Re: [edk2-devel] efi and ext4 and case sensitive file names

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

On Wed, Sep 27, 2023 at 1:09 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
>
>   Hi,
>
> I've noticed that the edk2 ext4 driver does case-insensitive filename
> matching.  I know the fat filesystem is case-insensitive, and the uefi
> spec describing the fat filesystem also explicitly says it is
> case-insensitive.  On a quick scan I can't find anything in the uefi
> spec requiring *all* filesystem drivers being case-insensitive though.
>
> So I'm wondering whenever the ext4 driver behavior is correct.  It
> certainly is different than the linux kernel's behavior which is (by
> default) case-sensitive.

Ok, so this is a funny problem. When I was originally writing it, I
noticed a good few consumers would expect case insensitiveness, so I
ended up needing case insensitive string comparison.

As an example, in my current system:
Boot0001* ARCHLINUX
HD(1,GPT,7e1b2e20-3013-4683-b8ab-bef1f9bfb1c8,0x800,0x32000)/File(\EFI\ARCHLINUX\GRUBX64.EFI)
whereas the GRUB tooling installed itself as:
/boot/efi/EFI/ARCHLINUX/grubx64.efi

I treated some good bits of the filesystem driver as "should remain
similar to FAT" due to compatibility reasons. I really really wouldn't
be surprised if lots of EFI code out there relied on case
insensitiveness or other FAT-ish stuff, instead of UNIX semantics.
(funnily enough, EFI also has the restriction where every filename
must be valid unicode, which is not the case on most UNIX systems out
there, that take the file name as "bag of bytes - '/'")

>
> Also note that the linux kernel ext4 driver recently got support for
> case-insensitive file names, which must be explicitly enabled for both
> filesystems (EXT4_FEATURE_INCOMPAT_CASEFOLD) and directories
> (EXT4_CASEFOLD_FL).

Ugh, this is annoying, they didn't even bother documenting it...
(https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#super-incompat)
This leaves me in the awkward spot where figuring the behavior out
would require me to read the fs/ext4 code and thus be legally dubious,
yay :^)

>
> On practical terms I've ran into actual problems due to Fedora mounting
> the ESP at /boot/efi[1] and UKIs (unified kernel images) should be
> placed in EFI/Linux on either ESP or XBOOTLDR partition, which on fedora
> translates to /boot/efi/EFI/Linux (ESP) or /boot/EFI/Linux (XBOOTLDR).
> So I have both /boot/efi and /boot/EFI ...

Oh boy, that seems fun. So Ext4Dxe can only open one of the two dirs, right?
Now that I think of it, there should be fun behavior when doing an EFI
readdir, where you could find two dirents with "different" names but
then opening both will lead to the same dirent being open twice...
yuck

-- 
Pedro


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