[PATCH] D43228: [ELF] Allow redefining LMA for a section inside a segment.

[Rafael Avila de Espindola via llvm-commits]

Igor Kudrin <ikudrin at accesssoftek.com> writes:

> The use case of a ROM image is a bit different. You don't have an OS, you don't have a loader, you even don't have an ELF file with ELF headers. All you have is an image, and a computer just starts execution from a predefined address. Everything else has to be done by the code in that image itself, including, in particular, initialization of R/W data by copying the corresponding block from ROM to RAM.

Another way of saying that is that the image is its own loader. That is
also true for ld.so too for example.

> When you prepare a ROM image, an ELF file is only an intermediate step. Other tools extract the final image, and it is much more convenient for them if there is only one PT_LOAD segment in the ELF file, because it is closer to the final layout.
>
> Please note, that if a linker script does not explicitly define desired segments for output sections, the generated output will still have separate PT_LOADs for each customized LMA. Nothing is changed here. The patch just allows a user to accurately specify their needs. As you may see, all the existing tests stay intact, and I merely add a new use case, which was not covered.

So what are the needs? If there are two areas in the file that the image
will process differently (like the example in the binutils manual), then
it is very reasonable to have them in different PT_LOADs.

Is there any reason why the linker script cannot be changed to have
multiple PT_LOADs?

>
> By the way, where does the idea of the uniform difference between
> VMAs, LMAs, and offsets within one segment come from? It might seem
> logical and natural, but I cannot find it as a requirement in the ELF
> specs.

>From the very idea of what a PT_LOAD is.

Cheers,
Rafael