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    Subject[PATCH 8/8] documentation: Update on livepatch elf format
    Date
    Add a section to Documentation/livepatch/module-elf-format.txt
    describing how klp-convert works for fixing relocations.

    Signed-off-by: Joao Moreira <jmoreira@suse.de>
    ---
    Documentation/livepatch/module-elf-format.txt | 47 ++++++++++++++++++++++++---
    1 file changed, 42 insertions(+), 5 deletions(-)

    diff --git a/Documentation/livepatch/module-elf-format.txt b/Documentation/livepatch/module-elf-format.txt
    index f21a5289a09c..6b0259dfab49 100644
    --- a/Documentation/livepatch/module-elf-format.txt
    +++ b/Documentation/livepatch/module-elf-format.txt
    @@ -2,7 +2,8 @@
    Livepatch module Elf format
    ===========================

    -This document outlines the Elf format requirements that livepatch modules must follow.
    +This document outlines the Elf format requirements that livepatch modules must
    +follow.

    -----------------
    Table of Contents
    @@ -25,8 +26,9 @@ Table of Contents
    3.3.2 Required name format
    3.3.3 Example livepatch symbol names
    3.3.4 Example `readelf --symbols` output
    -4. Architecture-specific sections
    -5. Symbol table and Elf section access
    +4. Automatic conversion of unresolved relocations
    +5. Architecture-specific sections
    +6. Symbol table and Elf section access

    ----------------------------
    0. Background and motivation
    @@ -293,8 +295,43 @@ Symbol table '.symtab' contains 127 entries:
    [*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
    "OS" means OS-specific.

    +--------------------------------------------------
    +4. Automatic conversion of unresolved relocations
    +--------------------------------------------------
    +Sometimes livepatches may operate on symbols which are not self-contained nor
    +exported. When this happens, these symbols remain unresolved in the elf object
    +and will trigger an error during the livepatch instantiation.
    +
    +Whenever possible, the kernel building infrastructure solves this problem
    +automatically. First, a symbol database containing information on all compiled
    +objects is built. Second, this database - a file named Symbols.list, placed in
    +the kernel source root directory - is used to identify targets for unresolved
    +relocations, converting them in the livepatch elf accordingly to the
    +specifications above-described. While the first stage is fully handled by the
    +building system, the second is done by a tool called klp-convert, which can be
    +found in "scripts/livepatch".
    +
    +When an unresolved relocation has as target a symbol whose name is also used by
    +different symbols throughout the kernel, the relocation cannot be resolved
    +automatically. In these cases, the livepatch developer must add annotations to
    +the livepatch, making it possible for the system to identify which is the
    +correct target amongst multiple homonymous symbols. Such annotations must be
    +done through a data structure as follows:
    +
    +struct KLP_MODULE_RELOC(object) data_structure_name[] = {
    + KLP_SYMPOS(symbol, pos)
    +};
    +
    +In the above example, object refers to the object file which contains the
    +symbol, being vmlinux or a module; symbol refers to the symbol name that will
    +be relocated and pos is its position in the object.
    +
    +When a data structure like this is added to the livepatch, the resulting elf
    +will hold symbols that will be identified by klp-convert and used to solve name
    +ambiguities.
    +
    ---------------------------------
    -4. Architecture-specific sections
    +5. Architecture-specific sections
    ---------------------------------
    Architectures may override arch_klp_init_object_loaded() to perform
    additional arch-specific tasks when a target module loads, such as applying
    @@ -305,7 +342,7 @@ be easily identified when iterating through a patch module's Elf sections
    (See arch/x86/kernel/livepatch.c for a complete example).

    --------------------------------------
    -5. Symbol table and Elf section access
    +6. Symbol table and Elf section access
    --------------------------------------
    A livepatch module's symbol table is accessible through module->symtab.

    --
    2.12.0
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