[Bug gas/27753] -mx86-used-note= defaulting to yes regresses

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27753

--- Comment #6 from Fangrui Song <i at maskray dot me> ---
A dynamic loader needs PT_GNU_PROPERTY to know the ISA usage of a component.
.note.gnu.property makes up PT_GNU_PROPERTY. Therefore, the section needs to
SHF_ALLOC to be part of the program image.

% cat a.c
int main() {}
% gcc -o a a.c -nostdlib -Wl,-N -static -Wl,-Ttext=0x10000 -fuse-ld=bfd
/usr/bin/ld.bfd: warning: cannot find entry symbol _start; defaulting to
0000000000010000

If you use objcopy -O binary, you will notice that the output as 4mib. Why?
Because the linker generates .note.gnu.build-id . You need -Wl,--build-id=none
to remove it.

  [Nr] Name              Type            Address          Off    Size   ES Flg
Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00     
0   0  0
  [ 1] .text             PROGBITS        0000000000010000 000190 00000b 00 WAX 
0   0  1
  [ 2] .note.gnu.property NOTE            0000000000400190 0001d8 000030 00   A
 0   0  8
  [ 3] .note.gnu.build-id NOTE            00000000004001c0 000208 000024 00   A
 0   0  4
  [ 4] .eh_frame         PROGBITS        0000000000010010 0001a0 000038 00   A 
0   0  8
  [ 5] .comment          PROGBITS        0000000000000000 00022c 00002e 01  MS 
0   0  1
  [ 6] .symtab           SYMTAB          0000000000000000 000260 000120 18     
7   7  8
  [ 7] .strtab           STRTAB          0000000000000000 000380 000022 00     
0   0  1
  [ 8] .shstrtab         STRTAB          0000000000000000 0003a2 00005a 00     
0   0  1

You may use -Wa,-mx86-used-note=no to remove .note.gnu.property

GNU ld -z separate-code has a separate issue, the segment layout isn't
particular good: https://sourceware.org/bugzilla/show_bug.cgi?id=24490 
two R around RX are wasteful. Ideally there is only one R and it precedes RX.

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