310 lines
14 KiB
ReStructuredText
310 lines
14 KiB
ReStructuredText
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===========================
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Livepatch module Elf format
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===========================
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This document outlines the Elf format requirements that livepatch modules must follow.
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.. Table of Contents
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.. contents:: :local:
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1. Background and motivation
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============================
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Formerly, livepatch required separate architecture-specific code to write
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relocations. However, arch-specific code to write relocations already
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exists in the module loader, so this former approach produced redundant
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code. So, instead of duplicating code and re-implementing what the module
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loader can already do, livepatch leverages existing code in the module
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loader to perform the all the arch-specific relocation work. Specifically,
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livepatch reuses the apply_relocate_add() function in the module loader to
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write relocations. The patch module Elf format described in this document
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enables livepatch to be able to do this. The hope is that this will make
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livepatch more easily portable to other architectures and reduce the amount
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of arch-specific code required to port livepatch to a particular
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architecture.
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Since apply_relocate_add() requires access to a module's section header
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table, symbol table, and relocation section indices, Elf information is
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preserved for livepatch modules (see section 5). Livepatch manages its own
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relocation sections and symbols, which are described in this document. The
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Elf constants used to mark livepatch symbols and relocation sections were
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selected from OS-specific ranges according to the definitions from glibc.
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Why does livepatch need to write its own relocations?
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-----------------------------------------------------
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A typical livepatch module contains patched versions of functions that can
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reference non-exported global symbols and non-included local symbols.
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Relocations referencing these types of symbols cannot be left in as-is
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since the kernel module loader cannot resolve them and will therefore
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reject the livepatch module. Furthermore, we cannot apply relocations that
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affect modules not yet loaded at patch module load time (e.g. a patch to a
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driver that is not loaded). Formerly, livepatch solved this problem by
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embedding special "dynrela" (dynamic rela) sections in the resulting patch
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module Elf output. Using these dynrela sections, livepatch could resolve
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symbols while taking into account its scope and what module the symbol
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belongs to, and then manually apply the dynamic relocations. However this
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approach required livepatch to supply arch-specific code in order to write
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these relocations. In the new format, livepatch manages its own SHT_RELA
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relocation sections in place of dynrela sections, and the symbols that the
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relas reference are special livepatch symbols (see section 2 and 3). The
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arch-specific livepatch relocation code is replaced by a call to
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apply_relocate_add().
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2. Livepatch modinfo field
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==========================
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Livepatch modules are required to have the "livepatch" modinfo attribute.
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See the sample livepatch module in samples/livepatch/ for how this is done.
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Livepatch modules can be identified by users by using the 'modinfo' command
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and looking for the presence of the "livepatch" field. This field is also
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used by the kernel module loader to identify livepatch modules.
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Example:
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--------
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**Modinfo output:**
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::
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% modinfo livepatch-meminfo.ko
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filename: livepatch-meminfo.ko
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livepatch: Y
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license: GPL
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depends:
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vermagic: 4.3.0+ SMP mod_unload
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3. Livepatch relocation sections
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================================
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A livepatch module manages its own Elf relocation sections to apply
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relocations to modules as well as to the kernel (vmlinux) at the
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appropriate time. For example, if a patch module patches a driver that is
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not currently loaded, livepatch will apply the corresponding livepatch
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relocation section(s) to the driver once it loads.
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Each "object" (e.g. vmlinux, or a module) within a patch module may have
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multiple livepatch relocation sections associated with it (e.g. patches to
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multiple functions within the same object). There is a 1-1 correspondence
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between a livepatch relocation section and the target section (usually the
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text section of a function) to which the relocation(s) apply. It is
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also possible for a livepatch module to have no livepatch relocation
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sections, as in the case of the sample livepatch module (see
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samples/livepatch).
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Since Elf information is preserved for livepatch modules (see Section 5), a
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livepatch relocation section can be applied simply by passing in the
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appropriate section index to apply_relocate_add(), which then uses it to
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access the relocation section and apply the relocations.
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Every symbol referenced by a rela in a livepatch relocation section is a
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livepatch symbol. These must be resolved before livepatch can call
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apply_relocate_add(). See Section 3 for more information.
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3.1 Livepatch relocation section format
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=======================================
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Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
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section flag. See include/uapi/linux/elf.h for the definition. The module
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loader recognizes this flag and will avoid applying those relocation sections
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at patch module load time. These sections must also be marked with SHF_ALLOC,
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so that the module loader doesn't discard them on module load (i.e. they will
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be copied into memory along with the other SHF_ALLOC sections).
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The name of a livepatch relocation section must conform to the following
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format::
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.klp.rela.objname.section_name
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^ ^^ ^ ^ ^
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|________||_____| |__________|
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[A] [B] [C]
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[A]
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The relocation section name is prefixed with the string ".klp.rela."
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[B]
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The name of the object (i.e. "vmlinux" or name of module) to
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which the relocation section belongs follows immediately after the prefix.
