ELF section flag SHF_EXCLUDE

[Fangrui Song]

On 2020-03-10, Alan Modra wrote:
>commit 18ae9cc1db made SHF_EXCLUDE generic.  Prior to that the flag
>was only defined for sparc, ppc, or32, and i370.  The problem is that
>the flag was left in the very limited SHF_MASKPROC processor specific
>range rather than being put in the generic range, and it clashes with
>other processor specific section flags.  aarch64, arm, hppa, mcore,
>microblaze, mips, mmix, nfp, score and v850 all define flags with the
>same value.  This means SHF_EXCLUDE as is really shouldn't be used on
>any of those machines.

Recent reference of the unfortunate fact:
https://sourceware.org/pipermail/binutils/2020-February/109567.html [PATCH] Support 'exclude' in objcopy --set-section-flags

>aarch64.h:#define SHF_COMDEF		0x80000000
>arm.h:#define SHF_COMDEF         	0x80000000
>hppa.h:#define SHF_PARISC_SBP		0x80000000
>mcore.h:#define SHF_MCORE_NOREAD	0x80000000
>microblaze.h:#define SHF_MICROBLAZE_NOREAD	0x80000000
>mips.h:#define SHF_MIPS_STRING		0x80000000
>mmix.h:#define SHF_MMIX_CANRELAX	0x80000000
>nfp.h:#define SHF_NFP_INIT		0x80000000
>score.h:#define SHF_SCORE_STRING	0x80000000
>v850.h:#define SHF_RENESAS_ABS		0x80000000
>
>SHF_PARISC_SBP, SHF_MCORE_NOREAD, SHF_MICROBLAZE_NOREAD,
>SHF_MIPS_STRING, SHF_MMIX_CANRELAX, SHF_SCORE_STRING, and
>SHF_RENESAS_ABS only appear in include/elf/*.h above.  I haven't
>searched any of the relevant ABI docs, if such exist.
>
>SHF_COMDEF is decoded by readelf (once you disable SHF_EXCLUDE for arm
>and aarch64), but is described in arm docs by: "the legacy SHF_COMDEF
>ELF section flag is deprecated".
>
>SHF_NFP_INIT is used in opcodes/nfp-dis.c but not set by
>bfd/elf64-nfp.c.
>
>So what to do?  Disabling the flag and assembler support for 'e' in
>flags is one possibility, but not a good idea for a 10 year old
>feature.

Arm Compiler 6.11 deprecates the following features:

Support for ELF sections that contain the legacy SHF_COMDEF ELF section flag has been deprecated

binutils does not use SHF_COMDEF.
LLVM does not have SHF_COMDEF at all.


We can probably recycle SHF_COMDEF for arm/aarch64.

Is SHF_MIPS_STRING in the same camp?


clang -gsplit-dwarf=single emits .debug_*.dwo in the object file and sets the SHF_EXCLUDE flag (https://reviews.llvm.org/D52303).
SHF_EXCLUDE is more useful than the probably obsoleted processor specific flags.