Re: [Qemu-devel] [PATCH v2 1/2] qga: flush explicitly when needed

[Eric Blake]

On 11/24/2015 11:04 AM, address@hidden wrote:
> From: Marc-André Lureau <address@hidden>
> 
> According to the specification:
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
> 
> "the application shall ensure that output is not directly followed by
> input without an intervening call to fflush() or to a file positioning
> function (fseek(), fsetpos(), or rewind()), and input is not directly
> followed by output without an intervening call to a file positioning
> function, unless the input operation encounters end-of-file."
> 
> Without this change, a write() followed by a read() may lose the
> previously written content, as shown in the following test.
> 
> Fixes:
> https://bugzilla.redhat.com/show_bug.cgi?id=1210246
> 
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
>  qga/commands-posix.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> index 0ebd473..d0228ce 100644
> --- a/qga/commands-posix.c
> +++ b/qga/commands-posix.c
> @@ -219,6 +219,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, 
> Error **errp)
>  typedef struct GuestFileHandle {
>      uint64_t id;
>      FILE *fh;
> +    bool writing;
>      QTAILQ_ENTRY(GuestFileHandle) next;
>  } GuestFileHandle;
>  
> @@ -460,6 +461,17 @@ struct GuestFileRead *qmp_guest_file_read(int64_t 
> handle, bool has_count,
>      }
>  
>      fh = gfh->fh;
> +
> +    /* explicitly flush when switching from writing to reading */
> +    if (gfh->writing) {
> +        int ret = fflush(fh);
> +        if (ret == EOF) {
> +            error_setg_errno(errp, errno, "failed to flush file");
> +            return NULL;
> +        }
> +        gfh->writing = false;
> +    }
> +
>      buf = g_malloc0(count+1);
>      read_count = fread(buf, 1, count, fh);
>      if (ferror(fh)) {
> @@ -496,6 +508,16 @@ GuestFileWrite *qmp_guest_file_write(int64_t handle, 
> const char *buf_b64,
>      }
>  
>      fh = gfh->fh;
> +
> +    if (!gfh->writing) {
> +        int ret = fseek(fh, 0, SEEK_CUR);
> +        if (ret == -1) {
> +            error_setg_errno(errp, errno, "failed to seek file");
> +            return NULL;
> +        }
> +        gfh->writing = true;
> +    }

Hmm.  This always attempts fseek() on the first write() to a file, even
if the file is not also open for read.  While guest-file-open is most
likely used on regular files (and therefore seekable), I'm worried that
we might have a client that is attempting to use it on terminal files or
other non-seekable file names.  Since the fseek() on first write is
unconditional, that means we would now fail to let a user write to such
a file, even if they could previously do so.  Should we add more logic
to only do the fseek() after a previous write (as in a tri-state
variable of untouched, last written, last read), so that we aren't
breaking one-pass usage of non-seekable files?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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