Re: [PATCH 3/9] block/vpc: Don't abuse the footer buffer for dynamic header

[Max Reitz]

On 17.12.20 17:19, Markus Armbruster wrote:
create_dynamic_disk() takes a buffer holding the footer as first
argument.  It writes out the footer (512 bytes), then reuses the
buffer to initialize and write out the dynamic header (1024 bytes).
Works, because the caller passes a buffer that is large enough for
both purposes.  I hate that.

Use a separate buffer for the dynamic header, and adjust the caller's
buffer.

Signed-off-by: Markus Armbruster <arm...@redhat.com>
---
  block/vpc.c | 22 ++++++++++++----------
  1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index d18ecc3da1..34186640ee 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -822,8 +822,9 @@ static int calculate_geometry(int64_t total_sectors, 
uint16_t *cyls,
  static int create_dynamic_disk(BlockBackend *blk, uint8_t *buf,
                                 int64_t total_sectors)
  {
+    uint8_t dyndisk_header_buf[1024];
      VHDDynDiskHeader *dyndisk_header =
-        (VHDDynDiskHeader *) buf;
+        (VHDDynDiskHeader *)dyndisk_header_buf;
      uint8_t bat_sector[512];
      size_t block_size, num_bat_entries;
      int i;
@@ -858,7 +859,7 @@ static int create_dynamic_disk(BlockBackend *blk, uint8_t 
*buf,
      }
  
      /* Prepare the Dynamic Disk Header */
-    memset(buf, 0, 1024);
+    memset(dyndisk_header_buf, 0, 1024);
  
      memcpy(dyndisk_header->magic, "cxsparse", 8);

I’d prefer something like

*dyndisk_header = (VHDDynDiskHeader){
    ...
};

but that isn’t possible before patch 5.  (And can be done on top now 
anyway, especially given that Kevin probably wants to send a pull 
request today.)

[...]

@@ -972,8 +974,8 @@ static int coroutine_fn vpc_co_create(BlockdevCreateOptions 
*opts,
      BlockBackend *blk = NULL;
      BlockDriverState *bs = NULL;
  
-    uint8_t buf[1024];
-    VHDFooter *footer = (VHDFooter *) buf;
+    uint8_t footer_buf[HEADER_SIZE];
+    VHDFooter *footer = (VHDFooter *)footer_buf;
      uint16_t cyls = 0;
      uint8_t heads = 0;
      uint8_t secs_per_cyl = 0;
@@ -1036,7 +1038,7 @@ static int coroutine_fn 
vpc_co_create(BlockdevCreateOptions *opts,
      }
  
      /* Prepare the Hard Disk Footer */
-    memset(buf, 0, 1024);
+    memset(footer_buf, 0, HEADER_SIZE);
  
      memcpy(footer->creator, "conectix", 8);

Same here, except here it’s patch 7.

Reviewed-by: Max Reitz <mre...@redhat.com>