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create-diff-object: support x86 NOP-padded functions
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Kernel v6.2+ commit bea75b33895f ("x86/Kconfig: Introduce function
padding") added 16 bytes of NOP padding in front of each function.

For objects built with --function-sections, this means that function
symbols no longer sit at the beginning of their respective ELF sections,
but 16 bytes offset.

In the same release, kernel v6.2+ commit 9f2899fe36a6 ("objtool: Add
option to generate prefix symbols") adds ELF function symbols with
prefix "__pfx_" to indicate the start of a function, inclusive of
NOP-padding.

For example:

  $ objdump -Dr -j.text.cmdline_proc_show fs/proc/cmdline.o
  ...
  Disassembly of section .text.cmdline_proc_show:

  0000000000000000 <__pfx_cmdline_proc_show>:
     0:   90                      nop
     1:   90                      nop
     2:   90                      nop
     3:   90                      nop
     4:   90                      nop
     5:   90                      nop
     6:   90                      nop
     7:   90                      nop
     8:   90                      nop
     9:   90                      nop
     a:   90                      nop
     b:   90                      nop
     c:   90                      nop
     d:   90                      nop
     e:   90                      nop
     f:   90                      nop

  0000000000000010 <cmdline_proc_show>:
    10:   e8 00 00 00 00          callq  15 <cmdline_proc_show+0x5>
                          11: R_X86_64_PLT32      __fentry__-0x4
    15:   55                      push   %rbp
    16:   48 8b 35 00 00 00 00    mov    0x0(%rip),%rsi        # 1d <cmdline_proc_show+0xd>
                          19: R_X86_64_PC32       saved_command_line-0x4
    1d:   48 89 fd                mov    %rdi,%rbp
    20:   e8 00 00 00 00          callq  25 <cmdline_proc_show+0x15>
                          21: R_X86_64_PLT32      seq_puts-0x4
    25:   48 89 ef                mov    %rbp,%rdi
    28:   be 0a 00 00 00          mov    $0xa,%esi
    2d:   e8 00 00 00 00          callq  32 <cmdline_proc_show+0x22>
                          2e: R_X86_64_PLT32      seq_putc-0x4
    32:   31 c0                   xor    %eax,%eax
    34:   5d                      pop    %rbp
    35:   e9 00 00 00 00          jmpq   3a <cmdline_proc_show+0x2a>
                          36: R_X86_64_PLT32      __x86_return_thunk-0x4

Kpatch-build needs to gracefully handle NOP-padding when it is present.
At the same time, it should include "__pfx_<function>" symbols when
their respective functions change, but not treat prefix such functions
as first-class functions.

This also adds support for CONFIG_CFI_CLANG, which also creates prefixed
symbols with the name "__cfi_<function>".

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
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@joe-lawrence @jpoimboe
joe-lawrence authored and jpoimboe committed Mar 17, 2023
1 parent 0d48a43 commit 3e54c63
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Showing 3 changed files with 111 additions and 6 deletions.
23 changes: 17 additions & 6 deletions kpatch-build/create-diff-object.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -236,14 +236,21 @@ static struct rela *toc_rela(const struct rela *rela)
static void kpatch_bundle_symbols(struct kpatch_elf *kelf)
{
struct symbol *sym;
unsigned int expected_offset;

list_for_each_entry(sym, &kelf->symbols, list) {
if (is_bundleable(sym)) {
if (sym->sym.st_value != 0 &&
!is_gcc6_localentry_bundled_sym(kelf, sym)) {
ERROR("symbol %s at offset %lu within section %s, expected 0",
if (sym->pfx)
expected_offset = 16;
else if (is_gcc6_localentry_bundled_sym(kelf, sym))
expected_offset = 8;
else
expected_offset = 0;

if (sym->sym.st_value != expected_offset) {
ERROR("symbol %s at offset %lu within section %s, expected %u",
sym->name, sym->sym.st_value,
sym->sec->name);
sym->sec->name, expected_offset);
}

sym->sec->sym = sym;
Expand Down Expand Up @@ -1913,6 +1920,8 @@ static int kpatch_include_changed_functions(struct kpatch_elf *kelf)
sym->type == STT_FUNC) {
changed_nr++;
kpatch_include_symbol(sym);
if (sym->pfx)
kpatch_include_symbol(sym->pfx);
}

if (sym->type == STT_FILE)
Expand All @@ -1927,7 +1936,8 @@ static void kpatch_print_changes(struct kpatch_elf *kelf)
struct symbol *sym;

list_for_each_entry(sym, &kelf->symbols, list) {
if (!sym->include || !sym->sec || sym->type != STT_FUNC || sym->parent)
if (!sym->include || !sym->sec || sym->type != STT_FUNC ||
sym->parent || sym->is_pfx)
continue;
if (sym->status == NEW)
log_normal("new function: %s\n", sym->name);
Expand Down Expand Up @@ -3922,7 +3932,8 @@ static void kpatch_find_func_profiling_calls(struct kpatch_elf *kelf)
struct rela *rela;
unsigned char *insn;
list_for_each_entry(sym, &kelf->symbols, list) {
if (sym->type != STT_FUNC || !sym->sec || !sym->sec->rela)
if (sym->type != STT_FUNC || sym->is_pfx ||
!sym->sec || !sym->sec->rela)
continue;

switch(kelf->arch) {
Expand Down
92 changes: 92 additions & 0 deletions kpatch-build/kpatch-elf.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -397,6 +397,97 @@ static void kpatch_create_section_list(struct kpatch_elf *kelf)
ERROR("expected NULL");
}

