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When a thread detects that a memory location was modified without its watchpoint being hit, the report notes that a change was detected, but does not provide concrete values for the change. Knowing the concrete values can be very helpful in tracking down any racy writers (e.g. as specific values may only be written in some portions of code, or under certain conditions). When we detect a modification, let's report the concrete old/new values, along with the access's mask of relevant bits (and which relevant bits were modified). This can make it easier to identify potential racy writers. As the snapshots are at most 8 bytes, we can only report values for acceses up to this size, but this appears to cater for the common case. When we detect a race via a watchpoint, we may or may not have concrete values for the modification. To be helpful, let's attempt to log them when we do as they can be ignored where irrelevant. The resulting reports appears as follows, with values zero-padded to the access width: | ================================================================== | BUG: KCSAN: data-race in el0_svc_common+0x34/0x25c arch/arm64/kernel/syscall.c:96 | | race at unknown origin, with read to 0xffff00007ae6aa00 of 8 bytes by task 223 on cpu 1: | el0_svc_common+0x34/0x25c arch/arm64/kernel/syscall.c:96 | do_el0_svc+0x48/0xec arch/arm64/kernel/syscall.c:178 | el0_svc arch/arm64/kernel/entry-common.c:226 [inline] | el0_sync_handler+0x1a4/0x390 arch/arm64/kernel/entry-common.c:236 | el0_sync+0x140/0x180 arch/arm64/kernel/entry.S:674 | | value changed: 0x0000000000000000 -> 0x0000000000000002 | | Reported by Kernel Concurrency Sanitizer on: | CPU: 1 PID: 223 Comm: syz-executor.1 Not tainted 5.8.0-rc3-00094-ga73f923ecc8e-dirty #3 | Hardware name: linux,dummy-virt (DT) | ================================================================== If an access mask is set, it is shown underneath the "value changed" line as "bits changed: 0x<bits changed> with mask 0x<non-zero mask>". Signed-off-by: Mark Rutland <mark.rutland@arm.com> [ elver@google.com: align "value changed" and "bits changed" lines, which required massaging the message; do not print bits+mask if no mask set. ] Signed-off-by: Marco Elver <elver@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
142 lines
3.5 KiB
C
142 lines
3.5 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
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* see Documentation/dev-tools/kcsan.rst.
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*
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* Copyright (C) 2019, Google LLC.
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*/
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#ifndef _KERNEL_KCSAN_KCSAN_H
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#define _KERNEL_KCSAN_KCSAN_H
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#include <linux/atomic.h>
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#include <linux/kcsan.h>
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#include <linux/sched.h>
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/* The number of adjacent watchpoints to check. */
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#define KCSAN_CHECK_ADJACENT 1
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#define NUM_SLOTS (1 + 2*KCSAN_CHECK_ADJACENT)
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extern unsigned int kcsan_udelay_task;
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extern unsigned int kcsan_udelay_interrupt;
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/*
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* Globally enable and disable KCSAN.
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*/
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extern bool kcsan_enabled;
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/*
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* Save/restore IRQ flags state trace dirtied by KCSAN.
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*/
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void kcsan_save_irqtrace(struct task_struct *task);
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void kcsan_restore_irqtrace(struct task_struct *task);
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/*
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* Statistics counters displayed via debugfs; should only be modified in
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* slow-paths.
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*/
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enum kcsan_counter_id {
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/*
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* Number of watchpoints currently in use.
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*/
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KCSAN_COUNTER_USED_WATCHPOINTS,
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/*
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* Total number of watchpoints set up.
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*/
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KCSAN_COUNTER_SETUP_WATCHPOINTS,
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/*
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* Total number of data races.
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*/
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KCSAN_COUNTER_DATA_RACES,
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/*
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* Total number of ASSERT failures due to races. If the observed race is
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* due to two conflicting ASSERT type accesses, then both will be
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* counted.
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*/
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KCSAN_COUNTER_ASSERT_FAILURES,
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/*
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* Number of times no watchpoints were available.
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*/
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KCSAN_COUNTER_NO_CAPACITY,
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/*
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* A thread checking a watchpoint raced with another checking thread;
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* only one will be reported.
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*/
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KCSAN_COUNTER_REPORT_RACES,
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/*
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* Observed data value change, but writer thread unknown.
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*/
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KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN,
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/*
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* The access cannot be encoded to a valid watchpoint.
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*/
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KCSAN_COUNTER_UNENCODABLE_ACCESSES,
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/*
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* Watchpoint encoding caused a watchpoint to fire on mismatching
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* accesses.
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*/
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KCSAN_COUNTER_ENCODING_FALSE_POSITIVES,
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KCSAN_COUNTER_COUNT, /* number of counters */
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};
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extern atomic_long_t kcsan_counters[KCSAN_COUNTER_COUNT];
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/*
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* Returns true if data races in the function symbol that maps to func_addr
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* (offsets are ignored) should *not* be reported.
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*/
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extern bool kcsan_skip_report_debugfs(unsigned long func_addr);
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/*
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* Value-change states.
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*/
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enum kcsan_value_change {
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/*
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* Did not observe a value-change, however, it is valid to report the
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* race, depending on preferences.
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*/
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KCSAN_VALUE_CHANGE_MAYBE,
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/*
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* Did not observe a value-change, and it is invalid to report the race.
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*/
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KCSAN_VALUE_CHANGE_FALSE,
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/*
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* The value was observed to change, and the race should be reported.
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*/
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KCSAN_VALUE_CHANGE_TRUE,
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};
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/*
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* The calling thread hit and consumed a watchpoint: set the access information
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* to be consumed by the reporting thread. No report is printed yet.
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*/
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void kcsan_report_set_info(const volatile void *ptr, size_t size, int access_type,
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int watchpoint_idx);
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/*
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* The calling thread observed that the watchpoint it set up was hit and
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* consumed: print the full report based on information set by the racing
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* thread.
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*/
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void kcsan_report_known_origin(const volatile void *ptr, size_t size, int access_type,
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enum kcsan_value_change value_change, int watchpoint_idx,
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u64 old, u64 new, u64 mask);
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/*
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* No other thread was observed to race with the access, but the data value
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* before and after the stall differs. Reports a race of "unknown origin".
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*/
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void kcsan_report_unknown_origin(const volatile void *ptr, size_t size, int access_type,
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u64 old, u64 new, u64 mask);
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#endif /* _KERNEL_KCSAN_KCSAN_H */
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