Home Original page

ELF section flag SHF_EXCLUDE

Alan Modra amodra@gmail.com
Thu Mar 12 02:05:39 GMT 2020 
On Wed, Mar 11, 2020 at 05:41:34PM -0700, Fangrui Song wrote:
> 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?

I'd say so.  It was likely superceded by SHF_STRINGS.

> 
> 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.

Yes, I'm not about to remove SHF_EXCLUDE for those targets where there
is a clash.

Ideally the flag should be moved to the generic SHF space, for example
#define SHF_EXCLUDE (1 << 12)
keeping support for the existing flag for some time.

-- 
Alan Modra
Australia Development Lab, IBM
More information about the Binutils mailing list