linux.git/tools/objtool/check.c, branch v5.18-rc2

objtool: Fix SLS validation for kcov tail-call replacement

2022-04-05T08:24:40+00:00

Since not all compilers have a function attribute to disable KCOV
instrumentation, objtool can rewrite KCOV instrumentation in noinstr
functions as per commit:

  f56dae88a81f ("objtool: Handle __sanitize_cov*() tail calls")

However, this has subtle interaction with the SLS validation from
commit:

  1cc1e4c8aab4 ("objtool: Add straight-line-speculation validation")

In that when a tail-call instrucion is replaced with a RET an
additional INT3 instruction is also written, but is not represented in
the decoded instruction stream.

This then leads to false positive missing INT3 objtool warnings in
noinstr code.

Instead of adding additional struct instruction objects, mark the RET
instruction with retpoline_safe to suppress the warning (since we know
there really is an INT3).

Fixes: 1cc1e4c8aab4 ("objtool: Add straight-line-speculation validation")
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lkml.kernel.org/r/20220323230712.GA8939@worktop.programming.kicks-ass.net

objtool: Fix IBT tail-call detection

2022-04-05T08:24:40+00:00

Objtool reports:

  arch/x86/crypto/poly1305-x86_64.o: warning: objtool: poly1305_blocks_avx() falls through to next function poly1305_blocks_x86_64()
  arch/x86/crypto/poly1305-x86_64.o: warning: objtool: poly1305_emit_avx() falls through to next function poly1305_emit_x86_64()
  arch/x86/crypto/poly1305-x86_64.o: warning: objtool: poly1305_blocks_avx2() falls through to next function poly1305_blocks_x86_64()

Which reads like:

0000000000000040 :
	 40:       f3 0f 1e fa             endbr64
	...

0000000000000400 :
	400:       f3 0f 1e fa             endbr64
	404:       44 8b 47 14             mov    0x14(%rdi),%r8d
	408:       48 81 fa 80 00 00 00    cmp    $0x80,%rdx
	40f:       73 09                   jae    41a 
	411:       45 85 c0                test   %r8d,%r8d
	414:       0f 84 2a fc ff ff       je     44 
	...

These are simple conditional tail-calls and *should* be recognised as
such by objtool, however due to a mistake in commit 08f87a93c8ec
("objtool: Validate IBT assumptions") this is failing.

Specifically, the jump_dest is +4, this means the instruction pointed
at will not be ENDBR and as such it will fail the second clause of
is_first_func_insn() that was supposed to capture this exact case.

Instead, have is_first_func_insn() look at the previous instruction.

Fixes: 08f87a93c8ec ("objtool: Validate IBT assumptions")
Reported-by: Stephen Rothwell 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lkml.kernel.org/r/20220322115125.811582125@infradead.org

objtool: Find unused ENDBR instructions

2022-03-15T09:32:47+00:00

Find all ENDBR instructions which are never referenced and stick them
in a section such that the kernel can poison them, sealing the
functions from ever being an indirect call target.

This removes about 1-in-4 ENDBR instructions.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.763643193@infradead.org

objtool: Validate IBT assumptions

2022-03-15T09:32:46+00:00

Intel IBT requires that every indirect JMP/CALL targets an ENDBR
instructions, failing this #CP happens and we die. Similarly, all
exception entries should be ENDBR.

Find all code relocations and ensure they're either an ENDBR
instruction or ANNOTATE_NOENDBR. For the exceptions look for
UNWIND_HINT_IRET_REGS at sym+0 not being ENDBR.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.705110141@infradead.org

objtool: Read the NOENDBR annotation

2022-03-15T09:32:46+00:00

Read the new NOENDBR annotation. While there, attempt to not bloat
struct instruction.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.586815435@infradead.org

objtool: Rework ASM_REACHABLE

2022-03-15T09:32:44+00:00

Currently ASM_REACHABLE only works for UD2 instructions; reorder
things to also allow over-riding dead_end_function().

To that end:

 - Mark INSN_BUG instructions in decode_instructions(), this saves
   having to iterate all instructions yet again.

 - Have add_call_destinations() set insn->dead_end for
   dead_end_function() calls.

 - Move add_dead_ends() *after* add_call_destinations() such that
   ASM_REACHABLE can clear the ->dead_end mark.

 - have validate_branch() only check ->dead_end.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.410010807@infradead.org

x86: Mark __invalid_creds() __noreturn

2022-03-15T09:32:44+00:00

vmlinux.o: warning: objtool: ksys_unshare()+0x36c: unreachable instruction

0000 0000000000067040 :
...
0364    673a4:	4c 89 ef             	mov    %r13,%rdi
0367    673a7:	e8 00 00 00 00       	call   673ac 	673a8: R_X86_64_PLT32	__invalid_creds-0x4
036c    673ac:	e9 28 ff ff ff       	jmp    672d9 
0371    673b1:	41 bc f4 ff ff ff    	mov    $0xfffffff4,%r12d
0377    673b7:	e9 80 fd ff ff       	jmp    6713c 

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lkml.kernel.org/r/Yi9gOW9f1GGwwUD6@hirez.programming.kicks-ass.net

exit: Mark do_group_exit() __noreturn

2022-03-15T09:32:43+00:00

vmlinux.o: warning: objtool: get_signal()+0x108: unreachable instruction

0000 000000000007f930 :
...
0103    7fa33:  e8 00 00 00 00          call   7fa38  7fa34: R_X86_64_PLT32   do_group_exit-0x4
0108    7fa38:  41 8b 45 74             mov    0x74(%r13),%eax

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.351270711@infradead.org

x86: Mark stop_this_cpu() __noreturn

2022-03-15T09:32:43+00:00

vmlinux.o: warning: objtool: smp_stop_nmi_callback()+0x2b: unreachable instruction

0000 0000000000047cf0 :
...
0026    47d16:  e8 00 00 00 00          call   47d1b        47d17: R_X86_64_PLT32   stop_this_cpu-0x4
002b    47d1b:  b8 01 00 00 00          mov    $0x1,%eax

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.290905453@infradead.org

objtool: Ignore extra-symbol code

2022-03-15T09:32:43+00:00

There's a fun implementation detail on linking STB_WEAK symbols. When
the linker combines two translation units, where one contains a weak
function and the other an override for it. It simply strips the
STB_WEAK symbol from the symbol table, but doesn't actually remove the
code.

The result is that when objtool is ran in a whole-archive kind of way,
it will encounter *heaps* of unused (and unreferenced) code. All
rudiments of weak functions.

Additionally, when a weak implementation is split into a .cold
subfunction that .cold symbol is left in place, even though completely
unused.

Teach objtool to ignore such rudiments by searching for symbol holes;
that is, code ranges that fall outside the given symbol bounds.
Specifically, ignore a sequence of unreachable instruction iff they
occupy a single hole, additionally ignore any .cold subfunctions
referenced.

Both ld.bfd and ld.lld behave like this. LTO builds otoh can (and do)
properly DCE weak functions.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.232019347@infradead.org