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SubjectRe: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()
Date
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@huawei.com/)

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.


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);
}

---
I also had objtool running for ever on
arch/powerpc/sysdev/xics/icp-hv.o, which I fixed with the below hack:

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 51b6dcec8d6a..ef2303ad6381 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -529,7 +529,7 @@ static struct instruction *find_last_insn(struct
objtool_file *file,
unsigned int offset;
unsigned int end = (sec->sh.sh_size > 10) ? sec->sh.sh_size - 10 : 0;

- for (offset = sec->sh.sh_size - 1; offset >= end && !insn; offset--)
+ for (offset = sec->sh.sh_size - 1; offset && offset >= end && !insn;
offset--)
insn = find_insn(file, sec, offset);

return insn;
---

Now I only have the following two warnings:

arch/powerpc/sysdev/xics/icp-hv.o: warning: objtool: can't find
unreachable insn at .text.unlikely+0x0
drivers/crypto/vmx/aesp8-ppc.o: warning: objtool:
aes_p8_set_encrypt_key+0x44: unannotated intra-function call

The first one is linked to the infinite loop I hacked. So I now have to
understand what the problem really is.

The second one is an assembly file aesp8-ppc.S which lacks the changes
you did in other assembly files in patch 12.

Christophe
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