Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()
From: Naveen N. Rao
Date: Thu Jun 30 2022 - 04:15:07 EST
Christophe Leroy wrote:
Hi Sathvika,
Adding ARM people as they seem to face the same kind of problem (see
https://patchwork.kernel.org/project/linux-kbuild/patch/20220623014917.199563-33-chenzhongjin@xxxxxxxxxx/)
Le 27/06/2022 à 17:35, Sathvika Vasireddy a écrit :
On 25/06/22 12:16, Christophe Leroy wrote:
Le 24/06/2022 à 20:32, Sathvika Vasireddy a écrit :
objtool is throwing *unannotated intra-function call*
warnings with a few instructions that are marked
unreachable. Remove unreachable() from WARN_ON()
to fix these warnings, as the codegen remains same
with and without unreachable() in WARN_ON().
Did you try the two exemples described in commit 1e688dd2a3d6
("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with
asm goto") ?
Without your patch:
00000640 <test>:
640: 81 23 00 84 lwz r9,132(r3)
644: 71 29 40 00 andi. r9,r9,16384
648: 40 82 00 0c bne 654 <test+0x14>
64c: 80 63 00 0c lwz r3,12(r3)
650: 4e 80 00 20 blr
654: 0f e0 00 00 twui r0,0
00000658 <test9w>:
658: 2c 04 00 00 cmpwi r4,0
65c: 41 82 00 0c beq 668 <test9w+0x10>
660: 7c 63 23 96 divwu r3,r3,r4
664: 4e 80 00 20 blr
668: 0f e0 00 00 twui r0,0
66c: 38 60 00 00 li r3,0
670: 4e 80 00 20 blr
With your patch:
00000640 <test>:
640: 81 23 00 84 lwz r9,132(r3)
644: 71 29 40 00 andi. r9,r9,16384
648: 40 82 00 0c bne 654 <test+0x14>
64c: 80 63 00 0c lwz r3,12(r3)
650: 4e 80 00 20 blr
654: 0f e0 00 00 twui r0,0
658: 4b ff ff f4 b 64c <test+0xc> <==
0000065c <test9w>:
65c: 2c 04 00 00 cmpwi r4,0
660: 41 82 00 0c beq 66c <test9w+0x10>
664: 7c 63 23 96 divwu r3,r3,r4
668: 4e 80 00 20 blr
66c: 0f e0 00 00 twui r0,0
670: 38 60 00 00 li r3,0 <==
674: 4e 80 00 20 blr <==
678: 38 60 00 00 li r3,0
67c: 4e 80 00 20 blr
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.
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?
- Naveen