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

["Pedro Falcato" <pedro.falcato@gmail.com>]

On Wed, Oct 18, 2023 at 6:24 PM Laszlo Ersek <lersek@redhat.com> 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>
> ---
>
> Notes:
>     context:-U4
>
>  OvmfPkg/VirtioFsDxe/SimpleFsOpen.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/OvmfPkg/VirtioFsDxe/SimpleFsOpen.c b/OvmfPkg/VirtioFsDxe/SimpleFsOpen.c
> index a13d4f6a1e2d..2ecf3d6c2325 100644
> --- a/OvmfPkg/VirtioFsDxe/SimpleFsOpen.c
> +++ b/OvmfPkg/VirtioFsDxe/SimpleFsOpen.c
> @@ -394,22 +394,33 @@ VirtioFsSimpleFileOpen (
>
>    //
>    // 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).
> +  // relative to a directory). See USWG Mantis ticket #2367.
>    //
>    if (!VirtioFsFile->IsDirectory) {
> +    BOOLEAN  BugCompat;
> +
> +    //
> +    // Tolerate this bug in the caller if FileName is absolute. If FileName is
> +    // absolute, then VirtioFsAppendPath() below will disregard
> +    // VirtioFsFile->CanonicalPathname.
> +    //
> +    BugCompat = (FileName[0] == L'\\');
> +
>      DEBUG ((
> -      DEBUG_ERROR,
> +      BugCompat ? DEBUG_WARN : DEBUG_ERROR,
>        ("%a: Label=\"%s\" CanonicalPathname=\"%a\" FileName=\"%s\": "
>         "nonsensical request to open a file or directory relative to a regular "
>         "file\n"),
>        __func__,
>        VirtioFs->Label,
>        VirtioFsFile->CanonicalPathname,
>        FileName
>        ));
> -    return EFI_INVALID_PARAMETER;
> +    if (!BugCompat) {
> +      return EFI_INVALID_PARAMETER;
> +    }
>    }
>
>    //
>    // Allocate the new VIRTIO_FS_FILE object.
>

Aww, you should've CC'd me.
Anyway, retroactive
Acked-by: Pedro Falcato <pedro.falcato@gmail.com>

If this is the new pseudo-sanctioned behavior for filesystem drivers,
I'll make sure to do the same adjustments for Ext4Dxe.

-- 
Pedro


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