[v2] file-posix: Clear out first sector in hdev_create

[Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

Posted by Fam Zheng 8 years, 6 months ago

People get surprised when, after "qemu-img create -f raw /dev/sdX", they
still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
header. While this is natural because raw doesn't need to write any
magic bytes during creation, hdev_create is free to clear out the first
sector to make sure the stale qcow2 header doesn't cause such confusion.

Signed-off-by: Fam Zheng <famz@redhat.com>

---

v2: Use stack allocated buffer. [Eric]
    Fix return value.
    (Keep qemu_write_full instead of switching to qemu_pwritev because
    the former handles short writes.)
    Fix typo "qemu-img". [Changlong]
---
 block/file-posix.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/file-posix.c b/block/file-posix.c
index f4de022ae0..a63bbf2b90 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
         ret = -ENOSPC;
     }
 
+    if (total_size) {
+        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
+        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
+        if (lseek(fd, 0, SEEK_SET) == -1) {
+            ret = -errno;
+        } else {
+            ret = qemu_write_full(fd, buf, zero_size);
+            ret = ret == zero_size ? 0 : -errno;
+        }
+    }
     qemu_close(fd);
     return ret;
 }
-- 
2.13.4

Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

Posted by Eric Blake 8 years, 6 months ago

On 08/11/2017 03:09 AM, Fam Zheng wrote:
> People get surprised when, after "qemu-img create -f raw /dev/sdX", they
> still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
> header. While this is natural because raw doesn't need to write any
> magic bytes during creation, hdev_create is free to clear out the first
> sector to make sure the stale qcow2 header doesn't cause such confusion.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> ---
> 
> v2: Use stack allocated buffer. [Eric]
>     Fix return value.
>     (Keep qemu_write_full instead of switching to qemu_pwritev because
>     the former handles short writes.)
>     Fix typo "qemu-img". [Changlong]
> ---
>  block/file-posix.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)

> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index f4de022ae0..a63bbf2b90 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
>          ret = -ENOSPC;
>      }
>  
> +    if (total_size) {
> +        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> +        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> +        if (lseek(fd, 0, SEEK_SET) == -1) {
> +            ret = -errno;
> +        } else {
> +            ret = qemu_write_full(fd, buf, zero_size);
> +            ret = ret == zero_size ? 0 : -errno;
> +        }
> +    }

Question: are we ever constrained by O_DIRECT where writing only 512
bytes would be too small for a block device that mandates 4k alignment?
If so, then we need MAX(minimum write size, MIN(BDRV_SECTOR_SIZE,
total_size)) - it would also mean we can't stack-allocate any more, but
that we have to do an aligned buffer allocation (where g_malloc is not
necessarily suitably aligned).

If O_DIRECT is not a problem, then this is okay:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

Posted by Fam Zheng 8 years, 6 months ago

On Fri, 08/11 09:42, Eric Blake wrote:
> On 08/11/2017 03:09 AM, Fam Zheng wrote:
> > People get surprised when, after "qemu-img create -f raw /dev/sdX", they
> > still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
> > header. While this is natural because raw doesn't need to write any
> > magic bytes during creation, hdev_create is free to clear out the first
> > sector to make sure the stale qcow2 header doesn't cause such confusion.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > 
> > ---
> > 
> > v2: Use stack allocated buffer. [Eric]
> >     Fix return value.
> >     (Keep qemu_write_full instead of switching to qemu_pwritev because
> >     the former handles short writes.)
> >     Fix typo "qemu-img". [Changlong]
> > ---
> >  block/file-posix.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> 
> > 
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index f4de022ae0..a63bbf2b90 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
> >          ret = -ENOSPC;
> >      }
> >  
> > +    if (total_size) {
> > +        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> > +        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> > +        if (lseek(fd, 0, SEEK_SET) == -1) {
> > +            ret = -errno;
> > +        } else {
> > +            ret = qemu_write_full(fd, buf, zero_size);
> > +            ret = ret == zero_size ? 0 : -errno;
> > +        }
> > +    }
> 
> Question: are we ever constrained by O_DIRECT where writing only 512
> bytes would be too small for a block device that mandates 4k alignment?
> If so, then we need MAX(minimum write size, MIN(BDRV_SECTOR_SIZE,
> total_size)) - it would also mean we can't stack-allocate any more, but
> that we have to do an aligned buffer allocation (where g_malloc is not
> necessarily suitably aligned).
> 
> If O_DIRECT is not a problem, then this is okay:

A few lines above:


    fd = qemu_open(filename, O_WRONLY | O_BINARY);

so there is no O_DIRECT issue.

Fam

Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

Posted by Fam Zheng 8 years, 5 months ago

On Fri, 08/11 16:09, Fam Zheng wrote:
> People get surprised when, after "qemu-img create -f raw /dev/sdX", they
> still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
> header. While this is natural because raw doesn't need to write any
> magic bytes during creation, hdev_create is free to clear out the first
> sector to make sure the stale qcow2 header doesn't cause such confusion.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>

Gentle ping as a reminder for 2.11 as we have now released 2.10.

Fam

Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

Posted by Kevin Wolf 8 years, 5 months ago

Am 11.08.2017 um 10:09 hat Fam Zheng geschrieben:
> People get surprised when, after "qemu-img create -f raw /dev/sdX", they
> still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
> header. While this is natural because raw doesn't need to write any
> magic bytes during creation, hdev_create is free to clear out the first
> sector to make sure the stale qcow2 header doesn't cause such confusion.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> 
> ---
> 
> v2: Use stack allocated buffer. [Eric]
>     Fix return value.
>     (Keep qemu_write_full instead of switching to qemu_pwritev because
>     the former handles short writes.)
>     Fix typo "qemu-img". [Changlong]
> ---
>  block/file-posix.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index f4de022ae0..a63bbf2b90 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
>          ret = -ENOSPC;
>      }

So the error paths above only set ret, but don't actually return or jump
to the end of the function.

> +    if (total_size) {
> +        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> +        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> +        if (lseek(fd, 0, SEEK_SET) == -1) {
> +            ret = -errno;
> +        } else {
> +            ret = qemu_write_full(fd, buf, zero_size);
> +            ret = ret == zero_size ? 0 : -errno;

Which means that an error above (like a too small block device or using
a regular file) can be overwritten with a success value if clearing the
first sector works. That's probably not quite right.

> +        }
> +    }
>      qemu_close(fd);
>      return ret;
>  }

Kevin

Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

Posted by Fam Zheng 8 years, 5 months ago

On Thu, 09/07 17:07, Kevin Wolf wrote:
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index f4de022ae0..a63bbf2b90 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
> >          ret = -ENOSPC;
> >      }
> 
> So the error paths above only set ret, but don't actually return or jump
> to the end of the function.
> 
> > +    if (total_size) {
> > +        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> > +        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> > +        if (lseek(fd, 0, SEEK_SET) == -1) {
> > +            ret = -errno;
> > +        } else {
> > +            ret = qemu_write_full(fd, buf, zero_size);
> > +            ret = ret == zero_size ? 0 : -errno;
> 
> Which means that an error above (like a too small block device or using
> a regular file) can be overwritten with a success value if clearing the
> first sector works. That's probably not quite right.

You're right, will fix.

Fam