Re: [PATCH] tools/libxg: Don't gunzip the guests initrd

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.09.2024 14:31, Andrew Cooper wrote:
> When booting guests, compressed first executables (the kernel itself for
> direct-boot, or firmware binaries) need decompressing in order to inspect the
> ELF notes or other relevant headers.
> 
> However for unclear reasons, libxenguest will also gunzip() all modules which
> happen to be compressed (typically a direct-boot kernel's initrd), before
> loading the contents into guest memory.

I suppose the simple reason was that grub also decompresse{s,d} everything.

> This was an unexpected discovery while trying to test Xen's gunzip()
> logic (Xen as a PVH guest, with a gzipped XTF kernel as dom0).
> 
> More generally, it is work that we should not be doing in the domain builder;
> it can and should be done in guest context.
> 
> This is unlikely to cause any issues; a kernel incapable of decompression
> wouldn't have a compressed initrd to start with,

Unless relying on the bootloader, which an OS may certainly do.

> and these days there are
> other algorithms typically preferred over gzip.
> 
> Furthermore, this change also fixes a bug with Linux guests.  Linux is happy
> consuming a initrd constructed of multiple CPIO fragments, including with
> differing compression, and Xen gunzip()'ing from the start would result in all
> subsequent fragments being silently dropped during load.

Now this and perhaps also ...

> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> 
> Furthermore, Linux's habit of prepending an uncompressed CPIO containing
> microcode will also stop it being recognised as a compressed blob.

... this are fair reasons on their own to get rid of that logic. So
let's hope there's not going to be any fallout.

Jan