Re: [PATCH v3 32/59] objtool: Allow STT_NOTYPE -> STT_FUNC+0 tail-calls

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 22, 2022 at 12:47:26PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 22, 2022 at 12:29:58PM +0200, Peter Zijlstra wrote:
> > I managed to reproduce with 12.2.0 -- my 12.1.0 compiler didn't
> > reproduce.
> > 
> > $ ./scripts/objdump-func vmlinux.o ftrace_replace_code
> 
> > 0000 0000000000012a83 <ftrace_replace_code.cold>:
> > 0000    12a83:	48 89 de             	mov    %rbx,%rsi
> > 0003    12a86:	89 c7                	mov    %eax,%edi
> > 0005    12a88:	5b                   	pop    %rbx
> > 0006    12a89:	5d                   	pop    %rbp
> > 0007    12a8a:	41 5c                	pop    %r12
> > 0009    12a8c:	41 5d                	pop    %r13
> > 000b    12a8e:	41 5e                	pop    %r14
> > 000d    12a90:	e9 62 fd ff ff       	jmp    127f7 <ftrace_bug>
> > 
> > 
> > Seems to suggest objtool is actually right; I cannot find a reference to
> > that cold symbol.
> 
> Ohhhh, ftrace_replace_cold is a weak function, so it could be the
> original weak symbol had a reference to the cold thing.
> 
> We have some code to deal with crap like that, lemme try and figure out
> what went wrong.

The original weak function:

  $ ./scripts/objdump-func kernel/trace/ftrace.o ftrace_replace_code
  0000 0000000000003bc0 <ftrace_replace_code>:
  0000     3bc0:  f3 0f 1e fa             endbr64
  0004     3bc4:  8b 15 00 00 00 00       mov    0x0(%rip),%edx        # 3bca <ftrace_replace_code+0xa>   3bc6: R_X86_64_PC32     .data..read_mostly+0xc
  000a     3bca:  41 56                   push   %r14
  000c     3bcc:  41 55                   push   %r13
  000e     3bce:  41 89 fd                mov    %edi,%r13d
  0011     3bd1:  41 54                   push   %r12
  0013     3bd3:  41 83 e5 02             and    $0x2,%r13d
  0017     3bd7:  55                      push   %rbp
  0018     3bd8:  53                      push   %rbx
  0019     3bd9:  85 d2                   test   %edx,%edx
  001b     3bdb:  75 67                   jne    3c44 <ftrace_replace_code+0x84>
  001d     3bdd:  4c 8b 35 00 00 00 00    mov    0x0(%rip),%r14        # 3be4 <ftrace_replace_code+0x24>  3be0: R_X86_64_PC32     .bss+0x54
  0024     3be4:  41 89 fc                mov    %edi,%r12d
  0027     3be7:  41 83 e4 01             and    $0x1,%r12d
  002b     3beb:  4d 85 f6                test   %r14,%r14
  002e     3bee:  74 54                   je     3c44 <ftrace_replace_code+0x84>
  0030     3bf0:  41 8b 46 10             mov    0x10(%r14),%eax
  0034     3bf4:  31 ed                   xor    %ebp,%ebp
  0036     3bf6:  85 c0                   test   %eax,%eax
  0038     3bf8:  7f 0b                   jg     3c05 <ftrace_replace_code+0x45>
  003a     3bfa:  eb 40                   jmp    3c3c <ftrace_replace_code+0x7c>
  003c     3bfc:  83 c5 01                add    $0x1,%ebp
  003f     3bff:  41 39 6e 10             cmp    %ebp,0x10(%r14)
  0043     3c03:  7e 37                   jle    3c3c <ftrace_replace_code+0x7c>
  0045     3c05:  48 63 dd                movslq %ebp,%rbx
  0048     3c08:  48 c1 e3 04             shl    $0x4,%rbx
  004c     3c0c:  49 03 5e 08             add    0x8(%r14),%rbx
  0050     3c10:  f6 43 0b 02             testb  $0x2,0xb(%rbx)
  0054     3c14:  75 e6                   jne    3bfc <ftrace_replace_code+0x3c>
  0056     3c16:  44 89 e6                mov    %r12d,%esi
  0059     3c19:  48 89 df                mov    %rbx,%rdi
  005c     3c1c:  e8 3f fb ff ff          call   3760 <__ftrace_replace_code>
  0061     3c21:  85 c0                   test   %eax,%eax
  0063     3c23:  0f 85 00 00 00 00       jne    3c29 <ftrace_replace_code+0x69>  3c25: R_X86_64_PC32     .text.unlikely+0x368

