Re: GNU-gABI: Alignment of .note.ABI-tag and .note.gnu.build-id sections

["H.J. Lu" <hjl.tools@gmail.com>]

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On Fri, Nov 17, 2017 at 5:59 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> According to gABI, each note entry should be aligned to 4 bytes in 32-bit
> objects or 8 bytes in 64-bit objects.  But Linux has been using 4 byte
> alignment for .note.ABI-tag note and .note.gnu.build-id note in 64-bit
> objects.  We can't change their alignment to 8 bytes.  Since all notes i
> n one PT_LOAD segment must have the same alignment which equals to p_align.
> Note parser can use p_align of PT_LOAD segment for note alignment, instead
> of assuming alignment based on ELF file class.
>
> Here is a patch to Linux Extensions to gABI.  I also regenerated PDF
> file at
>
> https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI
>
> H.J.
> ---
> Document difference in alignment of .note.ABI-tag and .note.gnu.build-id
> sections from gABI.
> ---
>  object-files.tex | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/object-files.tex b/object-files.tex
> index 64ce243..5b7b414 100644
> --- a/object-files.tex
> +++ b/object-files.tex
> @@ -518,6 +518,18 @@ identify OS and version targeted.  It can be merged with other
>     signifies a 2.2.5 kernel.
>  \end{description}
>
> +\subsection{Alignment of Note Sections}
> +
> +All entries in a \texttt{PT_LOAD} segment have the same alignment which
                                     ^^^^^^^^^^^^^ Typo.  It should PT_NOTE.
> +equals to the {\tt p_align} field in program header.
> +
> +According to gABI, each note entry should be aligned to 4 bytes in
> +32-bit objects or 8 bytes in 64-bit objects.  But \code{.note.ABI-tag}
> +section (see Section~\ref{sec_abi_tag}) and \code{.note.gnu.build-id}
> +section (see Section~\ref{sec_build_id}) are aligned to 4 bytes in
> +both 32-bit and 64-bit objects.  Note parser should use {\tt p_align}
> +for note alignment, instead of assuming alignment based on ELF file class.
> +
>  \section{Symbol Table}
>
>  \begin{table}[H]

Here is the updated patch.


-- 
H.J.

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From 9a7bc47bf065405c2e7b6c46d692a7d137d5f544 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Fri, 17 Nov 2017 05:41:09 -0800
Subject: [PATCH] Add a section for alignment of note sections

Document difference in alignment of .note.ABI-tag and .note.gnu.build-id
sections from gABI.
---
 object-files.tex | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/object-files.tex b/object-files.tex
index 64ce243..c33965d 100644
--- a/object-files.tex
+++ b/object-files.tex
@@ -518,6 +518,18 @@ identify OS and version targeted.  It can be merged with other
    signifies a 2.2.5 kernel.
 \end{description}
 
+\subsection{Alignment of Note Sections}
+
+All entries in a \texttt{PT_NOTE} segment have the same alignment which
+equals to the {\tt p_align} field in program header.
+
+According to gABI, each note entry should be aligned to 4 bytes in
+32-bit objects or 8 bytes in 64-bit objects.  But \code{.note.ABI-tag}
+section (see Section~\ref{sec_abi_tag}) and \code{.note.gnu.build-id}
+section (see Section~\ref{sec_build_id}) are aligned to 4 bytes in
+both 32-bit and 64-bit objects.  Note parser should use {\tt p_align}
+for note alignment, instead of assuming alignment based on ELF file class.
+
 \section{Symbol Table}
 
 \begin{table}[H]
-- 
2.14.3