[PATCH] iotests/108: probe loop device setup

[PATCH] iotests/108: probe loop device setup

[Ilya Leoshkevich]

Running iotests/108 in a Docker container, that has passwordless sudo
configured, fails with:

    losetup: /home/user/src/qemu/build/scratch/qcow2-file-108/t.qcow2: failed to set up loop device: No such file or directory

because it incorrectly assumes losetup is allowed to do loop device
setup, when in reality it can only list devices.

Fix by probing loop device setup, using the test script as a backing
file. With this, the test is correctly skipped.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 tests/qemu-iotests/108 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/qemu-iotests/108 b/tests/qemu-iotests/108
index 1de718188aa..adbaaf87095 100755
--- a/tests/qemu-iotests/108
+++ b/tests/qemu-iotests/108
@@ -53,7 +53,8 @@ _supported_os Linux
 _unsupported_imgopts 'refcount_bits=\([^1]\|.\([^6]\|$\)\)' data_file
 
 # This test either needs sudo -n losetup or FUSE exports to work
-if sudo -n losetup &>/dev/null; then
+if probe_dev=$(sudo -n losetup -r --show -f "$0" 2>/dev/null); then
+    sudo -n losetup -d "$probe_dev"
     loopdev=true
 else
     loopdev=false
-- 
2.56.0

Re: [PATCH] iotests/108: probe loop device setup

[Eric Blake]

On Tue, Oct 13, 2026 at 08:28:00PM +0200, Ilya Leoshkevich wrote:
> Running iotests/108 in a Docker container, that has passwordless sudo
> configured, fails with:
> 
>     losetup: /home/user/src/qemu/build/scratch/qcow2-file-108/t.qcow2: failed to set up loop device: No such file or directory
> 
> because it incorrectly assumes losetup is allowed to do loop device
> setup, when in reality it can only list devices.
> 
> Fix by probing loop device setup, using the test script as a backing
> file. With this, the test is correctly skipped.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  tests/qemu-iotests/108 | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tests/qemu-iotests/108 b/tests/qemu-iotests/108
> index 1de718188aa..adbaaf87095 100755
> --- a/tests/qemu-iotests/108
> +++ b/tests/qemu-iotests/108
> @@ -53,7 +53,8 @@ _supported_os Linux
>  _unsupported_imgopts 'refcount_bits=\([^1]\|.\([^6]\|$\)\)' data_file
>  
>  # This test either needs sudo -n losetup or FUSE exports to work
> -if sudo -n losetup &>/dev/null; then
> +if probe_dev=$(sudo -n losetup -r --show -f "$0" 2>/dev/null); then
> +    sudo -n losetup -d "$probe_dev"

Is 'sudo -n losetup -f' a simpler test?  It prints out the next
/dev/loopN value available only if a mount would succeed, which means
that it is a stronger test than the existing 'sudo -n losetup', but
also does not require a subsequent cleanup.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org

Re: [PATCH] iotests/108: probe loop device setup

[Ilya Leoshkevich]

On 10/13/26 20:53, Eric Blake wrote:
> On Tue, Oct 13, 2026 at 08:28:00PM +0200, Ilya Leoshkevich wrote:
>> Running iotests/108 in a Docker container, that has passwordless sudo
>> configured, fails with:
>>
>>      losetup: /home/user/src/qemu/build/scratch/qcow2-file-108/t.qcow2: failed to set up loop device: No such file or directory
>>
>> because it incorrectly assumes losetup is allowed to do loop device
>> setup, when in reality it can only list devices.
>>
>> Fix by probing loop device setup, using the test script as a backing
>> file. With this, the test is correctly skipped.
>>
>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>> ---
>>   tests/qemu-iotests/108 | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/tests/qemu-iotests/108 b/tests/qemu-iotests/108
>> index 1de718188aa..adbaaf87095 100755
>> --- a/tests/qemu-iotests/108
>> +++ b/tests/qemu-iotests/108
>> @@ -53,7 +53,8 @@ _supported_os Linux
>>   _unsupported_imgopts 'refcount_bits=\([^1]\|.\([^6]\|$\)\)' data_file
>>   
>>   # This test either needs sudo -n losetup or FUSE exports to work
>> -if sudo -n losetup &>/dev/null; then
>> +if probe_dev=$(sudo -n losetup -r --show -f "$0" 2>/dev/null); then
>> +    sudo -n losetup -d "$probe_dev"
> 
> Is 'sudo -n losetup -f' a simpler test?  It prints out the next
> /dev/loopN value available only if a mount would succeed, which means
> that it is a stronger test than the existing 'sudo -n losetup', but
> also does not require a subsequent cleanup.

I don't think 'losetup -f' attempts a mount, since it works fine without
privileges:

$ id -u
1000
$ losetup -f
/dev/loop0
$ losetup -r -f /bin/true
losetup: /bin/true: failed to set up loop device: Permission denied

A quick glance at the source code [1] seems to confirm this: the
iterator is immediately discarded after finding a free device name.
The loop-control logic above looks similar.

[1] 
https://github.com/util-linux/util-linux/blob/v2.42.4/lib/loopdev.c#L1760

Re: [PATCH] iotests/108: probe loop device setup

[Eric Blake]

On Tue, Oct 13, 2026 at 09:08:17PM +0200, Ilya Leoshkevich wrote:
> 
> 
> On 10/13/26 20:53, Eric Blake wrote:
> > On Tue, Oct 13, 2026 at 08:28:00PM +0200, Ilya Leoshkevich wrote:
> > > Running iotests/108 in a Docker container, that has passwordless sudo
> > > configured, fails with:
> > > 
> > >      losetup: /home/user/src/qemu/build/scratch/qcow2-file-108/t.qcow2: failed to set up loop device: No such file or directory
> > > 
> > > because it incorrectly assumes losetup is allowed to do loop device
> > > setup, when in reality it can only list devices.
> > > 
> > > Fix by probing loop device setup, using the test script as a backing
> > > file. With this, the test is correctly skipped.
> > > 
> > > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > > ---
> > >   tests/qemu-iotests/108 | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/tests/qemu-iotests/108 b/tests/qemu-iotests/108
> > > index 1de718188aa..adbaaf87095 100755
> > > --- a/tests/qemu-iotests/108
> > > +++ b/tests/qemu-iotests/108
> > > @@ -53,7 +53,8 @@ _supported_os Linux
> > >   _unsupported_imgopts 'refcount_bits=\([^1]\|.\([^6]\|$\)\)' data_file
> > >   # This test either needs sudo -n losetup or FUSE exports to work
> > > -if sudo -n losetup &>/dev/null; then
> > > +if probe_dev=$(sudo -n losetup -r --show -f "$0" 2>/dev/null); then
> > > +    sudo -n losetup -d "$probe_dev"
> > 
> > Is 'sudo -n losetup -f' a simpler test?  It prints out the next
> > /dev/loopN value available only if a mount would succeed, which means
> > that it is a stronger test than the existing 'sudo -n losetup', but
> > also does not require a subsequent cleanup.
> 
> I don't think 'losetup -f' attempts a mount, since it works fine without
> privileges:
> 
> $ id -u
> 1000
> $ losetup -f
> /dev/loop0
> $ losetup -r -f /bin/true
> losetup: /bin/true: failed to set up loop device: Permission denied

Thanks for testing. In that case, I think your longer version is reasonable.

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

> 
> A quick glance at the source code [1] seems to confirm this: the
> iterator is immediately discarded after finding a free device name.
> The loop-control logic above looks similar.
> 
> [1]
> https://github.com/util-linux/util-linux/blob/v2.42.4/lib/loopdev.c#L1760
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org