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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 04, 2024 at 04:43:09PM +0100, Marco Elver wrote:
> 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@google.com>
> ---
> 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);
>  }

OK, so this takes us back to kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)
and kcsan_atomic_next(0).

Which I suppose is safe, except it doesn't nest properly.

Anyway, these all look really nice, let me go queue them up.

Thanks!