Re: [PATCH 1/9] block/vpc: Make vpc_open() read the full dynamic header

[Markus Armbruster]

Max Reitz <mreitz@redhat.com> writes:

> On 17.12.20 17:19, Markus Armbruster wrote:
>> The dynamic header's size is 1024 bytes.
>> 
>> vpc_open() reads only the 512 bytes of the dynamic header into buf[].
>> Works, because it doesn't actually access the second half.  However, a
>> colleague told me that GCC 11 warns:
>> 
>>      ../block/vpc.c:358:51: error: array subscript 'struct 
>> VHDDynDiskHeader[0]' is partly outside array bounds of 'uint8_t[512]' 
>> [-Werror=array-bounds]
>> 
>> Clean up to read the full header.
>> 
>> Rename buf[] to dyndisk_header_buf[] while there.
>> 
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> ---
>>   block/vpc.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/block/vpc.c b/block/vpc.c
>> index 1ab55f9287..2fcf3f6283 100644
>> --- a/block/vpc.c
>> +++ b/block/vpc.c
>> @@ -220,7 +220,7 @@ static int vpc_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>       QemuOpts *opts = NULL;
>>       Error *local_err = NULL;
>>       bool use_chs;
>> -    uint8_t buf[HEADER_SIZE];
>> +    uint8_t dyndisk_header_buf[1024];
>
> Perhaps this should be heap-allocated, but I don’t know whether qemu has 
> ever established a (perhaps just inofficial) threshold on what goes on 
> the stack and what goes on the heap.

There is no official per-function limit.  I generally don't worry about
a few kilobytes unless it's recursive.  We have many, many functions
with stack frames in the order of a kilobyte.  Our coroutine and thread
stacks are reasonable (corotine stacks are 1MiB, the default thread
stack size 2MiB on x86-64, and I can't see code that sets a different
size).

>>       uint32_t checksum;
>>       uint64_t computed_size;
>>       uint64_t pagetable_size;
>> @@ -340,14 +340,14 @@ static int vpc_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>       }
>>   
>>       if (disk_type == VHD_DYNAMIC) {
>> -        ret = bdrv_pread(bs->file, be64_to_cpu(footer->data_offset), buf,
>> -                         HEADER_SIZE);
>> +        ret = bdrv_pread(bs->file, be64_to_cpu(footer->data_offset),
>> +                         dyndisk_header_buf, 1024);
>
> sizeof() would be better, but I see that’s what patch 6 is for.

Yes, but sizeof what?  VHDDynDiskHeader becomes usable for that only in
PATCH 5.

I chose to separate the buffers first, and only then tidy up their use.

>>           if (ret < 0) {
>>               error_setg(errp, "Error reading dynamic VHD header");
>>               goto fail;
>>           }
>>   
>> -        dyndisk_header = (VHDDynDiskHeader *) buf;
>> +        dyndisk_header = (VHDDynDiskHeader *)dyndisk_header_buf;
>>   
>>           if (strncmp(dyndisk_header->magic, "cxsparse", 8)) {
>>               error_setg(errp, "Invalid header magic");
>
> Reviewed-by: Max Reitz <mreitz@redhat.com>

Thanks!