target: support zero-size allocation lengths in transport_kmap_data_sg

In order to support zero-size allocation lengths, do not assert
that we have a scatterlist until after checking cmd->data_length.

But once we do this, we can have two cases of transport_kmap_data_sg
returning NULL: a zero-size allocation length, or an out-of-memory
condition.  Report the latter using sense codes, so that the SCSI
command that triggered it will fail.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index 7ddb0c33f644..4403477882b3 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -2190,7 +2190,6 @@ void *transport_kmap_data_sg(struct se_cmd *cmd)
         struct page **pages;
         int i;
 
-        BUG_ON(!sg);
         /*
          * We need to take into account a possible offset here for fabrics like
          * tcm_loop who may be using a contig buffer from the SCSI midlayer for
@@ -2198,13 +2197,17 @@ void *transport_kmap_data_sg(struct se_cmd *cmd)
          */
         if (!cmd->t_data_nents)
                 return NULL;
-        else if (cmd->t_data_nents == 1)
+
+        BUG_ON(!sg);
+        if (cmd->t_data_nents == 1)
                 return kmap(sg_page(sg)) + sg->offset;
 
         /* >1 page. use vmap */
         pages = kmalloc(sizeof(*pages) * cmd->t_data_nents, GFP_KERNEL);
-        if (!pages)
+        if (!pages) {
+                cmd->scsi_sense_reason = TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
                 return NULL;
+        }
 
         /* convert sg[] to pages[] */
         for_each_sg(cmd->t_data_sg, sg, cmd->t_data_nents, i) {
@@ -2213,8 +2216,10 @@ void *transport_kmap_data_sg(struct se_cmd *cmd)
 
         cmd->t_data_vmap = vmap(pages, cmd->t_data_nents,  VM_MAP, PAGE_KERNEL);
         kfree(pages);
-        if (!cmd->t_data_vmap)
+        if (!cmd->t_data_vmap) {
+                cmd->scsi_sense_reason = TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
                 return NULL;
+        }
 
         return cmd->t_data_vmap + cmd->t_data_sg[0].offset;
 }