x86/entry: Emit a symbol for register restoring thunk

Arnd found a randconfig that produces the warning:

  arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at
  offset 0x3e

when building with LLVM_IAS=1 (Clang's integrated assembler). Josh
notes:

  With the LLVM assembler not generating section symbols, objtool has no
  way to reference this code when it generates ORC unwinder entries,
  because this code is outside of any ELF function.

  The limitation now being imposed by objtool is that all code must be
  contained in an ELF symbol.  And .L symbols don't create such symbols.

  So basically, you can use an .L symbol *inside* a function or a code
  segment, you just can't use the .L symbol to contain the code using a
  SYM_*_START/END annotation pair.

Fangrui notes that this optimization is helpful for reducing image size
when compiling with -ffunction-sections and -fdata-sections. I have
observed on the order of tens of thousands of symbols for the kernel
images built with those flags.

A patch has been authored against GNU binutils to match this behavior
of not generating unused section symbols ([1]), so this will
also become a problem for users of GNU binutils once they upgrade to 2.36.

Omit the .L prefix on a label so that the assembler will emit an entry
into the symbol table for the label, with STB_LOCAL binding. This
enables objtool to generate proper unwind info here with LLVM_IAS=1 or
GNU binutils 2.36+.

 [ bp: Massage commit message. ]

Reported-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20210112194625.4181814-1-ndesaulniers@google.com
Link: https://github.com/ClangBuiltLinux/linux/issues/1209
Link: https://reviews.llvm.org/D93783
Link: https://sourceware.org/binutils/docs/as/Symbol-Names.html
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1408485ce69f844dcd7ded093a8

diff --git a/Documentation/asm-annotations.rst b/Documentation/asm-annotations.rst
index 32ea574..76424e0 100644 (file)
--- a/Documentation/asm-annotations.rst
+++ b/Documentation/asm-annotations.rst
@@ -100,6 +100,11 @@ Instruction Macros
 ~~~~~~~~~~~~~~~~~~
 This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.
 
+``objtool`` requires that all code must be contained in an ELF symbol. Symbol
+names that have a ``.L`` prefix do not emit symbol table entries. ``.L``
+prefixed symbols can be used within a code region, but should be avoided for
+denoting a range of code via ``SYM_*_START/END`` annotations.
+
 * ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the
   most frequent markings**. They are used for functions with standard calling
   conventions -- global and local. Like in C, they both align the functions to

diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index ccd3287..c9a9fbf 100644 (file)
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -31,7 +31,7 @@ SYM_FUNC_START_NOALIGN(\name)
        .endif
 
        call \func
-       jmp  .L_restore
+       jmp  __thunk_restore
 SYM_FUNC_END(\name)
        _ASM_NOKPROBE(\name)
        .endm
@@ -44,7 +44,7 @@ SYM_FUNC_END(\name)
 #endif
 
 #ifdef CONFIG_PREEMPTION
-SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
+SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore)
        popq %r11
        popq %r10
        popq %r9
@@ -56,6 +56,6 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
        popq %rdi
        popq %rbp
        ret
-       _ASM_NOKPROBE(.L_restore)
-SYM_CODE_END(.L_restore)
+       _ASM_NOKPROBE(__thunk_restore)
+SYM_CODE_END(__thunk_restore)
 #endif

diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index 5bcfbd9..dbf8506 100644 (file)
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -178,6 +178,11 @@
  * Objtool generates debug info for both FUNC & CODE, but needs special
  * annotations for each CODE's start (to describe the actual stack frame).
  *
+ * Objtool requires that all code must be contained in an ELF symbol. Symbol
+ * names that have a  .L prefix do not emit symbol table entries. .L
+ * prefixed symbols can be used within a code region, but should be avoided for
+ * denoting a range of code via ``SYM_*_START/END`` annotations.
+ *
  * ALIAS -- does not generate debug info -- the aliased function will
  */