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a1170bb00a
BOOT.CSV should be placed in fedora directory in order to locate the base directory of files recorded in $FILENAME column. Signed-off-by: Lans Zhang <jia.zhang@windriver.com>
61 lines
2.3 KiB
Plaintext
61 lines
2.3 KiB
Plaintext
Somebody mailed me this:
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> I am trying to understand how fallback.efi works. I have been reading
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> shim.c and see that fallback is called when
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> should_use_fallback(EFI_HANDLE image_handle) returns a 1. But in
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> should_use_fallback(EFI_HANDLE image_handle), there is a comparison of
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> bootpath to \\EFI\\BOOT\\BOOT. Why is it compare to \\EFI\\BOOT\\BOOT
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> since bootpath always return \EFI\Boot\shim.efi?
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And it seems like a common enough question that we need some
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documentation of how to properly use shim and fallback. Here's the
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basics of it:
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It doesn't always return \EFI\boot\shim.efi - in fact, not ever if
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installed correctly. The FS layouts are like this:
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for removable media:
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\EFI\BOOT\BOOTX64.EFI <-- shim
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\EFI\BOOT\MokManager.efi
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\EFI\BOOT\grubx64.efi
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\EFI\BOOT\grub.cfg
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for an installed system:
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\EFI\BOOT\BOOTX64.EFI <-- shim
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\EFI\BOOT\MokManager.efi
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\EFI\BOOT\fallback.efi
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\EFI\fedora\BOOT.CSV
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\EFI\fedora\shim.efi
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\EFI\fedora\MokManager.efi
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\EFI\fedora\grubx64.efi
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\EFI\fedora\grub.cfg
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When you boot removable media, it'll be in \EFI\BOOT , but fallback.efi
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won't be there, so it goes ahead and boots the normal bootloader
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(grubx64.efi). When you boot a normal system through a boot variable,
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the boot variable is configured to start \EFI\fedora\shim.efi (or
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whatever your distro's EFI directory is.) In that case it won't try to
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invoke fallback. But if the boot variables are missing or corrupted,
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the firmware will eventually try to boot the hard disk as removable
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media. In that case, it'll invoke \EFI\BOOT\BOOTX64.EFI (or whatever
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filename is right for your architecture.) In that case it'll be in
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\EFI\BOOT, so it'll check for fallback.efi , and it'll find it and run
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it. When it runs, fallback will look for every directory in \EFI\ with
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a BOOT${ARCH}.CSV in it, or BOOT.CSV if that's not found. It'll parse that,
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and create new boot variables from what it finds. Then it'll try to boot one
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of them.
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BOOT.CSV is a UCS-2 LE formatted CSV file. So it has the LE byte order
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marker, and after that it's just a series of lines, each having
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comma-separated date. It looks like this on Fedora:
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shim.efi,Fedora,,This is the boot entry for Fedora
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so basically it's:
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$FILENAME,$LABEL,$LOADER_DATA,$COMMENT0[,$COMMENT1[,...]]
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Where $FILENAME has to be the name of a file in the same directory as
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BOOT.CSV .
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