[PATCH v2 5/5] kcsan, seqlock: Fix incorrect assumption in read_seqbegin()

From: Marco Elver
Date: Mon Nov 04 2024 - 11:21:09 EST


During testing of the preceding changes, I noticed that in some cases,
current->kcsan_ctx.in_flat_atomic remained true until task exit. This is
obviously wrong, because _all_ accesses for the given task will be
treated as atomic, resulting in false negatives i.e. missed data races.

Debugging led to fs/dcache.c, where we can see this usage of seqlock:

struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
{
struct dentry *dentry;
unsigned seq;

do {
seq = read_seqbegin(&rename_lock);
dentry = __d_lookup(parent, name);
if (dentry)
break;
} while (read_seqretry(&rename_lock, seq));
[...]

As can be seen, read_seqretry() is never called if dentry != NULL;
consequently, current->kcsan_ctx.in_flat_atomic will never be reset to
false by read_seqretry().

Give up on the wrong assumption of "assume closing read_seqretry()", and
rely on the already-present annotations in read_seqcount_begin/retry().

Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
v2:
* New patch.
---
include/linux/seqlock.h | 12 +-----------
1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 45eee0e5dca0..5298765d6ca4 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -810,11 +810,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
-
- kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
- kcsan_flat_atomic_begin();
- return ret;
+ return read_seqcount_begin(&sl->seqcount);
}

/**
@@ -830,12 +826,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
*/
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
- /*
- * Assume not nested: read_seqretry() may be called multiple times when
- * completing read critical section.
- */
- kcsan_flat_atomic_end();
-
return read_seqcount_retry(&sl->seqcount, start);
}

--
2.47.0.163.g1226f6d8fa-goog