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Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold


On Wed, Sep 26, 2018 at 10:39 AM, Cary Coutant <ccoutant@gmail.com> wrote:
>
> Some requirements I'd like to see:

NT_GNU_PROPERTY_TYPE_0 is documented at

https://github.com/hjl-tools/linux-abi

> (1) An NT_GNU_PROPERTY_TYPE_0 note section must be named
> ".note.gnu.property" with type SHT_NOTE.

If it isn't specified this way, please send a patch.

> (2) A ".note.gnu.property" section must contain one and only one note,
> whose type must be NT_GNU_PROPERTY_TYPE_0.

True for linker output and all properties must be sorted.   For relocatable
inputs, there may be more than one NT_GNU_PROPERTY_TYPE_0 notes:

[hjl@gnu-cfl-1 tmp]$ touch n.c
[hjl@gnu-cfl-1 tmp]$ gcc -fcf-protection -c n.c -Wa,-mx86-used-note=yes
[hjl@gnu-cfl-1 tmp]$ readelf -n n.o

Displaying notes found in: .note.gnu.property
  Owner                 Data size Description
  GNU                  0x00000010 NT_GNU_PROPERTY_TYPE_0
      Properties: x86 feature: IBT, SHSTK
  GNU                  0x00000020 NT_GNU_PROPERTY_TYPE_0
      Properties: x86 ISA used: <None>
x86 feature used: x86
[hjl@gnu-cfl-1 tmp]$

> (3) A ".note.gnu.property" section must have sh_align of 8, and its
> length must be a multiple of 8.

If it isn't specified this way, please send a patch.

> (4) A linker is expected to combine ".note.gnu.property" sections in a
> manner described by the ABI documentation, and place the combined
> result, as a single note, in a unique SHT_NOTE section named
> ".note.gnu.property", and in a unique PT_NOTE segment.
>

Why is a unique PT_NOTE segment required? A PT_NOTE segment
may have more than one SHT_NOTE sections with the same section
alignment and each SHT_NOTE section is padded to section alignment.

-- 
H.J.


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