Documentation: livepatch: add section about arch-specific code

Document usage of arch-specific elf sections in livepatch as well
as implementation of arch-specific code.

[[email protected]: fix wording as suggested by Petr Mladek]
Signed-off-by: Jessica Yu <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Signed-off-by: Jiri Kosina <[email protected]>

Author: Jessica Yu

Documentation/livepatch/module-elf-format.txt

@@ -25,7 +25,8 @@ Table of Contents
        3.3.2 Required name format
        3.3.3 Example livepatch symbol names
        3.3.4 Example `readelf --symbols` output
-4. Symbol table and Elf section access
+4. Architecture-specific sections
+5. Symbol table and Elf section access
 
 ----------------------------
 0. Background and motivation
@@ -46,7 +47,7 @@ architecture.
 
 Since apply_relocate_add() requires access to a module's section header
 table, symbol table, and relocation section indices, Elf information is
-preserved for livepatch modules (see section 4). Livepatch manages its own
+preserved for livepatch modules (see section 5). Livepatch manages its own
 relocation sections and symbols, which are described in this document. The
 Elf constants used to mark livepatch symbols and relocation sections were
 selected from OS-specific ranges according to the definitions from glibc.
@@ -117,7 +118,7 @@ also possible for a livepatch module to have no livepatch relocation
 sections, as in the case of the sample livepatch module (see
 samples/livepatch).
 
-Since Elf information is preserved for livepatch modules (see Section 4), a
+Since Elf information is preserved for livepatch modules (see Section 5), a
 livepatch relocation section can be applied simply by passing in the
 appropriate section index to apply_relocate_add(), which then uses it to
 access the relocation section and apply the relocations.
@@ -292,8 +293,19 @@ Symbol table '.symtab' contains 127 entries:
 [*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
     "OS" means OS-specific.
 
+---------------------------------
+4. 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
+arch-specific sections. On x86 for example, we must apply per-object
+.altinstructions and .parainstructions sections when a target module loads.
+These sections must be prefixed with ".klp.arch.$objname." so that they can
+be easily identified when iterating through a patch module's Elf sections
+(See arch/x86/kernel/livepatch.c for a complete example).
+
 --------------------------------------
-4. Symbol table and Elf section access
+5. Symbol table and Elf section access
 --------------------------------------
 A livepatch module's symbol table is accessible through module->symtab.