kpatch-build: account for __pfx_-less NOP padding

Some kernel configurations generate function NOP padding, but without
associated __pfx_<function> symbols.  For example:

  $ git describe HEAD
  v6.4-rc7-72-gdad9774deaf1

  # Initial default config turns on Indirect Branch Tracking and 16-NOP padding bytes
  $ make defconfig
  $ grep -e CONFIG_X86_KERNEL_IBT -e CONFIG_FUNCTION_PADDING_BYTES .config
  CONFIG_X86_KERNEL_IBT=y
  CONFIG_FUNCTION_PADDING_BYTES=16

  # Test .o build generates 16 bytes of NOPs but no "__pfx_" symbol
  $ make -j$(nproc) fs/proc/cmdline.o
  $ objdump -Dr -j .text fs/proc/cmdline.o  | grep -e '<.*>:' -e 'Disassembly'
  Disassembly of section .text:
  0000000000000000 <cmdline_proc_show-0x10>:
  0000000000000010 <cmdline_proc_show>:

This is because objtool operation on the object code may be delayed
under certain configurations.  As such, create-diff-object should be
prepared to encounter NOP padded functions in object files before any
__pfx_ symbols are created.

Fixes: 3e54c63 ("create-diff-object: support x86 NOP-padded functions")
Closes: #1347 ("x86 NOP padded functions without __pfx_ symbol")
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>

kpatch-build/create-diff-object.c

@@ -228,6 +228,54 @@ static struct rela *toc_rela(const struct rela *rela)
 				   (unsigned int)rela->addend);
 }
 
+/*
+ * Some x86 kernels have NOP function padding, see upstream commit
+ * bea75b33895f ("x86/Kconfig: Introduce function padding").
+ * Check for any amount of NOPs between the beginning of a function's
+ * section and its offset value within that section.
+ */
+static bool insn_is_nop_pad(struct kpatch_elf *kelf, void *addr, void *end,
+			    unsigned long *insn_len)
+{
+	unsigned char *insn = addr;
+
+	switch(kelf->arch) {
+
+	case X86_64:
+		if (insn[0] == 0x90) {
+			*insn_len = 1;
+			return true;
+		}
+		break;
+
+	case PPC64:
+	case S390:
+		/* kernel feature not present on these arches  */
+		return false;
+
+	default:
+		ERROR("unsupported arch");
+	}
+
+	return false;
+}
+
+bool is_function_nop_padded(struct kpatch_elf *kelf, struct symbol *sym)
+{
+	unsigned long offset, insn_len;
+	void *end = sym->sec->data->d_buf + sym->sym.st_value;
+
+	if (sym->type != STT_FUNC)
+		return false;
+
+	for (offset = 0; offset < sym->sym.st_value; offset += insn_len) {
+		if (!insn_is_nop_pad(kelf, sym->sec->data->d_buf + offset, end, &insn_len))
+			return false;
+	}
+
+	return true;
+}
+
 /*
  * When compiling with -ffunction-sections and -fdata-sections, almost every
  * symbol gets its own dedicated section.  We call such symbols "bundled"
@@ -236,19 +284,21 @@ 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;
+	Elf64_Addr expected_offset;
 
 	list_for_each_entry(sym, &kelf->symbols, list) {
 		if (is_bundleable(sym)) {
 			if (sym->pfx)
 				expected_offset = 16;
 			else if (is_gcc6_localentry_bundled_sym(kelf, sym))
 				expected_offset = 8;
+			else if (is_function_nop_padded(kelf, sym))
+				expected_offset = sym->sym.st_value;
 			else
 				expected_offset = 0;
 
 			if (sym->sym.st_value != expected_offset) {
-				ERROR("symbol %s at offset %lu within section %s, expected %u",
+				ERROR("symbol %s at offset %lu within section %s, expected %lu",
 				      sym->name, sym->sym.st_value,
 				      sym->sec->name, expected_offset);
 			}

kpatch-build/kpatch-elf.c

@@ -399,7 +399,7 @@ static void kpatch_create_section_list(struct kpatch_elf *kelf)
 
 /*
  * Some x86 kernels have NOP function padding [1] for which objtool [2]
- * adds ELF function symbols with prefix "__pfx_" to indicate the start
+ * may add 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.
@@ -438,7 +438,7 @@ static void kpatch_create_section_list(struct kpatch_elf *kelf)
  * 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)
+ * (kpatch-build generated tmp.ko, multiple 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