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[C]
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The actual name of the section to which this relocation section applies.
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Examples:
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---------
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**Livepatch relocation section names:**
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::
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.klp.rela.ext4.text.ext4_attr_store
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.klp.rela.vmlinux.text.cmdline_proc_show
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**`readelf --sections` output for a patch
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module that patches vmlinux and modules 9p, btrfs, ext4:**
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::
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Section Headers:
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[Nr] Name Type Address Off Size ES Flg Lk Inf Al
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[ snip ]
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[29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
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[30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
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[ snip ]
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[34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
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[35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
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[36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
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[37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
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[ snip ] ^ ^
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[*] [*]
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[*]
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Livepatch relocation sections are SHT_RELA sections but with a few special
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characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
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not be discarded when the module is loaded into memory, as well as with the
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SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
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**`readelf --relocs` output for a patch module:**
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::
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Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
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Offset Info Type Symbol's Value Symbol's Name + Addend
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000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
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0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
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0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
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000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
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[ snip ] ^
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[*]
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[*]
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Every symbol referenced by a relocation is a livepatch symbol.
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4. Livepatch symbols
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====================
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Livepatch symbols are symbols referred to by livepatch relocation sections.
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These are symbols accessed from new versions of functions for patched
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objects, whose addresses cannot be resolved by the module loader (because
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they are local or unexported global syms). Since the module loader only
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resolves exported syms, and not every symbol referenced by the new patched
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functions is exported, livepatch symbols were introduced. They are used
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also in cases where we cannot immediately know the address of a symbol when
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a patch module loads. For example, this is the case when livepatch patches
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a module that is not loaded yet. In this case, the relevant livepatch
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symbols are resolved simply when the target module loads. In any case, for
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any livepatch relocation section, all livepatch symbols referenced by that
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section must be resolved before livepatch can call apply_relocate_add() for
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that reloc section.
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Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
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loader can identify and ignore them. Livepatch modules keep these symbols
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in their symbol tables, and the symbol table is made accessible through
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module->symtab.
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4.1 A livepatch module's symbol table
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=====================================
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Normally, a stripped down copy of a module's symbol table (containing only
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"core" symbols) is made available through module->symtab (See layout_symtab()
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in kernel/module/kallsyms.c). For livepatch modules, the symbol table copied
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into memory on module load must be exactly the same as the symbol table produced
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when the patch module was compiled. This is because the relocations in each
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livepatch relocation section refer to their respective symbols with their symbol
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indices, and the original symbol indices (and thus the symtab ordering) must be
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preserved in order for apply_relocate_add() to find the right symbol.
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For example, take this particular rela from a livepatch module:::
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Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
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Offset Info Type Symbol's Value Symbol's Name + Addend
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000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
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This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
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in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
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symbol index 94.
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And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
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[ snip ]
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94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
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[ snip ]
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4.2 Livepatch symbol format
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===========================
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Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
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that the module loader can identify them and not attempt to resolve them.
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See include/uapi/linux/elf.h for the actual definitions.
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Livepatch symbol names must conform to the following format::
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.klp.sym.objname.symbol_name,sympos
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^ ^^ ^ ^ ^ ^
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|_______||_____| |_________| |
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[A] [B] [C] [D]
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[A]
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The symbol name is prefixed with the string ".klp.sym."
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[B]
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The name of the object (i.e. "vmlinux" or name of module) to
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which the symbol belongs follows immediately after the prefix.
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[C]
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The actual name of the symbol.
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[D]
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The position of the symbol in the object (as according to kallsyms)
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This is used to differentiate duplicate symbols within the same
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object. The symbol position is expressed numerically (0, 1, 2...).
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The symbol position of a unique symbol is 0.
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Examples:
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---------
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**Livepatch symbol names:**
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::
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.klp.sym.vmlinux.snprintf,0
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.klp.sym.vmlinux.printk,0
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.klp.sym.btrfs.btrfs_ktype,0
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**`readelf --symbols` output for a patch module:**
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::
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Symbol table '.symtab' contains 127 entries:
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Num: Value Size Type Bind Vis Ndx Name
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[ snip ]
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73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
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74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
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75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
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76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
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[ snip ] ^
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[*]
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[*]
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Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
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"OS" means OS-specific.
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5. Symbol table and Elf section access
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======================================
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A livepatch module's symbol table is accessible through module->symtab.
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Since apply_relocate_add() requires access to a module's section headers,
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symbol table, and relocation section indices, Elf information is preserved for
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livepatch modules and is made accessible by the module loader through
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module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
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this struct is filled in by the module loader. Its fields are documented below::
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struct klp_modinfo {
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Elf_Ehdr hdr; /* Elf header */
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Elf_Shdr *sechdrs; /* Section header table */
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char *secstrings; /* String table for the section headers */
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unsigned int symndx; /* The symbol table section index */
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};
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