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Re: [PATCH] ELF program header

From: Andrei E. Warkentin
Subject: Re: [PATCH] ELF program header
Date: Fri, 12 Oct 2007 15:24:06 -0500

12.10.2007, в 11:19, Robert Millan писал(а):

It seems that grub-mkimage generates awkward ELF files, in which the Program header table is at the end of the file instead of right after the ELF header.

I know very little about ELF, but:

- This figure in ELF standard seems to indicate which is the "normal" (not
  sure if mandatory) location:

  - Our own ELF loader doesn't like phdroff > 0x2000 either, from

/* FIXME: Should we support program headers at strange locations? */ if (ehdr->e_phoff + ehdr->e_phnum * ehdr->e_phentsize > MULTIBOOT_SEARCH) return grub_error (GRUB_ERR_BAD_OS, "program header at a too high offset");

This breaks self-boot in the LinuxBIOS target. Moving the Program header (see attached patch) fixed it, with no apparent drawbacks or regressions in any of the ELF loaders around (tested on LinuxBIOS ELF loader and Efika OF).

I'm not completely sure of its correctness though, and would appreciate if someone with a better understanding of ELF can comment on it. In particular, I don't know if my proposed solution could overwrite valid data. Are the
segments garanteed to always leave room for the program header, do we
have to explicitly check for that, or perhaps we need to relocate the segments
when needed?

I don't think the ELF TIS says anything about location of PHDRs. That's the whole point of EHDR - so that the location, size of each entry, and count can be variable. The only invalid case would be for phoff or phoff + phentcount * phentsize to exceed end of file offset.

Robert Millan

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