Re: [avr-gcc-list] avr-objcopy not copying custom section...

[Erik Christiansen]

On 25.05.11 18:55, Boyapati, Anitha wrote:
> >On 26.02.11 20:22, Boyapati, Anitha wrote:

> >>
> >> Actually, looking back, I don’t get the meaning of this.
> Erik wrote:
> >Having just now seen this post, I'll offer the following thoughts.

...

> >> How can a section be ‘executable’ without being ‘allocatable’ in ELF?
> >
> >An overlay would fit that case. 
> 
> Ok! This is interesting.
> 
> > Perhaps the question relates instead to
> >the ELF format's ability to distinguish the cases. OK, the p_flags
> >member of the Phdr struct only provides for X, W, and R in its bitmask,
> >according to the elf manpage, so the question seems fair. 
> 
> Yes. That is where the confusion stemmed from.

Hi Anitha,

Last time, I didn't look exhaustively for flags in the ELF format. With
the aid of "readelf -e test.elf", we can see that there are more section
flags in the section header:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00000000 000094 00003a 00  AX  0   0  1
  [ 2] .data             PROGBITS        00800100 0000ce 000000 00  WA  0   0  1
  [ 3] .bss              NOBITS          00800100 0000ce 000000 00  WA  0   0  1
  [ 4] .noinit           PROGBITS        00800100 0000ce 000000 00   W  0   0  1
  [ 5] .eeprom           PROGBITS        00810000 0000ce 000000 00   W  0   0  1
  [ 6] .shstrtab         STRTAB          00000000 0000ce 00003c 00      0   0  1
  [ 7] .symtab           SYMTAB          00000000 000274 0002a0 10      8  28  4
  [ 8] .strtab           STRTAB          00000000 000514 000146 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings)
  I (info), L (link order), G (group), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

I overlooked that before. In the sh_flags member of the Shdr struct, the
ELF format does have separate explicit alloc and execute flags.

...

> 'objdump' program works on BFD structures. The set of BFD section
> flags for a super set to ELF section flags. So, the dump of section
> headers shows BFD flags but not ELF flags.

But it is an ELF file that we are dumping, so any explicit flags we read
must be in the ELF format. I just forgot about section headers, and
didn't look for sh_flags, and that's where the "missing" flags reside,
AIUI.

...

> >The only mystery seems to be whether any of these flags used earlier
> >in the toolchain become implicit in the ELF file, but that's just an
> >implementation detail.
> >

> I agree that there is an independence of attributes to certain extent.
> But this information pushed my curiosity further on how attributes are
> calculated for user-defined sections. I dug a little more. Binutils
> seems to work out the permutations and combinations of them.

> 
> For e.g, as you pointed out .eeprom has CONTENTS but is not 'ALLOC'.
> Obviously, this involves lot of implementation detail that possibly
> considers 'size' of the section to make it ALLOC. Since the size is 0,
> the .eeprom section is not ALLOC. (Making it CONTENTS is a little but
> may be that is default. I am sorry but this is just my wild guess, I
> could be wrong.)

Yes, ALLOC could in practical terms be implict, I agree, but the
SHF_ALLOC bit in sh_flags makes it explicit in ELF, we now see.

...

> I find this useful. 
> 
> http://ftp.gnu.org/pub/old-gnu/Manuals/bfd-2.9.1/html_node/bfd_13.html 

Thanks for the link. I've had a look around, and will read more on return
from a week on the family farm. (From tomorrow. Tools will be chainsaw
and bulldozer, not gnu tools.)

> Thanks for sharing your thoughts on overlays.

You are most welcome. After more than a quarter of a century of embedded
development, it is more interesting to explore odd corners of the
toolchain than it is to do yet more code development. (Though I still
enjoy tweaking linker scripts for odd cases.)

Erik

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