[PATCH v2 4/8] qemu: Validate firmware blob configuration

Michal Privoznik mprivozn at redhat.com
Tue Jun 9 14:44:46 UTC 2020

On 6/9/20 11:52 AM, Daniel P. Berrangé wrote:
> On Thu, Jun 04, 2020 at 08:44:05PM +0200, Michal Privoznik wrote:
>> There are recommendations and limitations to the name of the
>> config blobs we need to follow [1].
>> Firstly, we don't want users to change any value only add new
>> blobs. This means, that the name must have "opt/" prefix and at
>> the same time must not begin with "opt/ovmf" nor "opt/org.qemu"
>> as these are reserved for OVMF or QEMU respectively.
>> Secondly, there is a limit (FW_CFG_MAX_FILE_PATH in qemu.git) of
>> 56 characters for filename.
> Ewww, that is horrible. I'm have inclined to say we should leave the
> limit unchecked in libvirt, and file a BZ against QEMU. It should be
> using g_strdup_printf() with filenames and not allocating on the
> stack. We already see peoiple exceeding the 100 charater limit of UNIX
> sockets, so a 56 character limit is going to be trivially exceeded
> without even trying hard.

Ah, I got it wrong when reading the documentation. The limit is the name 
of the entry, not file name associated. For instance the following works:


but if @name would be too long QEMU would fail:

qemu-system-x86_64: -fw_cfg 
name=opt/com.example/some_very_long_...,file=...: name too long (max. 55 

And this comes from kernel's implementation of qemu_fw_cfg. However, the 
name can be up to PAGE_SIZE long (minus 2 for terminating \n and NUL), 
according to sysfs documentation (Documentation/filesystems/sysfs.rst:241):

- The buffer will always be PAGE_SIZE bytes in length. On i386, this
   is 4096.

Given this new realization, I think I'll just remove the check and not 
fill bug. I don't think we need longer names, do we?


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