From: "tip-bot2 for Marco Elver" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: Marco Elver <elver@google.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
x86@kernel.org, linux-kernel@vger.kernel.org
Subject: [tip: locking/core] kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state()
Date: Mon, 15 Feb 2021 14:55:55 -0000 [thread overview]
Message-ID: <161340095583.20312.15815240255763140896.tip-bot2@tip-bot2> (raw)
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 71a076f4a61a6c779794ad286f356b39725edc3b
Gitweb: https://git.kernel.org/tip/71a076f4a61a6c779794ad286f356b39725edc3b
Author: Marco Elver <elver@google.com>
AuthorDate: Tue, 24 Nov 2020 12:02:09 +01:00
Committer: Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Mon, 04 Jan 2021 14:39:07 -08:00
kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state()
Rewrite kcsan_prandom_u32_max() to not depend on code that might be
instrumented, removing any dependency on lib/random32.c. The rewrite
implements a simple linear congruential generator, that is sufficient
for our purposes (for udelay() and skip_watch counter randomness).
The initial motivation for this was to allow enabling KCSAN for
kernel/sched (remove KCSAN_SANITIZE := n from kernel/sched/Makefile),
with CONFIG_DEBUG_PREEMPT=y. Without this change, we could observe
recursion:
check_access() [via instrumentation]
kcsan_setup_watchpoint()
reset_kcsan_skip()
kcsan_prandom_u32_max()
get_cpu_var()
preempt_disable()
preempt_count_add() [in kernel/sched/core.c]
check_access() [via instrumentation]
Note, while this currently does not affect an unmodified kernel, it'd be
good to keep a KCSAN kernel working when KCSAN_SANITIZE := n is removed
from kernel/sched/Makefile to permit testing scheduler code with KCSAN
if desired.
Fixes: cd290ec24633 ("kcsan: Use tracing-safe version of prandom")
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/kcsan/core.c | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 3994a21..3bf98db 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -12,7 +12,6 @@
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
-#include <linux/random.h>
#include <linux/sched.h>
#include <linux/uaccess.h>
@@ -101,7 +100,7 @@ static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1];
static DEFINE_PER_CPU(long, kcsan_skip);
/* For kcsan_prandom_u32_max(). */
-static DEFINE_PER_CPU(struct rnd_state, kcsan_rand_state);
+static DEFINE_PER_CPU(u32, kcsan_rand_state);
static __always_inline atomic_long_t *find_watchpoint(unsigned long addr,
size_t size,
@@ -275,20 +274,17 @@ should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx *
}
/*
- * Returns a pseudo-random number in interval [0, ep_ro). See prandom_u32_max()
- * for more details.
- *
- * The open-coded version here is using only safe primitives for all contexts
- * where we can have KCSAN instrumentation. In particular, we cannot use
- * prandom_u32() directly, as its tracepoint could cause recursion.
+ * Returns a pseudo-random number in interval [0, ep_ro). Simple linear
+ * congruential generator, using constants from "Numerical Recipes".
*/
static u32 kcsan_prandom_u32_max(u32 ep_ro)
{
- struct rnd_state *state = &get_cpu_var(kcsan_rand_state);
- const u32 res = prandom_u32_state(state);
+ u32 state = this_cpu_read(kcsan_rand_state);
+
+ state = 1664525 * state + 1013904223;
+ this_cpu_write(kcsan_rand_state, state);
- put_cpu_var(kcsan_rand_state);
- return (u32)(((u64) res * ep_ro) >> 32);
+ return state % ep_ro;
}
static inline void reset_kcsan_skip(void)
@@ -639,10 +635,14 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
void __init kcsan_init(void)
{
+ int cpu;
+
BUG_ON(!in_task());
kcsan_debugfs_init();
- prandom_seed_full_state(&kcsan_rand_state);
+
+ for_each_possible_cpu(cpu)
+ per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();
/*
* We are in the init task, and no other tasks should be running;
reply other threads:[~2021-02-15 15:09 UTC|newest]
Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=161340095583.20312.15815240255763140896.tip-bot2@tip-bot2 \
--to=tip-bot2@linutronix.de \
--cc=elver@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-tip-commits@vger.kernel.org \
--cc=paulmck@kernel.org \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).