[v7,12/35] util: let qemu_fd_getlength support block device
diff mbox

Message ID 1446455617-129562-13-git-send-email-guangrong.xiao@linux.intel.com
State New
Headers show

Commit Message

Xiao Guangrong Nov. 2, 2015, 9:13 a.m. UTC
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 <guangrong.xiao@linux.intel.com>
---
 util/osdep.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

Comments

Vladimir Sementsov-Ogievskiy Nov. 2, 2015, 4:11 p.m. UTC | #1
On 02.11.2015 12:13, 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 <guangrong.xiao@linux.intel.com>
> ---
>   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;

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

just a question: is there any use for stat.st_size ? Is it always worse 
then lseek? Does it work for blk?

also, "This patch tries to add..". Hmm. It looks like this patch is not 
sure about will it success. I'd prefer "This patch adds", but this is 
not important
Xiao Guangrong Nov. 2, 2015, 4:21 p.m. UTC | #2
On 11/03/2015 12:11 AM, Vladimir Sementsov-Ogievskiy wrote:
> On 02.11.2015 12:13, 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 <guangrong.xiao@linux.intel.com>
>> ---
>>   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;
>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>
> just a question: is there any use for stat.st_size ? Is it always worse then lseek?

The man page says:
The  st_size field gives the size of the file (if it is a regular file or a symbolic link)
in bytes.  The size of a symbolic link is the length of the pathname it contains, without a
terminating null byte.

So it can not work on symbolic link.

> Does it work for
> blk?
>

Quickly checked with a program written by myself and 'stat' command, the answer is NO. :)

> also, "This patch tries to add..". Hmm. It looks like this patch is not sure about will it success.
> I'd prefer "This patch adds", but this is not important
>

Thanks for your sharing. I did not know the different, now, i got it. :)

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Eduardo Habkost Nov. 6, 2015, 3:44 p.m. UTC | #3
On Tue, Nov 03, 2015 at 12:21:30AM +0800, Xiao Guangrong wrote:
> On 11/03/2015 12:11 AM, Vladimir Sementsov-Ogievskiy wrote:
> >On 02.11.2015 12:13, 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 <guangrong.xiao@linux.intel.com>
> >>---
> >>  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;
> >
> >Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> >
> >just a question: is there any use for stat.st_size ? Is it always worse then lseek?
> 
> The man page says:
> The  st_size field gives the size of the file (if it is a regular file or a symbolic link)
> in bytes.  The size of a symbolic link is the length of the pathname it contains, without a
> terminating null byte.
> 
> So it can not work on symbolic link.

stat() and fstat() will get you information for the file pointed by the
symlink. You will get the symlink information only if you use lstat().
Eduardo Habkost Nov. 6, 2015, 3:48 p.m. UTC | #4
As this patch affects raw_getlength(), CCing the raw block driver
maintainer and the qemu-block mailing list.

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 <guangrong.xiao@linux.intel.com>
> ---
>  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 Habkost Nov. 6, 2015, 3:54 p.m. UTC | #5
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 <guangrong.xiao@linux.intel.com>

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
>
Xiao Guangrong Nov. 9, 2015, 5:58 a.m. UTC | #6
On 11/06/2015 11:54 PM, Eduardo Habkost wrote:
> 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 <guangrong.xiao@linux.intel.com>
>
> On which cases is this patch necessary? Do you know any examples of
> Linux block devices that won't work with lseek(SEEK_END)?

To be honest, i have not checked all block device, this patch was made
based on the man page. However, i do not mind drop this patch (and maybe
other patches) to make this pachset smaller. BLKGETSIZE64 can be added
in the future if we meet such device.


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Eduardo Habkost Nov. 9, 2015, 6:43 p.m. UTC | #7
On Mon, Nov 09, 2015 at 01:58:27PM +0800, Xiao Guangrong wrote:
> 
> 
> On 11/06/2015 11:54 PM, Eduardo Habkost wrote:
> >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 <guangrong.xiao@linux.intel.com>
> >
> >On which cases is this patch necessary? Do you know any examples of
> >Linux block devices that won't work with lseek(SEEK_END)?
> 
> To be honest, i have not checked all block device, this patch was made
> based on the man page. However, i do not mind drop this patch (and maybe
> other patches) to make this pachset smaller. BLKGETSIZE64 can be added
> in the future if we meet such device.

By looking at the Linux source code implementing BLKGETSIZE64, it looks
like it should work for all block devices where SEEK_END works:

* BLKGETSIZE64 returns i_size_read(bdev->bd_inode)
  (block/ioctl.c:blkdev_ioctl())
* llseek(SEEK_END) uses i_size_read(bd_inode) as the offset
  (fs/block_dev.c:block_llseek())

That's probably why raw_getlength() never needed a Linux-specific
BLKGETSIZE call.

Patch
diff mbox

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;