Re: [Qemu-devel] [PATCH 1/1] block/file-posix.c: fix not reopened lock file descriptor

[Dion Bosschieter]

Yeah I have a use case, before a last sync on a storage migration we suspend a 
VM -> send the last diffs -> mount the new storage server and after that we 
change a symlink -> call reopen -> check if all file descriptors are changed 
before resuming the VM.

Dion

> Op 22 mrt. 2018 om 18:39 heeft Kevin Wolf <address@hidden> het volgende 
> geschreven:
> 
> [ Cc: qemu-block ]
> 
> Am 22.03.2018 um 18:20 hat Dion Bosschieter geschrieben:
>> In commit 244a5668106297378391b768e7288eb157616f64 another
>> file descriptor to BDRVRawState is added. When we try to issue the
>> reopen command only s->fd is reopened; lock_fd could still hold an old
>> file descriptor "possibly" pointing to another file.
>> 
>> - change raw_reopen_prepare so it checks use_lock from BDRVRawState and
>> tries to reopen lock_fd accordingly
>> - change raw_reopen_commit so it closes the old lock_fd on use_lock
>> 
>> Signed-off-by: Dion Bosschieter <address@hidden>
> 
> bdrv_reopen() is not meant for opening a different file, it is meant to
> change the flags and options of the same file. Do you have a use case
> where you would actually need to switch to a different file?
> 
> As far as I know, lock_fd was specifically introduced _because_ it stays
> the same across reopen, so we don't need a racy release/reacquire pair.
> Fam (CCed) should know more.
> 
> In any case, doesn't your patch drop all the locks without reacquiring
> them on the new lock_fd?
> 
> Kevin
> 
>> block/file-posix.c | 25 +++++++++++++++++++++++++
>> 1 file changed, 25 insertions(+)
>> 
>> diff --git a/block/file-posix.c b/block/file-posix.c
>> index d7fb772c14..16d83fc49e 100644
>> --- a/block/file-posix.c
>> +++ b/block/file-posix.c
>> @@ -167,6 +167,7 @@ typedef struct BDRVRawState {
>> 
>> typedef struct BDRVRawReopenState {
>>     int fd;
>> +    int lock_fd;
>>     int open_flags;
>> } BDRVRawReopenState;
>> 
>> @@ -795,6 +796,7 @@ static int raw_reopen_prepare(BDRVReopenState *state,
>>     raw_parse_flags(state->flags, &rs->open_flags);
>> 
>>     rs->fd = -1;
>> +    rs->lock_fd = -1;
>> 
>>     int fcntl_flags = O_APPEND | O_NONBLOCK;
>> #ifdef O_NOATIME
>> @@ -820,6 +822,17 @@ static int raw_reopen_prepare(BDRVReopenState *state,
>>                 rs->fd = -1;
>>             }
>>         }
>> +
>> +        if (s->use_lock) {
>> +            rs->lock_fd = qemu_dup(s->lock_fd);
>> +            if (rs->lock_fd >= 0) {
>> +                ret = fcntl_setfl(rs->lock_fd, rs->open_flags);
>> +                if (ret) {
>> +                    qemu_close(rs->lock_fd);
>> +                    rs->lock_fd = -1;
>> +                }
>> +            }
>> +        }
>>     }
>> 
>>     /* If we cannot use fcntl, or fcntl failed, fall back to qemu_open() */
>> @@ -835,6 +848,14 @@ static int raw_reopen_prepare(BDRVReopenState *state,
>>                 error_setg_errno(errp, errno, "Could not reopen file");
>>                 ret = -1;
>>             }
>> +
>> +            if (s->use_lock) {
>> +                rs->lock_fd = qemu_open(normalized_filename, 
>> rs->open_flags);
>> +                if (rs->lock_fd == -1) {
>> +                    error_setg_errno(errp, errno, "Could not reopen file 
>> for locking");
>> +                    ret = -1;
>> +                }
>> +            }
>>         }
>>     }
>> 
>> @@ -861,7 +882,11 @@ static void raw_reopen_commit(BDRVReopenState *state)
>>     s->open_flags = rs->open_flags;
>> 
>>     qemu_close(s->fd);
>> +    if (s->use_lock) {
>> +        qemu_close(s->lock_fd);
>> +    }
>>     s->fd = rs->fd;
>> +    s->lock_fd = rs->lock_fd;
>> 
>>     g_free(state->opaque);
>>     state->opaque = NULL;
>> -- 
>> 2.14.2
>>