loaded image: add set_image function #266
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This allows overriding the data in `LoadedImage`. It's useful in the following scenario: 1. Secure boot is enabled, so images loaded with `LoadImage` must be signed with an appropriate key known to the firmware. 2. The bootloader has its own key embedded, and uses that key to verify the next stage. This key is not known to the firmware, so the next stage's image can't be loaded with `LoadImage`. 3. Since image handles are created by `LoadImage`, which we can't call, we have to make use of an existing image handle -- the one passed into the bootloader's entry function. By modifying that image handle (after appropriately verifying the signature of the new data of course), we can repurpose the image handle for the next stage. See [shim] for an example of this scenario in action. [shim]: https://github.com/rhboot/shim/blob/4d64389c6c941d21548b06423b8131c872e3c3c7/pe.c#L1143
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This allows overriding the data in
LoadedImage. It's useful in thefollowing scenario:
LoadImagemust besigned with an appropriate key known to the firmware.
verify the next stage. This key is not known to the firmware, so
the next stage's image can't be loaded with
LoadImage.LoadImage, which we can'tcall, we have to make use of an existing image handle -- the one
passed into the bootloader's entry function. By modifying that
image handle (after appropriately verifying the signature of the
new data of course), we can repurpose the image handle for the next
stage.
See shim for an example of this scenario in action.