On Tue, Jun 09, 2020 at 04:44:46PM +0200, Michal Privoznik wrote:
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:
-fw_cfg name=opt/com.example/blah,file=/tmp/some_very_long_path_that_is_more_than_fifty_six_characters_long_to_see_what_happens_if_I_do_that
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 char)
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?
Yep, fine with me. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|