Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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