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flat assembler > Compiler Internals > ELF section header alignment

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incertia



Joined: 21 Jun 2017
Posts: 2
With the rising popularity of the LLVM toolchain nowadays, I believe it would be beneficial to align data in a way that behaves nicely with with the LLVM linker. Most notably, LLVM will fail when e_shoff is not a multiple of two.
Code:
# fasm test.s test.o
flat assembler  version 1.71.60  (16384 kilobytes memory)
3 passes, 909 bytes.
# readelf -a test.o | grep -i 'start of section headers'
  Start of section headers:          525 (bytes into file)
# nm test.o
bfd plugin: LLVM gold plugin has failed to create LTO module: Invalid data was encountered while parsing the file
nm: test.o: no symbols
# ld.lld -o test test.o
ld.lld: error: invalid alignment of section headers
# vim test.s
# fasm test.s test.o
flat assembler  version 1.71.60  (16384 kilobytes memory)
3 passes, 910 bytes
# readelf -a test.o | grep -i 'start of section headers'
  Start of section headers:          526 (bytes into file)
# nm test.o
0000000000000000 T _start
# ld.lld -o test test.o
# exit    
Post 21 Jun 2017, 18:41
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6979
Location: Kraków, Poland
Even if not for the LLVM toolchain this would be a thing requiring correction, as it is a violation of ELF specification (which directly states that these fields need to be aligned). Thank you for pointing it out!

Would you be able to test this with the ELF formatting macros that come with fasmg? Because there I can fix it very quickly, while fixing it in fasm 1 core may take a bit longer.
Post 21 Jun 2017, 20:32
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6979
Location: Kraków, Poland
Anyway, I gave it a shot. Please try with 1.71.63 and see if the produced files are correct.
Post 21 Jun 2017, 21:37
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incertia



Joined: 21 Jun 2017
Posts: 2
Seems to be working. Thanks!
Post 22 Jun 2017, 17:45
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