Re: [edk2-devel] [PATCH] OvmfPkg/VirtioFsDxe: tolerate opening an abs. pathname rel. to a reg. file

["Gerd Hoffmann" <kraxel@redhat.com>]

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+tianocore@kernel.org>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>



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