/*
* Some x86 kernels have NOP function padding [1] for which objtool [2]
* adds ELF function symbols with prefix "__pfx_" to indicate the start
* of a function, inclusive of NOP-padding. Find the prefix symbols and
* link them to their corresponding function symbols at an expected
* offset.
*
* A few examples:
*
* Value Size Type Bind Vis Ndx Name
* (fork.o, simple case)
* 0000000000000000 0 SECTION LOCAL DEFAULT 31 .text.get_task_mm
* 0000000000000000 16 FUNC GLOBAL DEFAULT 31 __pfx_get_task_mm
* 0000000000000010 91 FUNC GLOBAL DEFAULT 31 get_task_mm
*
* (fork.o, multiple function aliases)
* 0000000000000000 0 SECTION LOCAL DEFAULT 190 .text.__do_sys_fork
* 0000000000000000 16 FUNC GLOBAL DEFAULT 190 __pfx___x64_sys_fork
* 0000000000000010 49 FUNC LOCAL DEFAULT 190 __do_sys_fork
* 0000000000000010 49 FUNC GLOBAL DEFAULT 190 __ia32_sys_fork
* 0000000000000010 49 FUNC GLOBAL DEFAULT 190 __x64_sys_fork
*
* (fork.o multiple functions in one section)
* 0000000000000000 0 SECTION LOCAL DEFAULT 59 .init.text
* 0000000000000000 16 FUNC LOCAL DEFAULT 59 __pfx_coredump_filter_setup
* 0000000000000010 40 FUNC LOCAL DEFAULT 59 coredump_filter_setup
* 0000000000000038 16 FUNC WEAK DEFAULT 59 __pfx_arch_task_cache_init
* 0000000000000048 10 FUNC WEAK DEFAULT 59 arch_task_cache_init
* 0000000000000052 16 FUNC GLOBAL DEFAULT 59 __pfx_fork_init
* 0000000000000062 357 FUNC GLOBAL DEFAULT 59 fork_init
* 00000000000001c7 16 FUNC GLOBAL DEFAULT 59 __pfx_fork_idle
* 00000000000001d7 214 FUNC GLOBAL DEFAULT 59 fork_idle
* 00000000000002ad 16 FUNC GLOBAL DEFAULT 59 __pfx_mm_cache_init
* 00000000000002bd 72 FUNC GLOBAL DEFAULT 59 mm_cache_init
* 0000000000000305 16 FUNC GLOBAL DEFAULT 59 __pfx_proc_caches_init
* 0000000000000315 192 FUNC GLOBAL DEFAULT 59 proc_caches_init
*
* (fork.o, function without nop padding / __pfx_ symbol)
* 0000000000000000 0 SECTION LOCAL DEFAULT 99 .text.unlikely.__mmdrop
* 0000000000000000 48 FUNC LOCAL DEFAULT 99 __mmdrop.cold
*
* (kpatch-build generated tmp.ko, multple functions in one section, no __pfx_ symbols)
* 0000000000000000 0 SECTION LOCAL DEFAULT 10 .text.unlikely.callback_info.isra.0
* 0000000000000010 65 FUNC LOCAL DEFAULT 10 callback_info.isra.0
* 0000000000000061 54 FUNC LOCAL DEFAULT 10 callback_info.isra.0
* 00000000000000a7 54 FUNC LOCAL DEFAULT 10 callback_info.isra.0
*
* CONFIG_CFI_CLANG uses something very similar, except the symbol is created
* by the compiler and its prefix is "__cfi_".
*
* [1] bea75b33895f ("x86/Kconfig: Introduce function padding")
* [2] 9f2899fe36a6 ("objtool: Add option to generate prefix symbols")
*/
static void kpatch_link_prefixed_functions(struct kpatch_elf *kelf)
{
struct symbol *func, *pfx;
bool found;

if (kelf->arch != X86_64)
return;

list_for_each_entry(pfx, &kelf->symbols, list) {
if (!pfx->name || pfx->type != STT_FUNC)
continue;

if (strncmp(pfx->name, "__pfx_", 6) &&
strncmp(pfx->name, "__cfi_", 6))
continue;

found = false;

list_for_each_entry(func, &kelf->symbols, list) {
if (func->type == STT_FUNC && func->sec == pfx->sec &&
func->sym.st_value == pfx->sym.st_value + 16) {

/*
* If a func has aliases, it's possible for
* multiple functions to have the same 'pfx'.
*/

pfx->is_pfx = true;
func->pfx = pfx;
found = true;
}
}

if (!found)
ERROR("missing func for %s", pfx->name);
}
}

static void kpatch_create_symbol_list(struct kpatch_elf *kelf)
{
struct section *symtab;
Expand Down Expand Up @@ -459,6 +550,7 @@ static void kpatch_create_symbol_list(struct kpatch_elf *kelf)
log_debug("\n");
}

kpatch_link_prefixed_functions(kelf);
}

struct kpatch_elf *kpatch_elf_open(const char *name)
Expand Down
2 changes: 2 additions & 0 deletions kpatch-build/kpatch-elf.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,7 @@ struct symbol {
struct list_head list;
struct symbol *twin;
struct symbol *parent;
struct symbol *pfx;
struct list_head children;
struct list_head subfunction_node;
struct section *sec;
Expand All @@ -91,6 +92,7 @@ struct symbol {
enum symbol_strip strip; /* used in the output elf */
};
int has_func_profiling;
bool is_pfx;
};

struct rela {
Expand Down

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