Christophe Leroy wrote:
Le 30/06/2022 à 10:05, Naveen N. Rao a écrit :
Christophe Leroy wrote:
The builtin variant of unreachable (__builtin_unreachable()) works.
How about using that instead of unreachable() ?
In fact the problem comes from the macro annotate_unreachable() which
is called by unreachable() before calling __build_unreachable().
Seems like this macro adds (after the unconditional trap twui) a call
to an empty function whose address is listed in section
.discard.unreachable
1c78: 00 00 e0 0f twui r0,0
1c7c: 55 e7 ff 4b bl 3d0
<qdisc_root_sleeping_lock.part.0>
RELOCATION RECORDS FOR [.discard.unreachable]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_REL32 .text+0x00000000000003d0
The problem is that that function has size 0:
00000000000003d0 l F .text 0000000000000000
qdisc_root_sleeping_lock.part.0
And objtool is not prepared for a function with size 0.
annotate_unreachable() seems to have been introduced in commit
649ea4d5a624f0 ("objtool: Assume unannotated UD2 instructions are dead
ends").
Objtool considers 'ud2' instruction to be fatal, so BUG() has
__builtin_unreachable(), rather than unreachable(). See commit
bfb1a7c91fb775 ("x86/bug: Merge annotate_reachable() into _BUG_FLAGS()
asm"). For the same reason, __WARN_FLAGS() is annotated with
_ASM_REACHABLE so that objtool can differentiate warnings from a BUG().
On powerpc, we use trap variants for both and don't have a special
instruction for a BUG(). As such, for _WARN_FLAGS(), using
__builtin_unreachable() suffices to achieve optimal code generation from
the compiler. Objtool would consider subsequent instructions to be
reachable. For BUG(), we can continue to use unreachable() so that
objtool can differentiate these from traps used in warnings.
Not sure I understand what you mean.
__WARN_FLAGS() and BUG() both use 'twui' which is unconditionnal trap,
as such both are the same.
On the other side, WARN_ON() and BUG_ON() use tlbnei which is a
conditionnel trap.
Objtool classifies 'ud2' as INSN_BUG, and 'int3' as INSN_TRAP. In x86
BUG(), there is no need for an annotation since objtool assumes that
'ud2' terminates control flow. But, for __WARN_FLAGS(), since 'ud2' is
used, an explicit annotate_reachable() is needed. That's _reachable_, to
indicate that the control flow can continue with the next instruction.
On powerpc, we should (eventually) classify all trap variants as
INSN_TRAP. Even in the absence of that classification today, objtool
assumes that control flow continues with the next instruction. With your
work to utilize asm goto for __WARN_FLAGS(), with no extra instructions
being generated, I think it is appropriate to just use
__builtin_unreachable() and to not use the annotation.
In any case, we are only hitting this since gcc is generating a 'bl' due
to that annotation. We are not yet enabling full objtool validation on
powerpc, so I think we can revisit this at that point.
The following changes to objtool seem to fix the problem, most warning
are gone with that change.
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 63218f5799c2..37c0a268b7ea 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -77,6 +77,8 @@ static int symbol_by_offset(const void *key, const
struct rb_node *node)
if (*o < s->offset)
return -1;
+ if (*o == s->offset && !s->len)
+ return 0;
if (*o >= s->offset + s->len)
return 1;
@@ -400,7 +402,7 @@ static void elf_add_symbol(struct elf *elf, struct
symbol *sym)
* Don't store empty STT_NOTYPE symbols in the rbtree. They
* can exist within a function, confusing the sorting.
*/
- if (!sym->len)
+ if (sym->type == STT_NOTYPE && !sym->len)
rb_erase(&sym->node, &sym->sec->symbol_tree);
}
Is there a reason to do this, rather than change __WARN_FLAGS() to use
__builtin_unreachable()? Or, are you seeing an issue with unreachable()
elsewhere in the kernel?
At the moment I'm trying to understand what the issue is, and explore
possible fixes. I guess if we tell objtool that after 'twui' subsequent
instructions are unreachable, then __builtin_unreachable() is enough.
Yes, see my explanation above. Since no 'bl' is emitted with the
builtin, objtool won't complain, especially for mcount.
I think we should also understand why annotate_unreachable() gives us a
so bad result and see if it can be changed to something cleaner than a
'bl' to an empty function that has no instructions.
Indeed. Not really sure. annotate_unreachable() wants to take the
address of the instruction after the trap. But, in reality, due to use
of asm goto for __WARN_FLAGS, no instructions would be generated. I
wonder if that combination causes such code to be emitted.
- Naveen
