Re: [Qemu-devel] [PATCH v7 12/35] util: let qemu_fd_getlength support block device

[Eduardo Habkost]

On Mon, Nov 02, 2015 at 05:13:14PM +0800, Xiao Guangrong wrote:
> lseek can not work for all block devices as the man page says:
> | Some devices are incapable of seeking and POSIX does not specify
> | which devices must support lseek().
> 
> This patch tries to add the support on Linux by using BLKGETSIZE64
> ioctl
> 
> Signed-off-by: Xiao Guangrong <address@hidden>

On which cases is this patch necessary? Do you know any examples of
Linux block devices that won't work with lseek(SEEK_END)?

> ---
>  util/osdep.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/util/osdep.c b/util/osdep.c
> index 5a61e19..b20c793 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -45,6 +45,11 @@
>  extern int madvise(caddr_t, size_t, int);
>  #endif
>  
> +#ifdef CONFIG_LINUX
> +#include <sys/ioctl.h>
> +#include <linux/fs.h>
> +#endif
> +
>  #include "qemu-common.h"
>  #include "qemu/sockets.h"
>  #include "qemu/error-report.h"
> @@ -433,6 +438,21 @@ int64_t qemu_fd_getlength(int fd)
>  {
>      int64_t size;
>  
> +#ifdef CONFIG_LINUX
> +    struct stat stat_buf;
> +    if (fstat(fd, &stat_buf) < 0) {
> +        return -errno;
> +    }
> +
> +    if ((S_ISBLK(stat_buf.st_mode)) && !ioctl(fd, BLKGETSIZE64, &size)) {
> +        /* The size of block device is larger than max int64_t? */
> +        if (size < 0) {
> +            return -EOVERFLOW;
> +        }
> +        return size;
> +    }
> +#endif
> +
>      size = lseek(fd, 0, SEEK_END);
>      if (size < 0) {
>          return -errno;
> -- 
> 1.8.3.1
> 

-- 
Eduardo