From: Kameron Carr <[email protected]> Sent: Monday, October 19, 2026 4:10 PM > > hv_get_ringbuffer_availbytes() reads the indices directly, so a caller > that also wants the index values has to read them a second time. The > host can change them in between, resulting in the byte counts and the > indices reflecting two different states of the ring. > > Replace it with hv_ringbuffer_avail_write() and > hv_ringbuffer_avail_read(), which take the indices as arguments and > return a single count. They are separate because most callers want only > one of the two values. Use them in hv_ringbuffer_write() in place of the > open-coded equivalent. hv_get_bytes_to_read(), hv_get_bytes_to_write() > and hv_pkt_iter_avail() duplicated the same arithmetic, so put the > helpers in include/linux/hyperv.h and use them there too. > > Add a hv_ringbuffer_indices_invalid() helper that bounds checks a pair > of read and write indices, and use it for the check in > hv_ringbuffer_write(). Convert the remaining callers to use a single > snapshot: > > - hv_ringbuffer_get_debuginfo() now reports the indices and the > computed byte counts from the same snapshot. > > - hv_pkt_iter_close() computes the free space from priv_read_index > instead of re-reading the shared read index. The two agree unless a > misbehaving host has rewritten the value. > > No functional change intended for a well-behaved host. > > Signed-off-by: Kameron Carr <[email protected]>
Reviewed-by: Michael Kelley <[email protected]> > --- > drivers/hv/ring_buffer.c | 68 > ++++++++++++++++-------------------------------- > include/linux/hyperv.h | 50 +++++++++++++++++++++++++++-------- > 2 files changed, 61 insertions(+), 57 deletions(-) > > diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c > index 7f466c5..cda957d 100644 > --- a/drivers/hv/ring_buffer.c > +++ b/drivers/hv/ring_buffer.c > @@ -107,35 +107,12 @@ static u32 hv_copyto_ringbuffer( > return start_write_offset; > } > > -/* > - * > - * hv_get_ringbuffer_availbytes() > - * > - * Get number of bytes available to read and to write to > - * for the specified ring buffer > - */ > -static void > -hv_get_ringbuffer_availbytes(const struct hv_ring_buffer_info *rbi, > - u32 *read, u32 *write) > -{ > - u32 read_loc, write_loc, dsize; > - > - /* Capture the read/write indices before they changed */ > - read_loc = READ_ONCE(rbi->ring_buffer->read_index); > - write_loc = READ_ONCE(rbi->ring_buffer->write_index); > - dsize = rbi->ring_datasize; > - > - *write = write_loc >= read_loc ? dsize - (write_loc - read_loc) : > - read_loc - write_loc; > - *read = dsize - *write; > -} > - > /* Get various debug metrics for the specified ring buffer. */ > int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info, > struct hv_ring_buffer_debug_info *debug_info) > { > - u32 bytes_avail_towrite; > - u32 bytes_avail_toread; > + u32 read_index; > + u32 write_index; > > mutex_lock(&ring_info->ring_buffer_mutex); > > @@ -144,13 +121,14 @@ int hv_ringbuffer_get_debuginfo(struct > hv_ring_buffer_info *ring_info, > return -EINVAL; > } > > - hv_get_ringbuffer_availbytes(ring_info, > - &bytes_avail_toread, > - &bytes_avail_towrite); > - debug_info->bytes_avail_toread = bytes_avail_toread; > - debug_info->bytes_avail_towrite = bytes_avail_towrite; > - debug_info->current_read_index = ring_info->ring_buffer->read_index; > - debug_info->current_write_index = ring_info->ring_buffer->write_index; > + read_index = READ_ONCE(ring_info->ring_buffer->read_index); > + write_index = READ_ONCE(ring_info->ring_buffer->write_index); > + debug_info->bytes_avail_toread = > + hv_ringbuffer_avail_read(ring_info, read_index, write_index); > + debug_info->bytes_avail_towrite = > + hv_ringbuffer_avail_write(ring_info, read_index, write_index); > + debug_info->current_read_index = read_index; > + debug_info->current_write_index = write_index; > debug_info->current_interrupt_mask > = ring_info->ring_buffer->interrupt_mask; > mutex_unlock(&ring_info->ring_buffer_mutex); > @@ -282,8 +260,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel, > > read_index = READ_ONCE(outring_info->ring_buffer->read_index); > old_write = READ_ONCE(outring_info->ring_buffer->write_index); > - if (unlikely(read_index >= outring_info->ring_datasize || > - old_write >= outring_info->ring_datasize)) { > + if (unlikely(hv_ringbuffer_indices_invalid(outring_info, read_index, > + old_write))) { > spin_unlock_irqrestore(&outring_info->ring_lock, flags); > pr_err_ratelimited("outbound ring indices out of range: relid > %u read %u write %u size %u\n", > channel->offermsg.child_relid, read_index, > @@ -291,9 +269,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel, > return -EIO; > } > > - bytes_avail_towrite = old_write >= read_index ? > - outring_info->ring_datasize - (old_write - read_index) : > - read_index - old_write; > + bytes_avail_towrite = hv_ringbuffer_avail_write(outring_info, > read_index, > + old_write); > > /* > * If there is only room for the packet, assume it is full. > @@ -445,10 +422,7 @@ static u32 hv_pkt_iter_avail(const struct > hv_ring_buffer_info *rbi) > */ > write_loc = virt_load_acquire(&rbi->ring_buffer->write_index); > > - if (write_loc >= priv_read_loc) > - return write_loc - priv_read_loc; > - else > - return (rbi->ring_datasize - priv_read_loc) + write_loc; > + return hv_ringbuffer_avail_read(rbi, priv_read_loc, write_loc); > } > > /* > @@ -568,6 +542,7 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) > { > struct hv_ring_buffer_info *rbi = &channel->inbound; > u32 curr_write_sz, pending_sz, bytes_read, start_read_index; > + u32 write_index; > > /* > * Make sure all reads are done before we update the read index since > @@ -607,11 +582,13 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) > return; > > /* > - * Ensure the read of write_index in hv_get_bytes_to_write() > - * happens after the read of pending_send_sz. > + * Ensure the read of write_index happens after the read of > + * pending_send_sz. > */ > virt_rmb(); > - curr_write_sz = hv_get_bytes_to_write(rbi); > + write_index = READ_ONCE(rbi->ring_buffer->write_index); > + curr_write_sz = hv_ringbuffer_avail_write(rbi, rbi->priv_read_index, > + write_index); > bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index); > > /* > @@ -627,8 +604,7 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) > * Exactly filling the ring buffer is treated as "not enough > * space". The ring buffer always must have at least one byte > * empty so the empty and full conditions are distinguishable. > - * hv_get_bytes_to_write() doesn't fully tell the truth in > - * this regard. > + * curr_write_sz doesn't fully tell the truth in this regard. > * > * So first check if we were in the "enough free space" state > * before we began the iteration. If so, the host was not > diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h > index 5c65820..09f25ad 100644 > --- a/include/linux/hyperv.h > +++ b/include/linux/hyperv.h > @@ -209,31 +209,59 @@ struct hv_ring_buffer_info { > }; > > > +/* > + * The indices live in memory shared with the untrusted host, so check a > + * snapshot of them before using it as an offset or to compute a byte count. > + */ > +static inline bool > +hv_ringbuffer_indices_invalid(const struct hv_ring_buffer_info *rbi, > + u32 read_loc, u32 write_loc) > +{ > + return read_loc >= rbi->ring_datasize || > + write_loc >= rbi->ring_datasize; > +} > + > +/* > + * Byte counts for a caller-supplied snapshot of the indices, so that the > + * counts and the indices the caller goes on to use describe one state of the > + * ring. > + */ > +static inline u32 > +hv_ringbuffer_avail_write(const struct hv_ring_buffer_info *rbi, > + u32 read_loc, u32 write_loc) > +{ > + u32 dsize = rbi->ring_datasize; > + > + return write_loc >= read_loc ? dsize - (write_loc - read_loc) : > + read_loc - write_loc; > +} > + > +static inline u32 > +hv_ringbuffer_avail_read(const struct hv_ring_buffer_info *rbi, > + u32 read_loc, u32 write_loc) > +{ > + return rbi->ring_datasize - > + hv_ringbuffer_avail_write(rbi, read_loc, write_loc); > +} > + > static inline u32 hv_get_bytes_to_read(const struct hv_ring_buffer_info *rbi) > { > - u32 read_loc, write_loc, dsize, read; > + u32 read_loc, write_loc; > > - dsize = rbi->ring_datasize; > read_loc = READ_ONCE(rbi->ring_buffer->read_index); > write_loc = READ_ONCE(rbi->ring_buffer->write_index); > > - read = write_loc >= read_loc ? (write_loc - read_loc) : > - (dsize - read_loc) + write_loc; > - > - return read; > + return hv_ringbuffer_avail_read(rbi, read_loc, write_loc); > } > > static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info > *rbi) > { > - u32 read_loc, write_loc, dsize, write; > + u32 read_loc, write_loc; > > - dsize = rbi->ring_datasize; > read_loc = READ_ONCE(rbi->ring_buffer->read_index); > write_loc = READ_ONCE(rbi->ring_buffer->write_index); > > - write = write_loc >= read_loc ? dsize - (write_loc - read_loc) : > - read_loc - write_loc; > - return write; > + return hv_ringbuffer_avail_write(rbi, read_loc, write_loc); > } > > static inline u32 hv_get_avail_to_write_percent( > > -- > 2.45.4