Jumps to the cold subfunction right here ^

  0069     3c29:  45 85 ed                test   %r13d,%r13d
  006c     3c2c:  74 ce                   je     3bfc <ftrace_replace_code+0x3c>
  006e     3c2e:  e8 00 00 00 00          call   3c33 <ftrace_replace_code+0x73>  3c2f: R_X86_64_PLT32    __SCT__cond_resched-0x4
  0073     3c33:  83 c5 01                add    $0x1,%ebp
  0076     3c36:  41 39 6e 10             cmp    %ebp,0x10(%r14)
  007a     3c3a:  7f c9                   jg     3c05 <ftrace_replace_code+0x45>
  007c     3c3c:  4d 8b 36                mov    (%r14),%r14
  007f     3c3f:  4d 85 f6                test   %r14,%r14
  0082     3c42:  75 ac                   jne    3bf0 <ftrace_replace_code+0x30>
  0084     3c44:  5b                      pop    %rbx
  0085     3c45:  5d                      pop    %rbp
  0086     3c46:  41 5c                   pop    %r12
  0088     3c48:  41 5d                   pop    %r13
  008a     3c4a:  41 5e                   pop    %r14
  008c     3c4c:  e9 00 00 00 00          jmp    3c51 <ftrace_replace_code+0x91>  3c4d: R_X86_64_PLT32    __x86_return_thunk-0x4
  0091     3c51:  66 66 2e 0f 1f 84 00 00 00 00 00        data16 cs nopw 0x0(%rax,%rax,1)
  009c     3c5c:  0f 1f 40 00             nopl   0x0(%rax)
  00a0     3c60:  90                      nop
  00a1     3c61:  90                      nop
  00a2     3c62:  90                      nop
  00a3     3c63:  90                      nop
  00a4     3c64:  90                      nop
  00a5     3c65:  90                      nop
  00a6     3c66:  90                      nop
  00a7     3c67:  90                      nop
  00a8     3c68:  90                      nop
  00a9     3c69:  90                      nop
  00aa     3c6a:  90                      nop
  00ab     3c6b:  90                      nop
  00ac     3c6c:  90                      nop
  00ad     3c6d:  90                      nop
  00ae     3c6e:  90                      nop
  00af     3c6f:  90                      nop
  0000 000000000000036c <ftrace_replace_code.cold>:
  0000  36c:      48 89 de                mov    %rbx,%rsi
  0003  36f:      89 c7                   mov    %eax,%edi
  0005  371:      5b                      pop    %rbx
  0006  372:      5d                      pop    %rbp
  0007  373:      41 5c                   pop    %r12
  0009  375:      41 5d                   pop    %r13
  000b  377:      41 5e                   pop    %r14
  000d  379:      e9 62 fd ff ff          jmp    e0 <ftrace_bug>

And yes, you're right this patch broke it.

Because the weak function symbol gets deleted by the linker, these
instructions will belong to no function and insn->func will be NULL.

Then the new: STT_NOTYPE -> STT_FUNC+0 logic kicks in and turns this
into a tail-call.

That means insn->jump_dest is unset in favour of insn->call_dest, and
then the unreachable logic falls on it's face because that doesn't deal
with call_dest.

Ho-Humm.. let me find a non hacky solution.