 additions and 0 deletions
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38
kernel/kcsan/report.c
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@ -422,6 +422,44 @@ static bool prepare_report(unsigned long *flags, const volatile void *ptr,
return false;
}
access_type |= other_info.access_type;
if ((access_type & KCSAN_ACCESS_WRITE) == 0) {
/*
* While the address matches, this is not the other_info
* from the thread that consumed our watchpoint, since
* neither this nor the access in other_info is a write.
* It is invalid to continue with the report, since we
* only have information about reads.
*
* This can happen due to concurrent races on the same
* address, with at least 4 threads. To avoid locking up
* other_info and all other threads, we have to consume
* it regardless.
*
* A concrete case to illustrate why we might lock up if
* we do not consume other_info:
*
* We have 4 threads, all accessing the same address
* (or matching address ranges). Assume the following
* watcher and watchpoint consumer pairs:
* write1-read1, read2-write2. The first to populate
* other_info is write2, however, write1 consumes it,
* resulting in a report of write1-write2. This report
* is valid, however, now read1 populates other_info;
* read2-read1 is an invalid conflict, yet, no other
* conflicting access is left. Therefore, we must
* consume read1's other_info.
*
* Since this case is assumed to be rare, it is
* reasonable to omit this report: one of the other
* reports includes information about the same shared
* data, and at this point the likelihood that we
* re-report the same race again is high.
*/
release_report(flags, KCSAN_REPORT_RACE_SIGNAL);
return false;
}
/*
* Matching & usable access in other_info: keep other_info_lock
* locked, as this thread consumes it to print the full report;