On 16.04.20 14:46, Samuel Thibault wrote:
Juergen Gross, le jeu. 16 avril 2020 14:27:31 +0200, a ecrit:
Using the standard -m elf_x86_64 for 64-bit mini-os results in the
first section (.text) to start only at offset 2MB in the binary file.
? I'm not seeing this on my system:
0 .text 0001933a 0000000000000000 0000000000000000 00001000 2**12
CONTENTS, ALLOC, LOAD, READONLY, CODE
# readelf -S mini-os
There are 19 section headers, starting at offset 0x245e88:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00200000
0000000000017d07 0000000000000000 AX 0 0 4096
[ 2] .rodata PROGBITS 0000000000017d20 00217d20
so only 4K offset in the file, the file ends up being 135K big after
stripping.
Lucky you. :-)
Might be specific to the linker used.
Using -m elf_i386 avoids that problem without any visible disadvantage.
Using a 32bit emulation for a 64bit binary? This looks very odd to me?
(and probably fragile)
That is only the final linking process, the option must not be used
earlier (I had linking errors in that case).
I'd like to know more where this 2MB binary file offset is coming from,
since AIUI it'd basically impact all binaries built by the toolchain of
your system, not just mini-os, and I don't think the maintainers of your
system want that :)
Andrew Cooper gave me the hint how to solve the problem. He has seen it
as well and told me (via IRC):
"I actually figured that out while hacking up a KVM-friendly version of
XTF for Andy Luto. The linking -m elf_i386/elf_x86_64 option sets the
"target emulation" which is more than just "what this is compiled for".
I haven't yet cleaned up the patch for XTF (which also suffers the same
problem), but linking an ELF64 using -m elf_i386 will DTRT with no
other ill effects.
Sadly, LD's documentation about details like this (and the linker
script for that matter) are poor at best. Specifically, 64bit emulation
appears to include "align primary sections to 2M so your OS can make
better use of superpages even when mmap()ing", with no way I can spot
to override this in the linker file."
Juergen
