On Wed, Oct 18, 2023 at 07:24:34PM +0200, Laszlo Ersek wrote: > Referring to a file relative to a regular file makes no sense (or at least > it cannot be implemented consistently with how a file is referred to > relative to a directory). VirtioFsSimpleFileOpen() has enforced this > strictly since the beginning, and a few months ago I reported USWG Mantis > ticket #2367 [1] too, for clearing up the related confusion in the UEFI > spec. > > Unfortunately, the shim boot loader contains such a bug [2] [3]. I don't > believe the shim bug is ever going to be fixed. We can however relax the > check in VirtioFsSimpleFileOpen() a bit: if the pathname that's being > opened relative to a regular file is absolute, then the base file is going > to be ignored anyway, so we can let the caller's bug slide. This happens > to make shim work. > > Why this matters: UEFI-bootable Linux installer ISOs tend to come with > shim and grub in the embedded (ElTorito) FAT image (ESP). Sometimes you > want to build upstream shim/grub binaries, but boot the same ISO > otherwise. The fastest way for overriding the ESP for this purpose is to > copy its original contents to a virtio filesystem, then overwrite the shim > and grub binaries from the host side. Note that this is different from > direct-booting a kernel (via fw_cfg); the point is to check whether the > just-built shim and grub are able to boot the rest of the ISO. > > [1] https://mantis.uefi.org/mantis/view.php?id=2367 > [2] https://bugzilla.redhat.com/show_bug.cgi?id=1966973 > [3] https://github.com/rhboot/shim/issues/382 > > Cc: Ard Biesheuvel <ardb+tianoc...@kernel.org> > Cc: Gerd Hoffmann <kra...@redhat.com> > Cc: Jiewen Yao <jiewen....@intel.com> > Cc: Jordan Justen <jordan.l.jus...@intel.com> > Signed-off-by: Laszlo Ersek <ler...@redhat.com>
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