Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] block/file-posix: File locking during creation

[Max Reitz]

On 2018-04-27 08:22, Fam Zheng wrote:
> On Sat, 04/21 00:09, Max Reitz wrote:
>> When creating a file, we should take the WRITE and RESIZE permissions.
>> We do not need either for the creation itself, but we do need them for
>> clearing and resizing it.  So we can take the proper permissions by
>> replacing O_TRUNC with an explicit truncation to 0, and by taking the
>> appropriate file locks between those two steps.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  block/file-posix.c | 41 +++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 39 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/file-posix.c b/block/file-posix.c
>> index c98a4a1556..ed7932d6e8 100644
>> --- a/block/file-posix.c
>> +++ b/block/file-posix.c
>> @@ -1992,6 +1992,7 @@ static int raw_co_create(BlockdevCreateOptions 
>> *options, Error **errp)
>>  {
>>      BlockdevCreateOptionsFile *file_opts;
>>      int fd;
>> +    int perm, shared;
>>      int result = 0;
>>  
>>      /* Validate options and set default values */
>> @@ -2006,14 +2007,48 @@ static int raw_co_create(BlockdevCreateOptions 
>> *options, Error **errp)
>>      }
>>  
>>      /* Create file */
>> -    fd = qemu_open(file_opts->filename, O_RDWR | O_CREAT | O_TRUNC | 
>> O_BINARY,
>> -                   0644);
>> +    fd = qemu_open(file_opts->filename, O_RDWR | O_CREAT | O_BINARY, 0644);
>>      if (fd < 0) {
>>          result = -errno;
>>          error_setg_errno(errp, -result, "Could not create file");
>>          goto out;
>>      }
>>  
>> +    /* Take permissions: We want to discard everything, so we need
>> +     * BLK_PERM_WRITE; and truncation to the desired size requires
>> +     * BLK_PERM_RESIZE.
>> +     * On the other hand, we cannot share the RESIZE permission
>> +     * because we promise that after this function, the file has the
>> +     * size given in the options.  If someone else were to resize it
>> +     * concurrently, we could not guarantee that. */
> 
> Strictly speaking, we do close(fd) before this function returns so the lock is
> lost and race can happen.

Right, but then creation from the perspective of file-posix is over.  We
are going to reopen the file for formatting, but that is just a normal
bdrv_open() so it will automatically be locked, no?

>> +    perm = BLK_PERM_WRITE | BLK_PERM_RESIZE;
>> +    shared = BLK_PERM_ALL & ~BLK_PERM_RESIZE;
>> +
>> +    /* Step one: Take locks in shared mode */
>> +    result = raw_apply_lock_bytes(fd, perm, shared, false, errp);
>> +    if (result < 0) {
>> +        goto out_close;
>> +    }
>> +
>> +    /* Step two: Try to get them exclusively */
>> +    result = raw_check_lock_bytes(fd, perm, shared, errp);
>> +    if (result < 0) {
>> +        goto out_close;
>> +    }
>> +
>> +    /* Step three: Downgrade them to shared again, and keep
>> +     *             them that way until we are done */
>> +    result = raw_apply_lock_bytes(fd, perm, shared, true, errp);
> 
> You comment it as "downgrade to shared" but what this actually does in 
> addition
> to step one is to "unlock" unneeded bytes which we haven't locked anyway. So 
> I'm
> confused why we need this stop. IIUC no downgrading is necessary after step 
> one
> and step two: only necessary shared locks are acquired in step one, and step 
> two
> is read-only op (F_OFD_GETLK).

Oops.  For some reason I thought raw_check_lock_bytes() would take the
locks exclusively.  Yes, then we don't need this third step.

Thanks for reviewing!

Max

>> +    if (result < 0) {
>> +        goto out_close;
>> +    }
>> +
>> +    /* Clear the file by truncating it to 0 */
>> +    result = raw_regular_truncate(fd, 0, PREALLOC_MODE_OFF, errp);
>> +    if (result < 0) {
>> +        goto out_close;
>> +    }
>> +
>>      if (file_opts->nocow) {
>>  #ifdef __linux__
>>          /* Set NOCOW flag to solve performance issue on fs like btrfs.
>> @@ -2029,6 +2064,8 @@ static int raw_co_create(BlockdevCreateOptions 
>> *options, Error **errp)
>>  #endif
>>      }
>>  
>> +    /* Resize and potentially preallocate the file to the desired
>> +     * final size */
>>      result = raw_regular_truncate(fd, file_opts->size, 
>> file_opts->preallocation,
>>                                    errp);
>>      if (result < 0) {
>> -- 
>> 2.14.3
>>
>>
> 
> Fam
>

signature.asc
Description: OpenPGP digital signature